Keithley 440 User manual

INSTRUCTION MANUAL
MODEL 440
DIGITAL PICOAMMETER

INSTRUCTION MANUAL
MODEL 440
DIGITAL PICOAMMETER
0 COPYRIGHT 1974

: . ,. - :
.I
CONTENTS
Section Psq:e
SpEClFIC*TIfJNS -___--__-___-____-__---------------------------- i"
1. GENEp&,DESCRIPTION--_-____------_------------------------- 1
2. OPWTlON --------------------__________I_________--------------- 3
3. Cl&-"lT DESCRIPTpJX--------------------______I_____________---
10
‘. *C(-ESSORlES -__------------------_----------I-------------------------- 25
5, CUp,UTION __-____----____----____________________I__------------ 27
lj. F\EP,dCEBLE pa?s ---------------------------------------------------- 32
SCHyJJICS -__--___-_---_-----_----------------------------- 53
i.

MODEL 440
ILLUSTRATIONS
Fip. NO. Title Page
1 Franc Panel. 1
2 Franc Panel Controls. 2
3 Rear Panel CmlfTOlS and Terminals. 2
4
Prinrer/Canerol Connector. 8
5 Timing Diagram. 9
6 Simplified Diagram of Feedback Ammeter. 10
7 Gain Calibration of Amplifier. 10
8 Damping for Anmeter. 11
9 Zero Check Operarion. 11
10 Block Diagram of AID Converter. 12
11 Delay Hold Circuit. 13
12 Incegratar Circuit. 13
13 Zero Crossing Decxtar. 13
14 "ode1 4401 Buffer Stage. 15
15 Chassis Top View, Showing Cal. Adjustments. 17
lb Location of Printed Circuit Boards. 18
17 Component Layout, K-207. 19
18 componenr layout, PC-208. 19
19 component Layour, PC-210. 21
20 Componene Layout, PC-217. 20
21
component Layout, PC-219.
20
22 component Layout, PC-222. 23
23 Componenr Layour, PC-223. 23
24 Componenf Layout, PC-229. 23
25 component I.ayouc, PC-209. 24
26
component Layout, PC-218. 24
27 Test Equipment Set-up far A/D Calibration. 30
28 AID Con~erfer Zero Adjusrmenrs. 31
29 Elechanical Assembly. 33
30 Template, Test Cover, Tap 51
31 Ternplace. Tesf cover, Bottom 52
0874 iii

SPKIFICATIOX
SPECIFICATIONS
XODEL 440
RANGE: 100 picoamperes full scale (0.1 picoampere.
least signdicant digIt) to 10 milliamperes in nine
decade ranges with 100% overrangingonall ranges.
DISPLAY: Four digits tram 000 to 1999; polarity and
overload indicabon.
POLARITY SELECTION: Automatic.
RANGE SELECTION: Manual with automatic decimal
point positmn,ng.
ACCURACY AND RESPONSE TIME:
DAMPING: Varies rise time from minimum value to
approximately 1 second on the lOO.nanoampere to
100.picoampere ranges.
ZERO DRIFT: Less than 0.5% of full Scale per week:
less than O.O54a/“C. after %-hour warmup with
source voltages greater than 2 volts.
DISPLAY RATE: 24 readings per second maximum
(20 per second with 50.M units) adjustable to
approximately two readings per manute.
INPUT VOLTAGE DROP: Less than 1 millivolt for lull.
scale display on all ranges when properly zeroed.
LINE FREQUENCY REJECTION: 60 dB (ratio of peak-
to-peak current of power line frequency or mul*lple
which will cause less than 1 digit of error. to that
error,. 100 dB on 100.p,coampere to 100.“ano.
ampere ranges with mawmum damping. Peak input
current should not exceed 20 milliamperes.
MAXIMUM INPUT OVERLOAD:
Transient: 1000 volts for up to 3 seconds.
Continuous: 600 V using a Keithley or other current
limited (up to 20 mA) Hugh Voltage Supply.
ANALOG OUTPUT: % 1 volt from a 500.ohm sourcefor
full~scale display. Maximum output, 1 millismpere.
Output polanty 3s oppxite input polarity.
PRINTER OUTPUTS AND OUTPUT CONTROLS: Model
4401 accessory provides BCD OUtput and external
cmtrois.
CONNECTORS: Input: Teflon-insulated UHF-type. Ana.
log output: Amphenol 8O.PCZF. Case ground: Bind.
,ng post.
POWER: 105.125 or 210-250 volts (switch selected).
60 Hr. 50.Hz models available. 30 watts.
DIMENSIONS, WEIGHT: 51,‘~” high x 19” wide x 10”
deep: “et weight. 15 pounds.
I - . _. t
iv
0874

Safety Precautions
The following safely precautions should be observed before using
this product and any associated instrumentation. Although some iw
strume”f~ and accessoties would normally be used with non-ha/-
ardous voltages, there are situarions where hazardous conditions
may be present.
This product is intended for use by qualified personnel who recog-
nize shock hazards and arc familiar with the safely prccaurions re-
quired to avoid possible injury Read the operating information
carefully before using rhe product.
The types of product users are:
Responsible body is the individual or group responsible for the use
and maintenance of equipment, for ensuring that the equipment is
operated within its specifications and operating limits. and lor ens
swing that operators are adequately trained.
Operators use the product for its intended funcrion. They must be
trained in electrical safety procedures and proper use of the insrru-
ment. They must be protected from electric shock and contact with
hazardous live circuits.
Maintenancepersonnelperform routine procedures on the product
to keep it operating. for example, setting the line voltage or replac-
ing consumable materials. Maintenance procedures are described in
the manual. The procedures explicitly state if the operator may per-
form them. Otherwise, they should be performed only by service
personnel.
Service personnel are trained to work on live circuits, and perform
safe installations and repairs of products. Only properly trained ser-
vice personnel may perform installation and service procedures.
Exercise extreme caution when a shock hazard is present. Lethal
voltage may be present on cable connector jacks or test fixtures. The
American National Standards Institute (ANSI) states that a shock
hazard exists when voltage levels greater than 30V RMS. 42.4V
peak, or 60VDC are present. A good safety practice is to expect
that hazardous voltage is present in any unknown circuit before
measuring.
Users of this producl muzl bc protected from elcclnc hhock a, nil
times. The responsible body musk enwrc Ihal users arc prcvenied
access andhr insulawd from every connection poini. In some cw5s.
~onneckms mw be expored to potential human contact. Producl
users in these circumslances must be trained to protect thanselves
from the risk of elearic shock. If the circuit is capable of operating
at or abwe 1000 volts. no conductive part of the circuit may be
exposed.
As described in the lnternaiional Electrorcchmcal Commissmn
(EC) Standard IEC 664, dtgiral muldmcler measuring cmam
(e.g.. Keithicy Mod& 175A. 199. 2000, 2001, 2002, and 2010~ arc
Installation Category II. All other inwuments agnai rcrminals are
lnsraliation Category I and mw no1 be connccxd 10 mani
Donotconnect switchingcards directly to unlimited powcrcircults,
They are intended to be used with impedance limited sources,
NEVER connect switching cards directly to AC mains. When cons
netting sources to switching cards, install prorective dcwces to llm-
it fault current and vollage to the card.
Before operating an instrument. make sure the line cord is connecr-
ed to a properly grounded power recepraclc. Inspea the connecring
cables, test leads. and jumpers for possible war. cracks, or breaks
before each use.
For maximum safety, do not touch lbe product. iest cables. or any
other instruments while power is appiicd to rhe circuit under [es!
ALWAYS remove power from the entire lesl system and discharge
my capacitors before: conneaing or disconnecrmg cables or jump-
us. installing or removing switching cards. or making internal
changes. such as installing or removing jumpers.
Do nor touch any ObJecr that could provide a conem path to Ihc
common side of the circuit under test or power line (earth) ground.
Always make measurements with dry hands while standing on a
dry, insulated surface capable of withstanding the voltage being
measured.

The instrument and accessories must be used in accordance with its
specifications and operating instructions or the safety of the equip-
ment may be impaired.
Do not exceed the maxjmum signal levels of the instruments and ac-
cessories, as defined in the specifications and operating informa-
tion, and as shown on the instrument or test fixture panels, or
switching card.
When fuses are used in a product, replace with same type and rating
for continued protection against fire hazard.
Chassis connections must only be used as shield connections for
measuting circuits, NOT as safety earth ground connections.
If you are using a test fixture, keep the lid closed while power is ap-
plied to the device under test. Safe operation requires the use of a
lid interlock.
Ifa@ SCI~Wis present, connect il to safety earth ground using the
wire recommended in the user documentation.
Then symbol on an instrumenr indicates that the user should re-
fer to the operating instructions located in the manual.
Then symbol on an instrument shows that it can source or mea-
sure 1000 volts or more, including the combined effect of normal
and common mode voltages. Use standard safety precautions to
avoid personal contact with these voltages.
The WARNING heading in a manual explains dangers that might
result in personal injury or death. Always read the associated infor-
maflon very carefully before performing the indicated procedure.
The CAUTION heading in a manual explains hazards that could
damage the instrument. Such damage may invalidate the warranty.
Instrumentation and accessories shall not be connected to humans.
Before performing any maintenance, disconnect the line cord and
all test cables.
To maintain protection from electric shock and fire, replacement
components m mains circuits. including the power transformer, test
leads, and input jacks, must be purchased from Keirhley Instru-
merits. Standard fuses, with applicable national safety approvals,
may be used if the rating and type are the same. Other components
that are not safety related may be purchased from other suppliers as
long as they are equivalent to the original component. (Note that se-
lected pans should he purchased only through Keithley Instruments
to maintain accuracy and functionality of the product.) If you are
unsure about the applicability of a replacement component, call a
Keirhley Instruments office for information.
To clean an instrument, use a damp cloth or mild, wafer based
cleaner Clean the exterior of the instrument only. Do not apply
cleaner directly to the instrument or allow liquids IO enter or spill
on the instrument. Products that consist of a circuit board with no
case or chassis (e.g., data acquisition board ior installation into a
computer) should never require cleaning if handled according to in-
structions. If the board becomes contaminated and operation is af-
fected, the board should be returned to the factory for proper
cleaning/servicing.
Rev. 2J99

MODEL 440
SECTION 1. GENERAL DESCRIPTION
Cancrol Functional Description Paragraph
RANGESwitch (S1202) Sees full range sensitiviq.
POWER SWifCh (S102) Controls power to
instrument.
ZERO CHECK (SL201)
Selects Zero Check Mode.
ZERO ADJUST (R1211) Adjusts the zero offset.
D.424PINC conrro1 (R1221) Adjusts damping.
DISPLAY RATE (R1201)
Adjusts the A/D Conversion rate.
2-3 a
2-l b
2-3 c
2-3 e
2-l d
2-l f
0874 1

GENERAL 0ESCR1PT10N MODEL 440
RANGE
Switch
ZERO CHECK
POWER DISPLAY RATE DAMPING ZERO Gl-0”“d INPOT
Suicch Switch S1203 Switch concro1 POSe Receptacle
s102
51203
It1211 51211 n210
FIGURE 3.
Rear Panel Controls and Terminals.
TABLE 1-2.
Rear Panel.
I Control or Terminal
Functional Description Paragraph
117-234V Swirch (SlOl)
FUSE (FlOl)
ANALOG OLTPVI (51213)
FRI>lTEI(/CONTKOL (31212)
S?.uE
set* 117 or 234V operation.
Type 3AG Slow Blow: 117V-3fSA
234V-3/16A
Provides a recorder ourput.
Provides SCD Ourputs from a 50-pin connector.
Cover plate for mounting a" additional SO-
pin Connector.
2-5
2-6
2-6
2

“ODE‘ 440
OPERATION
SECTION 2.
2-l. INPM CONNECTIONS.
a. Inp,,t Rece,,w.cle. The Input connecfor (51210)
is a Teflon-insulated UHF t.,w (Keithlev Part No. CS-
64). A mating (CS-49) conniCtor is supblied far mak-
ing custom cables. The center terminal of the connec-
tor is in the Input High terminal while the outer
shield is case ground. A separate grounding past “6”
is provided for system ground connections.
b. IDPUC Cables. Input connections should be made
using coaxial cables which are low-noise types with
graphite coaring beeween dielectric and shield braid.
Far eu~eom length cables Keithley Part No. SC-9 Low
Noise Coaxial Cable should be used.
1. Model 2611 Coaxial Cable. This cable is a
pre-assembled cable 24 inches long having a IMF male
connector on each end.
2. Part No. 19072C Coaxial Test Cable. This
cable is a pre-assembled cable 30 inches long having
a UHF male connecmr an one end with alligator clips
on the ocher.
C. Insulaeio”.
Use high resisrance, low-loss mafe-
rials such as sapphire, reflan, polyethylene or poly-
scyrene for insulation of the input circuit.
NOTE
The input terminal should be protected from con-
raminacion $0 chat the insulation will not be
degraded, Clean, dry connections and cables are
very important to maincain the value of all in-
sulation materials. Even the best insulation
can be compromised by due, dirt, solder flux,
films of oil or water vapor. A good cleaning
agent is methyl alcohol, which dissolves most
comon dirt without chemically atracking the
insulation.
2-2. MEAS”RENENT CONSloEiWTIONS
a. Noise. The limit of resolurion in voltage and
current measuremen?.s is determined largely by rhe
noise penerared in rhe source. stray law-level noise
is presene in some form in nearly all electrical cir-
cuirs. The instrumenr does nor distinguish between
stray and signal currents since it measures the net
current. When using the picoampere ranges, consider
the presence of low-level electrical phenomena such
as thermocouples (thermoelectric effete), flexing of
coaxial cables (triboelecrric effect), apparent re-
sidual charges on capacitors (die-leccric absorption),
and baCCery action of two terminals (galvanic action).
OPERATION
1. Thermal EMUS. ThemaeLectric potentials
(thermal emfs) are generated by thermal gradients
between two junctions of dissimilar metals. These
can often be large compared to the signal to be
measured. To minimize the drift caused by therma!
emfs, use pure copper leads “herever possible in
the source circuit. Drift can also
be minimized by
mainmining constant junction temperatures espec-
ially by using a large hear sink near the connec-
tions.
The Keifhley accessory Model 1483 Lou Ther-
mal Connection Kit contains ali necessary materials
far making very low rhemal copper crimp connections
far minimizing Chermal effects.
2. AC Electric Fields. The presence of electric
fields generated by paver lines or ocher sources
can have an effect on instrument operacion. AC
voltages which are very large with respect co the
full-scale range sensitivity could drive the ac
amplifier into sacuracion, thus producing an erron-
eous dc output. Proper shielding as described in
paragraph 2-1, d can minimize noise picl-up when
the instrument is in the presence of large ac fields
or when very sensitive measurements are being made.
3. Magnetic Fields.
The presence of scrang nag-
neck fields can be a potential source of ac noise.
Magnetic flux lines which CUL a conductor can pro-
duce large ac noise especially at power line fre-
quencies. The voltage induced due co magnetic flux
is prnpor~ional co ehe area enclosed by the circuit
as well as the rate of change of magnetic flux. For
example, the motion of a l-inch diameter loop in the
earth’s magnetic field will induce a signal of sev-
era1 tenths of a microvolt. One way to minimize
magneric pickup is to arrange all wiring so that the
loop area enclosed is as small as possible (such as
rwiscing input leads). A second way to minimize
magnetic pickup is co use shielding as described in
paragraph 2-1, b.
b. ShieldinK.
1.‘ Electric Fields. Shielding is usually neces-
sary’when the instrumnt is in the presence of very
large ac fields or when very sensitive measurements
are being made. The shields of the measurement cir-
cuit and leads should be connected together co ground
a~ only one point. This provides a “tree” config-
uration, which minimizes ground loops.
2. Magnetic Fields. Magnetic shielding is useful
where very large magnetic fields are present. Shield-
ing, which is available in the farm of plates. foil
or cables, can be used to shield the measuring cir-
cuir,, the lead wires, or the instrmenr itself.
0874 3

OPERATION MODEL 440
C.
Damping. The amount of high frequency ooise
which will be observed on the picoammerer is deter-
mined by: I) the noise pickup ae the input, and 2)
the bandwidth of the amplifier circuir. The front
panel DAMPING Confrol (R1221) controls the amount of
filtering from a minimum (NW) value (as stated in
the rise time specification) fo a maximum value
(approximately 1 second on PICOAMP range) when see
fully clockwise.
d. Accuracy. The accuracy is specified in terme
of a percent of reading on eech range. An additional
fl digit is specified since the A/D conversion has an
&herenc +I digir uncertainty. Noise and source re-
sistance conditions should be evaluated as additional
measurement considerations.
e. Source Resistance. The value of source resisf-
ante em affect the measurement if the loading effect
of the picoammecer is significant. To avoid a degrad-
ation of zero drift. rhe picoammeter range should be
selected so that the range feedback resistor is much
less than the source resistance. The zero drift
specification is valid only for source voltages greac-
er than 2 volts. The suggested minimum source resisc-
ante for each range is given in Table 2-l. The amounf
of degradation of the drift specification is given by
the following equation.
Drift = %/week x Nominal Gain =
Effective Gain
where Nominal Gain = K
Effective Gain = (w)(q
Example: If RANGE = 1 NICROAMP
RF = 1.02 x 106
RS = 2 x 106
Then Rs I' RF = 3 = 1.5
RS -i-
TABLE 2-l.
Suggested Minimum Source Resistances.
RANGE
Selected RANGE Source Resistance
Resistance (ohms)
100 PICO-AMPS 1010
1 NANO-AMP 109
10 NANO-AMPS 108
100 t&NO-AMPS 107
1 MICRO-AMP 1.02 x 106
10 MICRO-AMPS 1.05 x 105
100 MICRO-AMPS 1.05 x 104
1 MILLI-AMP 1050
10 MILLI-AMPS 105
2 x 1010
2 x 109
2 x 108
2 x 107
2 x 106
2 x 105
2 x 104
2 x 103
2 x 102
f. Overloads. A unique input circuit provides
complete overload protection with fast recovery. The
maximum transiene overload is 1000 voles for up co
3 seconds. The maximum continuous overload is 600
volts using a current limited supply (up to 20 mA)
such as Keichley Models 240A, 245, or 246.
2-3. FRONT PANEL CONTROLS.
a. RANGE Switch (51202). This switch selects the
full scale displsy range in nine decade steps. The
dial is designated in engineering units, that is,
PICO AMPS, NAN0 AMPS, MICRO AMPS, and MILL1 AMPS.
b. POWERSwitch (SlOZ). This swirch controls the
line power to the instrument.
C. OPERATE/ZERO CHECK Swifch (51201). This swirch
selects either normal operation or zero check opera-
tion.
d. DAMPING Control (R1221). This control varies
the response time of the picoanrmeter on the PICO AMP
and NAN0 AMP ranges.
e. ZERO Control (R1211). This control adjusrs the
zero display. The ZERO Control should be used when
in ZERO CHECK mode.
f. DISPIAY RATS Control (R1201). *his control ad-
justs the A/D converter conversion rate from 24 reed-
ingslsec. (MAX) to 2 readingslmin (appron).
2-4. OPERATING PROCEDURE.
a. Preliminary Procedure.
1. Check the 117-234V Switch (SlOl) dit the rear
panel for proper line voltage.
2. Check for proper rated fuse.
3. Connecr the power cord, place the POWERswitch
ON, and allow a 30 minure warmup for critical meas-
urements.
4. Adjust the ZERO Control (R1211) as necessary.
Zero is indicated by alternately flashing + polarity
lights.
5. Connect the source es described in paragraph
2-l.
b. Measurements. The Model 440 measures current
over a full-scale range from 10 milliamperes (10m2 A)
fo 10C1picoampere (lo-10 A) with resolution to 0.1
picoarhpere (lo-l3 A). The display will indicate either
a positive or negative input current automatically.
(A positive curreot is defined as a positive "conveo-
rional current" applied ac Input High with respect co
case ground.)
4 0874

1 x 10-4 A 100 MTCKO-AMPS 100.0 x 10-6
1 x m-3 A 1 +llLLIlAME 1.000
x10-3
LIxlo-?h 10MILLI-AMPS10.00
x10-3
2-5. ANALOG OUTPUT. The analog output on the rear
2-6. DIGITAL OUTPUT.
a. cenera1.
1. The Model 440 has provision for the installa-
tion of OUtpUt buffer printed circuit boards co
&rain Binary Coded Decimal (NO) outputs. Two 44-
pin card-edge cOn*eCeOrS are installed and complete-
ly wired on Che main PC board.
CO”“eCrOr: so-pin Amphenol xicro-Ribbon mounted on
Model 440. “ueput mating connector supplied with
4401.
0874

1. Full Scale Magnitude. The magnitude of the
reading is indicated by BCD outputs which correspond
to the three front panel display lights as shown in
Table 2-5.
TABLE 2-5.
Full Scale Magnitude
:onneccar Dee ima 1
Pin No. OUCp”t Digits
1
2
26
27
1 x 100
2 x 100
4 Y 100
8 x 100
3 1 x 101
4 2 x 101
28 4 x 10’
29 ‘- a x 101
5 1 x 102
6
2 I 102
30 4 x 102
31 8 x 102
2. overrange Indicarian. Overrange is indicated
by the faurrh (from the right) display light and
corresponding BCO output as shown in ~ahle 2-6.
Overload is indicated by a blanked display and
corresponding BCD output. The output at pi” 33
will he a logic “1” when the magnitude of ehe digital
display exceeds 1999.
TABLE 2-6.
Overrange and Overload Oucpurs.
3. Polarify Indication. The polariey is indicated
automatically by ehe Polarity Indicator and corres-
ponding BCO output as shown in Table 2-7. Addition-
al BCO levels are available at pins 14 and 39 for
use with some printers. The zero check made is in-
dicaced by a BCD output from pin 38. Four pins may
be used to obcain BCD polarity codes for external
printers, where 1010 = + and 1011 = - prineer chsr-
acters.
TABLE 2-7.
PolariCy Output
4. ~<mtx:c Indication (Exponent). The range or
exponent has a corresponding BCD oufpue as shown in
Table 2-8. The print-auf of the RANGE Swifch ex-
i
ponent uses 2 coiumns to represent information far
exponents from 00 thr” 10.
TABLE 2-8.
Range or Exponent Indication.
CO”“eCCOC
Pin No.
9
10
34
35
Decimal
OUtpUt Digits
1 x 100 Range 1
2 x 100 Range 2
4 x 100 Range 4
8 x 100 Range 8
d. External cancro1.
1. General. To obtain optimum *ystem p%ffbrm-
ante, if is often desirable CO operate Che~Nodel
440 synchronously with other digital equipment,
such as printers, paper tape punches, computers and
other daea handling devices. The Model 440 with
4401 Printer Cards installed provides several print-
er control connnands for the purpose of synchronizing
external equipment to achieve maximum conversion
rates.
2. Applications. Several alrernate approaches
may he used in designing the overall system control
scheme.
a) The Model 440 can be used to provide master
conerol of external devices so that the maximum
possible conversion race* can be obtained.
h) An external device can also he used for
master control such as a high speed printer.
c) A completely independent “master clock” can
be used for system control far maximum flexibiliry.
3. Description of external controls.
=I ‘71OI.C 1”. This control inhibits A to D
conversion at the instant a closure to ground is
made. The conv’rsion cycle will resume immediate-
ly when the “HO:D 1” line is opened.
6

5. Nigh and Low Keference. The PRINTEKICONTROL
connecror pro”ide* f”0 reference voltages, High
(t8”) and Low (+ZV). These levels may be used to
define the ‘~HICII” and “LOW” digital output states
1ar exeerna1 printing or computer devices.
1. Standard Outpue Codes and Levels. The stand-
ard o rpue code for Hodel 4401 Printer Output Cards
is l-2-4-8 Binary Coded Decimal (BCD). A binary
coded decimal digit is represented by a four-bit
binary code as show,, in Table 2-12. refer to figure
for a circuit diagram oI the Model 4401 Stand=,rd
Prineer outpue buffer stage.
2. PKpmx/CONTROL connector. nie PKINTEK/CONTROL
Connector used o,, the Model 440 provides for connec-
tions to 50 pins as shown in Table z-11. The mating
connecfor supplied with Model 4401 is an hmphcnoi
Part Number 57-,050o or Keithley Part Number cs-220.
available on special order.
3. Analog-fo-Digicai Conversion Cycle.
a) The analog-Co-digital conversion cycle can
be inifiafed in any one of three ways.
1.) DISPUY RATE concro1 set at MAX. With the
DISPLAY RATE Control set at MAX, the end of one
compleLe COnverSion triggers a second conversion
to obrain the maximum co”“ersio” rate of ZL read-
ings per second.
2.) DISPLAY RATE ConLrol SeC at Other man EWY.
WiCh etle DISPLAY RATE Control set at soale psi-
tion other than MAX, (uncalibrated control set-
ting) the end of one complete conversion rriggers
a second conversion which is delayed by a speci-
fic time interval (DELAY). The time delay is a
funceio” of he position ai a continuously vari-
able control to provide a co”“ersio” rate from
24 readings per second to 2 readings per minute.
3.) ‘HOLD 2” with TRIGGER Control. With the
“HOLD 2” command grounded, a ciosure to ground
of the “TRIGGER” command initiates one complete
conversion cycle. A second conversion viii
foilow only if the TRIGGER cormand is removed
and m-applied a second time. The maximum con-
version rate using an external trigger is 24
readings per second.
TABLE Z-10.
Typical Digital Outputs.
Front Panel RANGE Significant Range
Digital Display Sefting Polarity Digif & Overload Nag. EXP. InrerprelJrio”
-093.6 PICO-AMP 0 936 -0.936 x 10‘10
+0.275 NANO-AMP + 0 275 09 +0.275 x ld
-17.31 NAN”-AMP 1 731 08 -1.731 x 10-a
c122.3 NANO-AMP + 1 223 07 +1.223 x 10-7
10.096 MICRO-AMP + 0 096 06 to.096 Y 10-6
-07.81 MICRO-AMP 0 781 05 -0.781 x 10-5
+165.2 MICRO-AMP + 1 652 04 hL.652 x 1V4
-L.921 MILLI-AMP i 92, 03 -1.921 7. 10-3
-06.17 MILLI-AMP 0 637 02 -0.637 x lo-*
+(bk*k) MICRO-AMP + 8 000 OS positive overloac
0874 7

OPERAT‘ION
TABLE 2-11.
PRINTER/CONTROL Connector Pin Identification.
Pin No. output Function Pin NO. O"tp"r Funcrion
13 1 x 100 Polarity 38 1 x 10 Zero Check
14 + 15" _-_ 39 + 15" ---
15 Blank --_ LO Blank ---
lb ELIkInk ___ 41 Blank ---
17 b 15" -__ 42 - 15" ---
18 + 3.6V --- 43 Common __-
19 Blank ---
20 Blank --- 44 Grounded
45 Grounded
21 Blank --- 46 Grounded Trigger
22 Blank ___ 47 Blank ___
23 i lb" Pulse Print Command 48 Blank ---
24 + av Hi Reference 49 Blank ---
25 /' 2v LO" Reference 50 Blank --_
L FICURF. 4. Printer/Control Connector.
0:;;;

HOOEL 440
OPERATION
b) Conversion Cycle Timing. The Conversion
Cycle is composed Of three timing periods, namely.
Integrator zero, Integrator Sampling, and A-D
Counting period. Refer to Timing Diagram Figure
1.) Integrator Zero Period (ZERO). “hen a
trigger pulse initiates a new conversion cycle,
the Inregracar circuit is zeroed for a period
not to exceed 8.33 milliseconds for 60 Hz aper-
ation. The Integrator Zero Period is 10.00
milliseconds for 50 Hz operarim.
2.) Integrator Sampling Period (INTEGRATE).
The Ineegraeor Sampling Period follows automat--
ically the Integrator Zero Period and lasts for
a duration of 16.67 milliseconds for 60 Hz op-
eration. The Integrator Sampling Period lasrs
for a duration of 20.00 milliseconds for 50 Hz
operation.
3.) A-D Counting Period (COUNT). The A-D
Gunring Period is initiated immediately follow-
ing the Integrator Sampling Period. The actual
counring rime durarion vi11 depend on the actual
incegraror voltage up to a maximum of 2000 clock
pulses or 16.67 milliseconds. Following the
counting period a Buffer/Storage cormand is
automatically generated in order to store the
new reading in the output registers.
4.) PRINT COMMAND. The PRINT COEPWNO signal
is used to trigger external printers or paper
tape punches. The PRINT CObDL\ND signal is de-
layed 10 microseconds to allow the Storage Reg-
isters to settle. The PRlNT COMMAND pulse width
is approximately 100 microseconds with a I volt/
microsecond rise time into a 1 kilohm load. The
pulse smplitude is approximated by ehe follow-
ing equation:
% = 14R/(R + 2200~
where R is the output load resistor.
The “OFF” state is less ihan co.4 volt with
approximately 1 milliampere sink current.
NOTE
The data stared in the Output Registers will not
change for at leasr 25 milliseconds for 60 Hz op-
eration. If the front panel controls are charmed.
the Zero Check BCD o”tput vi11 be
changed only.
FIGURE 5.
Timing Diagram.
0874
9

CIRCUIT DESCRIPTION MODEL 440
SECTION 3. CIRCUIT DESCRIPTION
3-l. GENERAL. The Model 440 Digifal Picoamerer con-
sisfs of two separate sections packaged together in
one chassis for optimum performance and convenience:
a sensitive picoammerer and an analog-to-digital con-
verrer.
a. Picoameeer. The picoameter is a linear dc
amplifier connected as a feedback ammeter with nine
CUrrent ranges.
b. A/D Converter. The analog-to-digital converter
is a dual slope, integrating type converrer with med-
ium conversion race. cold cathode readout tubes, BCD
output options and external control.
3-2. PICOAMMETER
a. Ooerarion. The picoamerer consists of a sms-
itive, linear dc amplifier with a 1 volt full scale
sensitiviey. The amplifier provides an analog output
up to 2 volts for*a 100% overrange display. The
RANGE resistors are connected across the feedback of
the amplifier. A simplified diagram of a feedback
anmeter is shown in Figure 6.
C-
NPUT
FIGURE 6. Simplified Diagram of Feedback &meter.
b. Circuitry. The amplifier input stage is a pair
of insulated-gate, field-effect transistors (IGFET)
designated Q1201 and Q1202 connected in a differential
configurarion. The circuit designared 218508 is a
special overload protection circuit on the Input FET
board (which connects to 51201). The gate of Q1201 is
connected;to the input through 10M ohms. The gate of
Q1202 is referenced to ground. Poteneiometer RI205 is
an internal COARSE ZERO adjusment. Potentiometer
R1206 is an internal Balance control. Transistors
Q1203-Q1204 form a second differential amplifier stage.
Potentiometer R1211 is a fronr panel ZERO adjustment.
Transistor Q1205 and emitter follower transistor Q1206
provide sufficient gain for the analog output and A/D
converter. The analog output is connected through
R614 (499n) fo the dc amplifier output (the junction
of R1220 and D1203). The full scale current sensitiv-
ity is determined by ehe range resistor connected
across the feedback. The range resistance RF is com-
posed of a fixed resistance Rl plus a calibrarion ad-
justment resistance R2 as shown in Figure 7. Potent-
iomerer Rl221 controls the amount of damping on rhe
PICO AMP and NAN0 AMP ranges only. Switch S1203 de-
feats the damping feature when set to 'WIN" or open
position. The damping circuir is shown in Figure 8.
When switch S1201 is ser fo ZERO CHECK the picoamerer
is connected as shown in Figure 9. Ie should be noted
that the feedback resistor RF is shorted out & the
inpur High terminal is shorted to ground.
GAIN
ADJ.
FIGURE 7. Gain Calibration of Amplifier.
10

CIRCUIT DESCRIPTIOX
OUTPUT
m GROUND DAMPING \;
CONTROL <
,,‘-.
cu.-,- I
1
/-k! GROUNC
3-4. ANALOG-TO-DIGITAL CONVERTER OPERATION.
a. General. A detailed block diagram of the A/D
co"ver~er is shown in Ffgure 10. The analog-to-digital
converter operates using a dual slope integration tech-
nique which has inherent line frequency noise rejection.
The analog signal is applied to the integrator for one
complete line frequency cycle. The analog signal is
then removed from the integrator input. The voltage
on the integrator is then driven to zero fo complete
the voltage-co-rime conversion. The time interval to
reach a "Zero Crossing" is counted and displayed on
the "Readout" in proportion to the original analog
signal. The sequence is then repeated for a second
reading. A Timing Diagram is show in Figure 5.
b. Circoifs. The AID Converter is composed of nine
major circuits which perform the analog-to-digital con-
versions and provide various control commands.
1. Oscillator or Clock
2. BCD counter
3. Delay Hold
4. Program/Decoder
5. 1nregraror
6. Zero Crossing !&rector
7. Buffer/Storage Register
8. Decoder/Driver
9. Numerical Readout
C. Oscillator or Clock. The Oscillaror produces
oulses at a rate of 120 kilohertz for Electrometer
using 60 Hz line power.
The 50 Hz "nice have a pulse
rate of 100 kilohertz.
d. BCD Counter. The BCD Counter counfs ?he Clock
pulses with a roral range of 5000 counts. The Counter
is composed of 4 individual coyntere designated 1, 10,
100, and 1000.
1. The "l", "lO", and "100" coun?ers have a ca-
pacity of ten counts each.
2. The "1000" counter has a capacity of five
cooncs.
3. The tocal capacity of all four co""ters is
5000 CO""tS.
e. Delay Hold. The Delay Hold circuit conrrols the
DISPLAY RATE function and external Hold and Trrgger
connrands as shown in Figure 11. It determines the
length of time between A/D conversio"s when the front
panel DISPLAY RATE Control is set to any position
other than MAX. The clock is stopped at the beginning
of the ZERO (2) period for a time determined by the
rocafion of the DISPLAY RATE Control. re ensures thaL
when the Hold 2 is grounded the conversion in process
will be completed and new data will be stored in the
output storage register. Then the clock will be in-
hibited at the beginning of the ZERO period (2). The
instrument will remain in this condition indefinitely
until Hold 2 is released or until Trigger is shorted
to ground. Afrer conversion, the insrrumenr will again
be inhibited at the beginning of rhe period (2). If
bath Swifches Sl and S2 are closed, the conversion
cycle works in the following manner. After the pre-
vious conversion has been completed, the leading edge
of the program comrt~nd (2) resets the flip-flop. In
this new condition Q is high end, therefore, rhe clock
gives no ootpuf. Af that time, rhe unijunction timer
begins ifs cycle and. after the appropriate rime, pro-
duces a pulse chat sets rhe flip-flop. This changes
Q to a low scare and a new conversion cycle begins.
After the reading has bee" completed, the (2) co-and
again resets the flip-flop end the timer again issues
a new pulse Co set the flip-flap.
0874
11

A-TO-0
CONVERTER
I I I I
! I I I I I ’