DATA PRECISION 938 User manual

Instruction Manual
for
MODEL 938
Hand-Held, Pocket-Portable
3 1 /2 Digit
CAPACITANCE METER
Publication No 42-5124
■□ATA PRECISION

42-5124
MODEL 938
PROPRIETARY NOTICE
The information contained in this publication is derived in part
from proprietary and patent data of the Data Precision Corporation.
This information has been prepared for the express purpose of
assisting operating and maintenance personnel in the efficient use
of the instrument described herein. Publication of this information
does not convey any rights to use or reproduce it or to use for any
purpose other than in connection with the installation, operation
and maintenance of the equipment described herein.
First Edition
arch, 1979
Copyright 1979 DATA PRECISION CORPORATION
Printed in USA all rights reserved.
42-5124
MODEL 938
CONTENTS
1 INTRODUCTION
1.1 General Description
1.2 Operating Features
2 SPECIFICATIONS
2.1 Capacitance easurement
2.2 Functional Characteristics
2.3 Power
2.4 Accessories Supplied
2.5 Options / Accesories
3 OPERATION AND RECALIBRATION
3.1 General
3.2 Unpacking & Repacking odel 938
3.3 Installing (or Replacing) the Battery
3.4 Operating Controls & Indicators
3.5 aking easurements with odel 938
3.6 Recalibrating odel 938
3.7 Operation
4 PRINCIPLES OF OPERATION
4.1 Basic Concepts
4.2 System Block Diagram
4.3 Signal Conditioning Details
4.4 Signal Conditioning Control Logic
4.5 A/D Converter Details
4.6 Display
4.7 Power Supply
5 MAINTENANCE
5.1 General
5.2 Trouble Shooting Strategy
5.3 Safety Precautions and Procedures
5.4 Closed-Case Trouble Shooting
5.5 Open-Case Details
6 PARTS LISTS
6.1 Ordering Information
Reference Schematic

42-5124
MODEL 938
mtispiece
Model 938
Digital
Capacitan
ce Meter
MODEL 938
INTRODUCTION
Chapter 1
INTRODUCTION
1 1 GENERAL DESCRIPTION
Data Precision Model 938 Digital Capacitance Meter (Frontis 
piece) is a hand-held, portable, field and laboratory, 3 1 /z-digit
capacitance meter. It employs a unique circuit (patent pending)
that provides accuracy, stability, and noise immunity not
previously obtainable in low-cost capacitance meters.
The meter's eight (8) range scales, providing the user with
nominal full scale values from 200 pF to 2000 gF (in decade
steps), with corresponding resolutions from 0.1pF to 1 gF, make
this instrument a useful tool in a wide range of applications.
Reduced component count, the use of low-powered C OS IC ’ s,
and the use of a liquid crystal display all contribute to reliable
performance over an extended battery life (100 to 200 hours with
normal usage). Optionally, an accessory Battery Eliminator (Data
Precision odel BE9) may be used to power the instrument from
local ac sources. The BE9 is available for each of the standard
supplies (100VAC, 117VAC, 200VAC, or 240VAC).
The odel 938 is assembled in the new Data Precision standard
hand-held mini case configuration, molded of high-impact Noryl*
plastic, and resistant to shock and vibration in even the most
stringent of field environmental applications.
The odel 938 is supplied with battery and spare fuse, and is
ready to use as soon as the battery iS installed in the case (see
Chapter 3). The front panel includes standard banana-jack recep 
tacles, as well as recessed spring-clip connectors with select 
able spacing for capacitor leads of varying separations. A stan 
dard banana-plug to alligator clips test lead set (T9) is included in
the standard configuration.
‘ Registered trademark.
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MODEL 938
1 2 OPERATING FEATURES
odel 938 input circuitry is protected by a 1/4-amp Fast-Bio
fuse against input overloads. The liquid crystal display
assembly is built around large (0.5") digits, and is easily read
under a wide range of ambient lighting conditions, indoors or
out. The decimal point is automatically positioned in the display
by the action of selecting the range scale, so that the results
may be read directly in the units established by the range scale
selected. The full scale range values are printed adjacent to the
range-scale -select pushbuttons, and include the capacitance
units, as well as the values.
Out-of-range values are indicated by blanking all but the SD
and decimal point. This prevents erroneous readings,
while at the same time providing an indication of an
operable meter. The meter also contains a sense and display cir 
cuit to Indicate when the battery (or ac source power) is low. Ap 
pearance of the LO BAT display indicates that the battery is
below 7 volts, or that the ac source is at a lower voltage than
specified.
One internal trimmer is used to calibrate the meter for accuracy
on all ranges. One external thumbwheel adjusts the zero and
simplifies the meter use from one application to another where
different levels of stray capacitance to affect the measurement
results.
1-2
MODEL 938
SPECIFICATIONS
Chapter 2
SPECIFICATIONS
2 1 CAPACITANCE MEASUREMENT
NOMINAL
RANGE
MAX IN-RANGE
DISPLAY
RESOLUTION
200 pF
199.9 pF
0.1 pF
2 nF
1.999 nF
1 pF
20 nF
19.99 nF 10 pF
200 nF
199.9 nF
100 pF
2 m F
1.999 m F
1000 pF
20 m F
19.99 m F
0.01 m F
200 m F
199.9 m F
0.1 m F
2000 m F
1999 m F
1 m F
Accuracy (1 yr , 23°C ± 5°C):
±(0.1% Reading + 1 digit + 0.5pF)
Except ±(1.0% reading + 1 digit) on 2000 m F range).
Temperature Coefficient:
±(0.01% input + 0.01 %range)/°C
Except ±(0.02% input + 0.02% range)/°C on 200 pF and
2000 m F range.
Excitation Voltage:
2.8 volts peak, maximum
High input terminal is always more positive than low ter 
minal
Measurement Technique:
Dual-slope integrating A/D converter; measures ratio of
change in charge to change in voltage (Capacitance).
Overload:
Input protected by 1/4 amp 250V fuse
Controls:
Eight range push buttons, On/Off, and zero adjust.
2-1

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MODEL 938
2 2 FUNCTIONAL CHARACTERISTICS
Display
0.5" (13mm) liquid crystal
Reading Rate:
2.0 readings /sec.
Overload Indication:
Blanking of all digits except SD and decimal point
Calibration Adjustments:
One internal adjustment for accuracy
One front-panel adjustment for zero
2.3 POWER
Power Supply:
9V standard battery or optional battery eliminator BE9
Battery Life:
Approximately 200 hours on alkaline or 100 hours on car 
bon zinc battery with normal usage
2 4 PHYSICAL & ENVIRONMENTAL
Size:
6.75"L x 3.5"W x 1.5"H (170mm x 89mm x 38mm)
2 4 ACCESSORIES SUPPLIED
Operator ’ s and aintenance anual
Warranty Card, Certificate of Conformance , and Final Test
and Calibration Certification in a combined document
9V Battery
Spare Fuse
Banana plug to alligator clips Test Leads (T9)
2 5 OPTIONS/ACCESSORIES
Test Leads with alligator clips ........................................................ T9
Banana plug mounted test clip ................................................... LC1
Battery Eliminator ......................................................................... BE9
2-2
MODEL 938 OPERATION & RECALIBRATION
Chapter 3
OPERATION AND RECALIBRATION
3 1 GENERAL
Chapter 3 contains instructions for installing (or replacing) the
battery, for connecting the odel 938 to make a measurement,
for recalibrating the capacitance-meter, and hints for obtaining
the high performance designed into the instrument.
3 2 UNPACKING AND REPACKING THE MODEL 938
a As shown in Figure 2-1, the odel 938 is placed in a water 
proof plastic wrapper, securely fitted between molded plastic
foam shock isolators, and placed with the documentation and
battery in the shipping carton. When unpacked, you should find:
(1) odel 938 Digital Capacitance-meter
(2) Certificate of Conformance, Factory Test Certification,
and Warranty Card (combined in one document)
(3) Spare 1/4A 250V 3AG Fast Bio Fuse, inside multimeter
bottom cover
(4) 9V zinc-carbon battery, outside of multimeter.
(5) Test lead set T9
(6) Instruction manual
b. Carefully examine these articles, noting especially the match 
ing serial numbers of the instrument and the test certification.
Inspect the packing case and the instrument for any signs of
damage during shipment and report immediately to the carrier.
Fill out and return the warranty card to register your instrument
and to establish your warranty service interval.
c Accessories, such as test leads and battery eliminator, may
have been ordered. These will be shipped in their own con 
tainers.
d Retain the packing material for reshipment. When shipping
odel 938, use foam liners and pack in original carton, if
available. If new wrapping and shipping container must be used,
wrap the instrument in at least 3 inches of shock isolating
material, such as bubble plastic, and fasten securely in the con 
tainer.
3-1

42-5124
MODEL 938
Figure 3-1 Model 938 Shipping Configuration
3 3 INSTALLING (OR REPLACING) THE BATTERY
Figure 3-2 illustrates the procedure for installing (or replacing)
the battery in odel 938 Capacitance-meter. Proceed as follows:
(1) Be sure power switch is OFF.
(2) Remove any input connections.
(3) Remove the fastening screw holding the bottom cover in
place.
(4) Remove the bottom cover.
(5) Remove old battery (if replacing it).
(6) Attach the battery clip to the 9V battery.
(7) Place the battery in the battery space.
(8) Replace the bottom cover and its fastening screw.
3-2
MODEL 938
OPERATION & RECALIBRATION
Figure 3-2 Installing (or Replacing) Battery in Model 938
3 4 OPERATING CONTROLS & INDICATORS
Figure 3-3 illustrates the operating features of the odel 938
C- eter. The range switches are interlocked so that only one
range selection may be operative at any one time. (Note,
however, that all switches may be placed in the “ out ” position if
the selected range pushbutton is not pushed all the way in.)
Table 3-1 describes the items referenced in the illustration and
the functions performed by each.
3-3

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MODEL 938
Figure 3-3 Model 938 Front Panel Features
3-4
MODEL 938
OPERATION & RECALIBRATION
MODEL 938 OPERATING CONTROLS & FEATURES
Table 3-1
FIG
BEE
NAME/DESCRIPTION
FUNCTION
1.
Display value
3 full decades plus 1 SD (3Vz
digits); liquid crystal;
automatic positioning of
decimal point.
2.
LO BAT
Display message when battery
(or line voltage when using
Battery Eliminator) is less than
the present level. Normal
operation is possible for a few
hours after the LO BAT indica 
tion first appears.
3. Eight range pushbut 
tons
Interlocked switches. Depress 
ing one switch automatically
releases another in the set of
eight.
4.
Capacitor lead con 
nectors
Spring contacts spaced for
convenient insertion of
capacitor leads. Polarized for
use with polarized capacitors.
5. Test Lead input con 
nectors
Standard banana jack input
connectors. Polarized for use
with polarized capacitors.
6.
Range Scale Decal Range scale units and max 
imum display (+ 1) associated
with each of the range selec 
tion buttons. Note that the
scale values are expressed as
1 count more than the max 
imum value of capacitance
(2.000, 20.00 , 200.0, 2000)
although the maximum display
is 1999 counts in each case.
7. ZERO
Thumbwheel adjustment of the
zero value of the disolav.
8.
ON
Two-position slide switch.
3-5

42-5124
MODEL 938
3 5 MAKING MEASUREMENTS WITH MODEL 938
CAUTION
A maximum excitation voltage of 2.8 volts (peak) is applied to
the capacitor under test. Observe the polarity conventions for
polarized capacitors.
3 5 1 General
Follow the sequence below when making measurements.
1. Select the range switch for the maximum expected
capacitance.
2. Turn power ON.
3. Check the zero display. Before connecting the capacitor
under test (but after inserting any test leads or test fixtures),
rotate the front-panel thumbwheel for a zero display. Note that
the maximum offset that may be introduced with the thumbwheel
is about 20pF, and that a negative value is possible with the
thumbwheel use.
4. Insert the capacitor leads into the spring contact connec 
tors (or into the test fixture, or connect alligator clips to the
capacitor leads.)
Do not hold the capacitor
5. Read the display. The value is direct reading in the electrical
units (picofarads, nanofarads, or microfarads) indicated at the
selected range switch. If the three less significant digits are
blanked (indicating an Out-of-Range measurement), select the
next higher range scale. If the display indicates one or more
leading zero ’ s, shift to the next lower range scale to improve the
resolution of the measurement.
3 5 2 Powering the Model 938
a. The odel 938 is normally powered from a standard 9V
carbon-zinc battery of the transistor radio type. A Data Precision
Battery Eliminator, odel BE9, may be used to convert the in 
strument to line operation, and its use effectively removes the
battery from the circuit.
b The odel 938 incorporates a battery (or battery eliminator)
voltage sense circuit. A low voltage output from these power
sources is indicated in the display by LO BAT Appearance of
the LO BAT indication when using the battery eliminator is in 
dicative of a low ac source voltage. When using internal battery,
a LO BAT display indicates a nominal battery output of less than
7V and that approximately 20% of the battery life remains. The
3-6
MODEL 938
OPERATION & RECALIBRATION
battery should be replaced (or shift to use of a Battery
Eliminator) as soon as possible after the appearance of the LO
BAT indication. However, in-spec measurements may still be
made for several hours after the LO BAT appears before the in 
strument becomes inaccurate.
A weak or dead battery should be removed promptly to prevent
possible damage because of leakage
3 6 RECALIBRATING THE MODEL 938
3 6 1 General
The odel 938 requires two adjustments to obtain in-spec perfor 
mance on all range scales. One of the adjustments is internal
gain trimmer R4. It calibrates the reference voltage that is
developed by capacitor discharge for the dual slope integrating
A/D converter. The second adjustment is a front-panel-
accessible zero potentiometer R8, by which up to 20pF offset
may be introduced to compensate for interal and lead stray
cpacitances.
The design incorporates sufficiently high quality, precise , and
stable components to assure overall specified performance with
these two adjustments.
The odel 938 is designed to remain in calibration for at least 12
months. Recalibration, when required, should be performed
as described in the paragraphs that follow.
3 6 2 Access to Calibrating Adjustment R4
Remove the back cover of the multimeter and locate the access
hole in the inner shield cover. See Figure 3-2, referenced earlier
when installing the battery. The adjusting potentiometer is
mounted on the pcv board, and its shaft is accessible through the
hole that is identified by the label R4 printed on the shield cover.
3 6 3 Recalibrating Procedure
(1) Set up the meter with the lead configuration to be used in
the recalibration. Insert any test fixture (Kelvin clip, e.g.) or con 
nect any test leads to be used later, t Remember that even very
short leads may have a significant effect.)
(2) Select the 200pF measurement range.
(3) Adjust the front-panel control R8 for 000 display. Back off
the thumbwheel position one half turn from a negative display
(4) Connect a standard capacitor whose value is near full scale
on one of the higher ranges, and is known to within ±0.05%.
(5) Select the appropriate range scale.
(6) Adjust R4 for a display equal to the known capacitance.
(7) Remove the standard capacitor. Replace bottom cover.
3-7

42-5124 MODEL 938
3 7 OPERATION
3 7 1 Operating Controls & Indicators
Table 3-1 described the odel 938 C- eter controls and in 
dicators that are shown in Figure 3-3. Refer to paragraph 3.4.
3 7 2 Hints for Improved Operation
The ideal capacitor exhibits no electrical performance except
that of capacitance; it would convert all the “ storing ” energy into
charge, would be able to hold that charge indefinitely, and would
be able to deliver all the energy represented by its charge into a
load without loss. While many laboratory standard capacitors
approach this ideal with more or less success, all capacitors
contain elements intrinsic to their design that cause departures
from ideal in all realizable capacitor elements.
Figure 3-4 illustrates two of the more important equivalent
resitance elements that affect capacitor performance, — - and
measurement of capacitance values. These resistances exist in
a distributed fashion, but are drawn as shown in the illustration
for ease in analysis. The series resistance Rs introduces losses
in the charge and discharge actions, while the parallel resistance
Rp serves to drain the stored charge.
The odel 938 measurement technique is hardly affected by the
series resistance Rs because the Capacitance eter measures
changes in the stored charge and changes in the charging
voltage, and does not depend upon the absolute values, except
when radically different from ideal. However, the parallel
resistance Rp parallels the range-selected precision resistor in
the odel 938 measuring circuit, and, depending upon the
relative values of the leakage resistance and that of the selected
range, may have a visible effect on the display value when
measuring leaky capacitors. In most cases, the resulting
measurement remains within the performance specification.
easured values that exceed the capacitor specified tolerances
will, therefore, be valid indicators of degraded components.
Measurements of very low capacitances should be performed us 
ing extremely short leads and a Kelvin clip in order to avoid in 
troducing any stray capacitance.
When using the optional test leads, remember that the leads in 
troduce a measurable capacitance to the measurement. As a
first approximation, the test lead capacitance may be measured
by opening the leads at the tips, recording the open circuit value,
3-8
MODEL 938
OPERATION & RECALIBRATION
and subtracting that value value from the display results.
However, in order for this correction to be valid, the succeeding
capacitor measurements must be made with all other measure 
ment conditions exactly the same.
The existence of a leaky capacitance may be detected if the
value changes significantly as the scales are changed. Since the
shift to a lower range scale introduces more resistance in the
discharge cycle (see principles of operation), the effect of the in 
ternal leaky resistance is minimized on the lower range scales.
9 Q
d0
Figure 3-4 Equivalent Capacitor Schematic
3-9
■3

42-5124
MODEL 938
NOTES:
3-10
MODEL 938
PRINCIPLES OF OPERATION
Chapter 4
PRINCIPLES OF OPERATION
4 1 BASIC CONCEPTS
4 1 1 Capacitance
The electrical property of capacitance describes the ability of a
component to store charge when subject to an electric potential
(voltage). All matter exhibits this property to a greater or lesser
extent. Personnel on an assembly line, for example, may store a
charge when they introduce friction while walking across a
carpet. That charge poses a potential danger to some integrated
circuits if it should be discharged through the components of the
IC, and care must be taken to “ ground ” personnel working with
charge-sensitive components. Capacitors are electronic com 
ponents specifically designed to store charge.
Figure 4-1 Charging a Capacitor through a Resistor
4-1
I-3

42-5124
MODEL 938
If there are no charge-storing units in series with the capacitor,
the capacitor will develop a charge equal to the potential applied
to it, as shown in Figure 4-1. The illustration includes a resistive
component in series with the capacitor that acts to impede the
flow of current, or the rate of storing charge. However, as long
as there is a relatively long charging time, the charge on the
capacitor will develop a potential across the capacitor equal to
the applied voltage. This charging phenomenon is also il 
lustrated graphically in Figure 4-1. In an ideal capacitor (with no
resistive components) the charge will remain without change un 
til the capacitor is connected in a circuit that allows the flow of
current, and the reduction in charge. Figure 4-2 illustrates the
capacitor discharge when connected to a resistance component.
The measurement of capacitance (the ability to store charge
under the “ pressure ” of a voltage “ force ” ) is often accomplished
by observing the effects on the capacitor voltage when the
capacitor stored charge is connected to a precisely known charg 
ing or discharging circuit for a precise time interval. (The
measurement of the amount of stored charge is also possible,
but with more elaborate and expensive equipment.) As shown in
Figure 4-3, the reduction in charge is directly proportional to the
time of the current flow and to the amount of capacitance. Or,
to obtain the capacitance from this relationship, the capacitance
is the change in charge per change in voltage
Figure 4-2 Charge and Discharge Cycles of a Capacitor
4-2
MODEL 938
PRINCIPLES OF OPERATION
4 1 2 Capacitance Measurement
The ratio definition as stated in the previous paragraph, is the
basis for the measurement as performed in the odel 938
Capacitance- eter. That is, the instrument uses a dual-slope
analog-to-digital converter that digitizes the ratio of the change
in charge to the change in voltage. The charge reduction is ac 
complished by connection to a circuit that allows current to flow,
and this current is averaged over an integral number of
cycles as a measure the value of the change in charge. The
voltage difference between the values across the capacitor at
the start and at the end of the discharge cycle is the
denominator in the ratio meaurement. The A/D converter
develops the digitized value of this ratio for display in the liquid
crystal display.
AQ1
CURRENT THROUGH R IN *3
Figure 4-3 Relationship of Capacitance to the Change of Charge
and Change of Voltage
4 2 SYSTEM BLOCK DIAGRAM
4 2 1 General
Figure 4-4 is a simplified block diagram of the instrumentation of
the odel 938 Capacitance- eter. The illustration is prepared in
some detail with respect to those elements that instrument the
basic concepts of capacitance measurement described briefly in
the previous paragraphs. Other functioning elements are includ 
ed in block form. Their circuit operation will be described in
greater detail in later paragraphs of this chapter.
4-3

42-5124
MODEL 938
Figure
4-4
System
Block
Diagram
4-4
MODEL 938
PRINCIPLES OF OPERATION
As shown in the illustration, the A/D converter operates on the
values of voltage connected to two pairs of inputs: the INPUT
and the REFERENCE A sequence of switching actions perform 
ed by the four output signals of a control logic programming
block determine the the circuit configurations that develop
values of the voltage signals for each of these two inputs to the
A/D converter. The four phases in the signal conditioning pro 
cess are: 4>1, 4>2, <i>3, and 4>4
<i>1: During which the capacitor to be measured (Cy) is charg 
ed to an initial nominal value depending upon the value of
capacitance and the range scale selected (but not exceeding 2.8
volts).
$2: During which Capacitor C4 and C5 are charged in series
to the initial value of the capacitor under test.
4>3: During which the capacitor under test Cy is discharged
through a range-selected resistor. The resulting current is averaged
over a number of cycles of this interval and converted into a
voltage on C3 by the filtering action of R9 and C3.
4>4: During which the value of the C t voltage at the end of the
discharge interval of 4>3 is stored on C4. As a result, the voltage
on C5 represents the difference between the initial and final
values of capacitance on C t - It is the REFERENCE input to the
A/D. Figure 4-5 derives the result in a more rigorous fashion.
4-5

42-5124
MODEL 938
4.2.2 The Capacitor initial Charge
When Switch A (Figure 4-4) is closed during 4>1, the capacitor
under test, Cy is charged rapidly to some nominal value, limited
to 2.8 volts.
The voltage level on the capacitor is connected to the non 
inverting input of amplifier Z1-1, which draws no current (and
therefore no charge) from the capacitor. The amplifier output
drives a FET follower that is cut off during 4>3. The voltage ap 
pears across the voltage divider R4, R5, and R6 (of which R4 is a
variable resistance with which to adjust the gain of the stage).
During <i>2, Switch B is closed, and capacitors C4 and C5 in
series are charged with the scaled value of the initial Cy voltage.
4 2 3 The Capacitor Discharge Interval
During <t>3, Switch C is closed (Figure 4-4), connecting the charg 
ed capacitor to the range-selected value of load resistance
through which to partially discharge the capacitor. During
repeated cycles of this phase, the filter action of R9 and C3 acts
as an averaging operation, storing the average value of the cur 
rent as a voltage across 03. This value is connected as the IN 
PUT to the A/D converter.
4.2.4 The Voltage Change easurement
During T4, Switch D closes (Figure 4-4), connecting the scaled
output from amplifier Z1-1 to C4. Since C4 and C5 in series had
previously been charged with the initial value of the charged
capacitor, capacitor C5 now retains the value of the difference
between the two samplings. This becomes the input to the
REFERENCE terminals of the A/D converter.
4 2 5 Zero Adjustment
Adjustment to zero on all ranges is accomplished by subtracting
a current proportional to the Reference voltage from the
discharge current flowing in the range-selected load resistance.
This is limited to ±20pF.
4 2 6 Range Scale Selection
Range scale conditioning of the two signals (difference in charge
and difference in voltage) is accomplished by several methods.
For full scale values of 200pF, 2nF, 20nF, and 200nF, the
resistance divider introduces a proportionate load for the
discharge cycle of the capacitor, and, therefore, a proportionate
current to be averaged for the A/D INPUT signal. For the higher
end of the range scales (2uF, 20uF, 200uF, and 2000uF), the
charge-discharge timing cycle is changed by range-alteration of
4-6
MODEL 938
PRINCIPLES OFOPERATION
the control logic timing circuit. The interval is lengthened by fac 
tors of 10 to obtain a greater voltage change during the
discharge phase for large value capacitors.
4 2 6 Display
The display is driven directly from the A/D converter integrated
circuit. Each of the three full digits is configured by a combina 
tion of 1 to 7 segments, each driven by a separate line from the
A/D. The liquid crystal back plane is driven by a square wave
signal, and the appropriate sgment in each digit is driven by an
out-of-phase component at the same frequency as the back
plane. A single line from the A/D drives the most significant
digit “ 1 ” , and another line drives the polarity sign (not used in
the C- eter). Range selection activates Exclusive OR gates for
the decimal point drives; the other input to each gate is the back 
plane drive signal.
A low battery sensing circuit, comparing the battery (or Batter
Eliminator) output with an internal reference, develops a drive
signal for the LO BAT indicator in the display. Insertion of the
jack for the Battery Eliminator removes the internal battery from
the power supply circuit.
4 3 SIGNAL CONDITIONING DETAILS
4.3.1 Charging and Discharging the Capacitor
Refer to the complete schematic at the back of this manual.
Note that the input unknown capacitance is paralleled internally
with a capacitor (C1) which, with the stray capacitance of the
assembly, always provides some “ zero ” capacitance for the
meter. This small capacitance always charges up in parallel
with the capacitance under test, and is the amount that is zeroed
out with the thumbwheel adjustment before connecting the
capacitor to be measured.
The capacitor under test is connected to the HI and LO terminals
(keeping the poarities correct for polarized components). Q1 and
Q2 are clamps, keeping the voltage at safe levels, and protecting
internal components. Q3 is a switching transistor that is turned
ON during <I>1 to connect the rail value to the HI terminal. The
power supply circuit described later keeps the voltage to be
developed across the unknown capacitance to a safe limit of 2.8
volts (nominal).
For measurment of low capacitance values, range selection of
100pF to 100nF opens the base drive to transistor Q6, and only
4-7

42-5124 MODEL 938
the FET Q4 is turned ON during 4>3 to connect the stored charge
in the unknown capacitance to the discharge circuit.
For measurement of higher capacitances, the <i>3 signal turns ON
04 and then 06, so that the discharge now takes place through
R16 and/or R1 1. ( Note that the measurement of voltage through
R11 is, effectively a 4-wire connection.) Changes in range scale
for the higher ranges is now accomplished by changing the
number of repetition cycles for each of the charge/discharge ac 
tions averaged. This is accomplished by switching connections
to the timing logic element, Z5.
4 3 2 Measuring the Change in Charge
In each $1 interval, the capacitor reaches some nominal charged
value (not necessarily the applied voltage), and the exact value is
not critical to the accuracy of measurement. In each *3 interval
the discharging capacitor generates a current in the range-
selected resistance paralleling and charging C3. Over a number
of such cycles, C3 stores a voltage proportional to the average
value of the current generated by the discharging capacitor.
4 3 3 Measuring the Difference in Voltage
The non-inverting input to Z1-1 is always connected to the high
side of the input connector, and to the unknown capacitor. The
operational amplifier drives a FET follower to develop a current
in the output resistor divider load so that the voltage across R4,
R5, and R6 in series is equal to the charge on the capacitor.
(The output “ follows ” the input capacitor voltage.) When the <i>2
control signal is active, the scaled output (at the voltage divider
junction of R5 and R6) appears across C4 and 05 in series.
When the 4>4 control signal is active, the scaled output appears
across C4 only. Since the value across 04 and 05 at <i>2 is the
value before discharge, and the value across 04 at 4>4 is the
value after discharge, then the value remaining on 05 is the dif 
ference.
4 3 4 Zero Adjustment
Transistor Q7, R7 and R8 provide a path for current flow from the
REFERENCE of the A/D converter to the input. The current in this
path is controlled by the value of the voltage at the non-inverting
input to Z1. Current will flow through this circuit until the
voltage developed across the two resistances R7 and R8 equals
that input voltage. Thus, a current proportional to the reference
is drawn from the discharge current. That current is calibrated
by adjustment of R8. It remains a constant count, even though
each measuring operation with difference capacitors develops
different INPUT and REFERENCE voltages.
4-8
MODEL 938 PRINCIPLES OF OPERATION
Figure 4-6. Signal Conditioning Waveforms (Z5)
4 4 SIGNAL CONDITIONING CONTROL LOGIC
Control signals for the signal conditioning circuits are developed in
Z5. The input to Z5 is the output from a master oscillator, crystal-
controlled to generate 32.768 KHz. It is counted down to develop
timing waveforms depending upon the switch-selected jumpering
shown in the reference schematic, and illustrated in Figure 4-6.
4 5 A/D CONVERTER DETAILS
4.5.1 Analog Section.
The odel 938 A/D Converter is a monolithic C OS chip contain 
ing both analog and digital sections on the same chip. Figure
4-7- illustrates the analog section, and includes the interfaces
with discrete elements on the main pc-board. As shown in the il 
lustration, the converter consists of a buffer, integrator, and
comparator, configured by internal switches programmed by the
digital section. The switches are identified by the names of the
step intervals during which they are “ on ” . For example, the iNT
switches are closed during Step 2, Integration of the Conditioned
Input; The A/Z switches are closed during Step 1, the Autozeroing
step; the DE switches are closed during Step 3, Integration of the
Reference Voltage. Note that the DE switches are closed in
pairs: DE( + ) when a positive conditioned input is sensed at the
end of Step 1, and DE(-) when a negative conditioned input is
sensed at the end of Step 2.
(The steps of the A/D conversion are not synchronized with the
four phases of the signal conditioning described earlier.)
During Step 1 (Autozeroing; A/Z switches closed), the conditioned
input is removed (INT is open); the A/D input is shorted to Analog
Common, the integrator-comparator loop is closed, and capacitor
C9 is charged with compensating offset for the automatic zero 
ing during Steps 2 and 3 to follow. Capacitor C6 is charged with
the reference voltage for later use in Step 3.
4-9

42-5124
MODEL 938
During Step 2 (INT switches are closed), the A/D input is con 
nected to the buffer amplifier, whose output is connected in
series with the zero compensation of the previous step, and the
net value charges capacitor C7.
During Step 3 (DE switches are closed), the pair of DE switches
are closed to connect the voltage across C6 to the A/D input with
the proper polarity convention in order to reduce the voltage
previously built up on integrating capacitor C7 in Step 2.
4.5.2 Digital Section of the A/D Converter.
The digital section of the A/D converter contains counters and
logic circuitry with which to develop the 3-step timing signals
and to generate driving signals for each of the seven segments
of the 3 full display decades, as well as the driving signal for the
SB “ 1 ” . This section of the IC also includes the circuitry to
determine and drive the negative polarity sign, when required, in
the display, to sense an out-of-range condition, and to blank the
digits. The chip also provides the driving signal for the liquid
crystal display back plane in proper synchronism with the
signals for the display elements.
Figure 4-7 Analog Section of the A/D Converter
R14
4-10
MODEL 938 PRINCIPLES OF OPERATION
4.5.3 Converter Timing
The three steps in the A/D conversion cycle, that occur asyn 
chronously with the 4 phases of the charge/discharge cycles in
the signal conditioning circuits, complete an A/D conversion at
the nominal rate of 2 per second. A typical waveform pattern for
the A/D timing waveforms, referred to the output of the in 
tegrator, is shown in Figure 4-8.
4 6 DISPLAY
The liquid crystal display used in the odel 938 is a low power
device, whose back plane power is furnished directly by the A/D
converter chip, while the digit segments are synchronously
driven by independent lines from the A/D chip (7 segment drivers
for each of 3 digits, 1 driver for the SD, and 1 for the negative
polarity sign). Drivers for the decimal points (3) and for the “ LO
BAT ” indicator are gated with the back plane power drive
through Exclusive OR gates.
4-11

42-5124
MODEL 938
4 6 POWERSUPPLY
a. Power for the odel 938 is derived from either a 9-volt tran 
sistor radio type battery or from a Battery Eliminator (BE9).
When a battery eliminator is used, the internal battery is discon 
nected from the circuit.
b A low battery sense circuit in the odel 938 continuously com 
pares a fraction of the battery voltage (or BE9 output) against a
5.8V reference and generates the “ LO BAT ” drive signal when
the input is approximately 7 volts.
Figure 4-9 illustrates the generation of excitation sup 
plies and voltage levels in the model 938.
Figure 4-9 • Model 938 Power Sourcing Schematic
4-12
MODEL 938
MAINTENANCE
Chapters
MAINTENANCE
5 1 GENERAL
CAUTION
odel 938 is covered by a one-year warranty, and
it should be returned to the factory for
maintenance within the warranty period. At 
tempts to make any extensive repairs within the
warranty period may invalidate the warranty. If
repairs are needed after the warrany period, only
qualified technicians should attempt to effect
such repairs and should use test instruments and
standards calibrated within the accuracy and
tolerances of the instrument specifications.
Note that the warranty does not cover damage
caused by leaky batteries.
odel 938 has been designed with a minimum number of com 
ponents, and is intended to deliver years of reliable service.
In addition, all components have been screened and tested, prior
to use, in an extensive quality control procedure, and the final
assembly has been burned in prior to final test and shipment.
5 2 TROUBLE SHOOTING STRATEGY
Should the meter performance indicate the need for repair, a
well-defined strategy should be used to isolate the cause of
malfunction. Such a strategy begins with an analysis of the
symptoms of malfunction in order to keep to a minimum the
repair and replacement operations within the meter assembly.
uch can be learned from an analysis of the meter performance
under varying input conditions. Figure 5-1 illustrates the type of
5-1

42-5124
MODEL 938
Figure 5-1 Trouble Shooting Strategy
5-2
MODEL 938
MAINTENANCE
analysis that may be used to isolate the probable cause of
malfunction to specific circuits. As shown in the illustration,
some checks may be made without opening the meter, while
others may require access to internal circuitry.
CAUTION
Do not attempt any internal repairs before observ 
ing the cautions and warnings in paragraph 5.3.
Check all input and line (if BE9 is used) connec 
tions before assuming an internal circuit cause of
the malfunction symptom. Read and understand
Chapter 4, Principles of Operation, before attemp 
ting any repair.
5 3 SAFETY PRECAUTIONS AND PROCEDURES
a. Remove input signal connections before opening the case.
b. Take care that static discharges do not contact static 
sensitive IC components. Use grounding straps and anti-static
bags and devices when handling and storing these components.
5 4 COMPONENTS
Figures 5-2 identifies and locates the components for con 
venient trouble shooting. Refer to chapter 4 for waveform
and voltage levels in obtaining test comparisions.
5-3

42-5124
MODEL 938
Figure
5-2
Component
Location
5-4
MODEL 938
PARTS LIST
Chapter 6
PARTS LIST
6 1 ORDERING REPLACEMENT PARTS
a. Replaceable parts for odel 938 are listed in paragraph 6.2.
That list identifies the reference designation, description of
significiant parameters, and the Data Precision Part Number.
b. Refer to the list when a replacement is required. All orders
for parts should include at least the following:
odel Number ( ODEL 938) and case serial number.
Drawing No. and Rev Ltr of pc board (See Figure 5-2).
Reference Designation of each part.
Description of the part, as in the table.
Data Precision part number.
c. Orders for replacement parts may be placed with an authoriz 
ed Data Precision Service Representative or with the factory.
Consult the list of authorized Service Representatives (printed on
the inside back cover of this manual) for the address of the one
nearest you.
6-1

42-5124
MODEL 938
6 2 MAIN ASSEMBLY PARTS LIST
REF
DES
DESCRIPTION
DP PART NO
A1 RESISTOR NETWORK, ATTENUATOR
22-48-1112-02
A2
RESISTOR NETWORK, 6-PIN, SIP 22-740000
A3 RESISTOR NETWORK, 8-PIN SIP 22-731009
A4 RESISTOR NETWORK, 6-PIN SIP
22-721004
A5
RESISTOR NETWORK. 8-PIN SIP 22-731010
C1 CAPACITOR, 51pF ica 23-310006
C2 CAPACITOR, 0.001uF,150 ±20%DISC
23-118201
C3
CAPACITOR, 0.047uF,100V ±20%
23-210304
C4-8 CAPACITOR, 0.22uF,100V 2 %
23-210315
C9
CAPACITOR. 39pF.10%,CER
23-110014
C10
CAPACITOR, 68pF,10%, CER 23-110019
C11
CAPACITOR, 4. 7uF, 20%, 35V,TANT
23-441052
C12-14
CAPACITOR, 15uF,10V. TANT
23-441003
C15 CAPACITOR, 0.1uF,100V, 20%
23-210302
CR1,2 DIODE, 1N4733A.5.1V 24-121001
CR2
DIODE, 1N4744.15V, ZENER
24-121103
DS1
DISPLAY
48-1129
F1
FUSE, 3AG, 1/4 A P
25-500009
Q1,2 TRANSISTOR, PSU06
24-230U06
Q3,10,1 1
TRANSISTOR, GES5819 24-245819- A
Q4,5 TRANSISTOR. E305 FET
24-210305
Q6,8,9 TRANSISTOR, GES-5818
24-235818- A
Q7 TRANSISTOR, 2N3904 4-233904
R1 RESISTOR, 10.1/4W, 5%
22-021009
R4
POTENTIO ETER. 5K
22-6f73502
R5 RESISTOR, 100K.1 %,RN55C
22-341003
R6 RESISTOR, 9.09K, 1%RN 55 C 22-349091
R7
RESISTOR. 1 ,1/4W.5% 22-021059
R8 POTENTIO ETER, 1
22-683105
R9.14
RESISTOR, 100K,1/4W,5% 22-021049
R11
RESISTOR, 10,0.1 %,RN55C
22-48-1124
R12
RESISTOR. 240K, FSV 22-022449
R13 RESISTOR, 2 ,1/4W,5%
22-022059
R15 RESISTOR, 100K, %W, 5%, CC 22-021049-C
R16 RESISTOR, 90,0.1 %RN55C
22-48-1123
S1
SWITCH ODULE, 8-STATION 48-1117
S2
SWITCH, SLIDE
25-441008
Y1
CRYSTAL, 32.768 KHZ
25-48-1130
Z1,2 I.C., TL062 24-400062
Z3 I.C., ICL 7106 24-G07106
Z4
I.C. 4030B, QUAD OR GATE 24-L04030
Z5 I.C., CUSTO HYBRID 48-1114
Z6 I.C., 4016
24-L04016
6-2

2012
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THESE DnAWInuS AND SPECIFICATIONS ARE
THE PROPERTY OF DATA PRECISION CORPORA 
TION ANO SHALL NOT BE REPRODUCED OR
COPIED OR USED AS THE BASIS FOR MANU 
FACTURE OR SALE OF APPARATUS WITHOUT
EXPRESSLY WRITTEN AUTHORIZATION FROM
DATA PRECISION CORPORATION "
5. EG (4OIG) PINT To D>IC»- y PIN I
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1. A.U_ RESISTOR VAUUES A.RE IN OH S, '/4 ,5°/
2. ALL. CAPACITOR VALUES ARE IN ICROFARADS.
3. ALL. NPN TRANSISTORS ARE GES S%>ie>.
GSSSB19