Keithley 445 User manual

INSTRUCTION MANUAL
Automatic Ranging Digital Picoamneter
Model 445
COPYRIGHT 1974, KEITHLEY INSTRUMENTS, INC.
PRINTED JULY, 1978, CLEVELAND, OHIO U. S. A.
DOCUMENT NO. 29105

CONTENTS
SBCCiOU
SPEClFIMTI(JNS ---------------_--------------------------
1. GENERAL DESCRIPTION ------------ -------------------
2. 'PEmTION ---------------_-_--_______________I____---
3. CIRCUIT DESCRIPTION __-_-------------------------------
4. ~p~‘-~LE PARTS ________________----_____I________-_
5. C&TBmTl,,N __-______---__-__----------------------------------
SCHEMATICS __-_----_------__---______ll______l_____------
PM,.2
ii
1
3
11
29
51
57
MODEL 445
PRINTrn JUL 74
0174

iLLUSTF.ATIONS
ILLUSTRATIONS
Fiu. No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1B
19
20
21
22
23
lb
25
26
27
25
:i
31
32
33
::
Title Page
Front Panel. 1
Front Panel Conrrols. 2
Rear Panel. 2
Triaxial Receptacle. 3
Reading Time. 4
Digital Display. 5
Prfnter/Control ConneccLr. 9
Timing Diagram. 10
Feedback titer. 11
Range Calibration. 11
Overall Block Diagram. 12
Autoranging Block Diagram. 13
A-Lo-D Converter Diagram. 14
Delay Hold Diagram. 15
Integrator Block Diagram. 15
zero cro**ing DetacLor.
15
Model 4401 Buffer stage. 17
Chassis, Top View. 19
PC Board Locationa. 20
Componanr Layout, PC-207. 21
cMnponenc Layout, PC-208. 21
COmponent Leyout, PC-209. 22
Component Layout, PC-212. 22
Component Layour. PC-214. 23
cmponent Layout, PC-217. 23
Colnponent LayoUr, PC-218. 24
component Layout, PC-219. 24
component Layout, PC-221. 25
corQpon.nt Layout. PC-229. 27
Componenr Layout, PC-230. 27
Mechanical Assembly. 31
Test Equipment Set-up for A/D Calibration. 54
AID Converter Zero Adjuetmenrs 55
Test Cover Template, Top Panel. 57
Teat Cover Template, Bottom Panel. 58
0714

SPECIFICATIONS HODEL445
SPECIFICATIONS
RANQE: lo-’ amper. full scale (lo-” ampere. least
significant digit) to 104 ampsra in eight decade
ranges with 100% ov.rr.nging on all ranges.
DISPLAI: Four digits from 000 to 1999: range ~4.
pon.nt digit from 2 lo 9: paerity. overload and
Range Changing indication.
POLARITY SELECTION: Automatic.
RANQE SELECTION:
Automatk: Range change possible after each A to
D conversion. An undrrrangscondition (<lOOdigits)
causer one range change to next more sewdive
rsng.. An overr.nge condition (21999 digit.) on
any range ca”ses rang. change to IO-’ A range.
Mmu~l: Front panel swi1ch.s permit manual r.nga
control.
ACCURACY AND RESWNSE TIME:
I~ C,llbr.l~.l1.c1011 IllT .n*.nlb. bl.m,l Id,“Itmmnt.On10.. to ,w
&lw,,. ran,,, to, loc.1,~1.llO”.
1. I1 dW1“4” ‘1l1.1WI .“d 100Wol.l.dS Y”Ml”, m. mow
1. WI” “0 10 $00 P’cOlll.as %h”nlm, tn. l”D”l l * rllt” wt.
FILTER: Improves .c rsiection by Ikngthening rise tim.
to approximately 3/4 son lO-’ to IO-* A ranger.
LEN0 DRIFT: Lass than 0.5% of full scale per week;
le.1 than O.OS%/‘C. after %.hour warm-up with
source volt.ge greater than 2 volts.
DISPLAY RATE: 24 r..dings per second maximum (20
per second with 50+lz units), adiusbble toapproxi.
mstely 1 reading per two seconds.
With Filter in. m.ximum display rate is abeut l/s.
INPUT VOLTAQE DROP: Less than 1 millivolt for full.
scale display on all r.ng.s when properly zeroed.
LINE FREQUENCV REJECTION: 60 dS (ratio of peak.
to.pe.k current of power line frequency or multiple
which will cause less than 1 digit of error, to that
error). 100 dEon LO-rto 10-v A rmges with Filter
in. P..k input current should not .xc.ed 20 mA.
MAXIMUM INPUT OVERLOAD: 1000 volt. using .
Keithlay or other current lim#t.d (up to 20 mifli.
amperes) High Voltage Supply with Model 445 in
autorange mod.. Inst.ntaneous input current must
never exceed 125 m~lliamp.res.
ANALOG OUTPUT: tl volt from . 500.ohm sourc.
for full.scale display. Maximum output. 1 millian.
per*. Output polarity is opposit. input polarity.
PRINTER OUTPUTS AN0 REMOTE CONTROLS:
Printer Output.: SCD positive output represents
each of the four digits, rang.. polarity. overrange.
r.nge changing and zero check. Standard cod. is
1.2.4.8. “0”<+0.4 volt: “l”>+ 10 volts at up to
one milliampw.: O=OOOO.
Print bmm.nd A: Positive pulse of 14 volts from.
2200.ohm source with 1 volt per mlcros.cond rise
time. 100 mkroseconds minimum pulse width. Print
commrnd given after each A to 0 conversion.
Print Commend 0: Same .s Print Command A ax.
cept pres.nt only when displaying an onrsnge
reading.
Remote Control.:
Hold # 1: Closure* to ground inhibits A to D
conversion .t that instant.
Hold # 2: Closure* to ground inhibits A to D
conversion .fter r..ding h.s been compl.t.d.
Trigger: Cbsure* to ground initiates one conv.rsn)n
when in Hold #2. Integration period starts 8.3
millisecond. (10 milliseconds on 50.H~ models)
.fter “Trigger” or r.le.se of Hold # 2.
R.nSr HoM: Closure* to ground prevents range
change.
2ero Check: Closure* to ground places instrument
in zero check mode and prewnts range change.
101 Range: Cbsure’ to ground places instrument
in 10-a ampere r.nge and holds until lo-* Range
rolnsed.
ISOLATION: Circuit ground to chassis ground: Greatsr
th.n l(r ohm. shunted by .02 microfarad. Circuit
ground mey bi floated up to 100 volts with respect
to ch.s.is ground.
COMMON MODE REJECTION: 100 V de or 200 V
pe.k.to.p..k rt line frequency will not affect resdmg.
CONNLCTORS: Input: Teflon.insul.ted triaxi.1. Analog
output: Amphenol SO.PCZF. Printer outputs and
rrmote controls: 50.pin Amphenol Micro.Ribbon..
Low md C.S. ground: Binding posts.
FOWEL 105.125 or 210.240 volts (switch selectsd).
60 Hz. 50.H~ models .v.il.bl.. 30 watts.
DIMENSIONS. WEIQHT: 5’1,” high x 19” wd. by lo”
deep; nrt weight, I5 pounds.
ACCESSORIES SUPPLIED:
Model 4451 Input Cable: 48” triaxial cable wth
triaxi.1 connector and 3 alligator clips.
M.ting digit.1 output connector.
IV
0874

MODEL445
SECTION 1. GENERAL DESCRIPTION
GENERALDESCRIPTION
1-1. GENERAL. The Model 445 is B sensitive Automatic d. Digital Display. The digital display provides
Ran
lo- 8
ing Digital Picoammeter with eight ranges from
to 10-2 ampere plus 100%cwerranging. 3 digits with lo-= ampere resolution.
e. Variable Dieplay Kate. A front panel DISPLAY
RATEcontrol adjusts from 24 readingslsec. to 1 read-
l-2. FEATURES. ing/Z eec, to accommodate the mode of data retrieval.
a. Stability. Zero drift with time is less than 1 f. Autorsnning. Autorange circuit sense8 the in-
digit per day; drift with temperature is less than one put current and automatically selects the proper
digit per O/C, making frequent adjustment unnecessary. range far each meaeurement.
b. Overload Protection. A transient overload of g. Complete BCDOutput and Control Lines. Binary-
1000 volts at up to 20 r,A will not damage the instru- Coded-Decimal Outputs are provided on a rear panel
nlent. connector for significant digits, range, polarity,
overrange, and zero check. Cantrr,l lines are also
C. Damping. A front panel FILTER Switch selects
additional damping of noisy signals on the 10-7. lo:*. provided for remote control of ranging and A/D conver-
*ions.
and 1O-g amp& r&es. - - ,
TABLE l-l.
Front Panel Contr018.
Control Functional Description
POWERwitch (S102) Controls power to instrument.
ZEROCHgCK(S1204) Selects Zero Check Made.
ZEROADJUST(R1207) Adjusts the zero offset.
FILTER Switch (S1201) Selects additional filtering.
RANGE-HOLD/AUTOMATIC(51203) Selects HOLDor AUTOMATICmodes.
RANGE- lo-Z/DOWN (51202) Permits manual range selection.
DISPLAY FATE (R1237) Adjusts the AID conversion rate.
Paragraph
2-3, B
2-3, b
2-3, c
2-3, d
2-3, a
2-3, f
2-3, 8
0971 1

GENERALDESCRIPTION MODEL445
ZEROADJUST ZEROCHECK
Control R1207 Switch S1204
I I
PO&R DISPLA; RATE PI<TER RANGE P&&E
Switch Switch 51205 Switch CHANGING HOLD/AUTO 10-2/DowN~
s102 51201 Lamp DS1201 Switch 51203 Switch S1202
FIGURE2. Front Panel Controls.
TABLE 1-2.
Rear Panel Terminals.
Terminal
INPUT Receptacle (51213)
LO Binding Post (51215)
GNDBinding Post (51217)
PRINTER/CONTROLConnector (51214)
ANALOGOUTPU'I(51218)
Functional Description
Input connector, triaxial type.
Input LO connector.
Chassis Ground connector
Provides BCDoutputs and control lines; 50 pins.
Analog Output connector, Mates with Keithley
Part NO. a-33.
Paragraph
2-1, a
2-1, a
2-1, a
2-8, a
2-7 I
2 0971

MODEL445 OPERATION
SECTION 2. OPERATION
1NPUT
LOW
HIGH
CASE

OPERATION
!L
of a
al 2
Accurecv. The accuracy is specified in terme
percent of reading on esch range. A” addition-
1 digit is specified since the A/o conversion
has-e” inherent 2 1 digit uncertainty. Noise and
saurce~resistance conditions should be evaluated es
additianal meeeurement considerations.
a. Source Resistance.
The value of eowce resist-
ance can affect the measurement if the loading effect
of the picoammeter is significant. To avoid a de-
gradation of zero drift, the picoammetee range should
be selected so that the range feedback resisrar is
much lesa than the e~urce resistance. The zero drift
epecificseio” is valid only for e~urce voltagee greaf-
er the” 2 volts.
f. Overloads. A unique input circuit provides
complete overload protection with feet recovery. The
maximumoverload is 1000 volts using e current limit-
ed supply (up to 2OmA)such as Keithley Models 240.4,
245, or 246.
Instantaneous input current mwt “ever
exceed 125 milliamperes. With a” input voltage up
to 1000 volts use e current limited supply or a ser-
ies limiting resistor (8000 ohms minimum).
MODEL445
g. Reading Time.
The time interval for a new
reading to take place depends on the Model 445 front
panel Settings as well as the chareceeristics of the
input signal. The various time intervals which co”-
etitute the total reading time are described es
follows. The intervals are graphically shown in
Figure 5.
1. T1 = A/D Conversion Time until first range change.
2. T2 = Total Time for all successive range changes.
3. Tj = A/D Conversion Time to first an-scale reading.
4. Tq = Total Settling Time until reading is final.
h. Off-Ground Measurements. The Model 445 can be
used far off-ground meeeurements with the circuit low
floated up to 100 volts with respect to chassis ground.
Circuit low to chassis ground isolation is greater
then 106 ahms shunted by .02 microfarad. Whenmaking
off-ground meeeuremente discannect the LO to GND
shorting link on the rear panel. A triaxial cable
should be used to maintain noise shieldine and to
provide ueer safety., The BCDautputs are”co”“ected
to circuit low and therefore muet be used with de-
vices which ce” be Operated off-ground.
TABLE 2-1.
Typical Reading Time With Filter “OUT”.
Input
Initial Sampling Range Ranging Final Range Settling Total
Signal Range Time Tl Sec. Changes Time T2 Sec. Time Tg Sec. Time Tq Sec. Time Sec.
10-9 A 10-2 A .042 7 .294 10-g 10-2 (1) (7) .042 (1) .120 (3) .498 (12)
A A .042
(1) 6 .252 (6) ,042 (1) ,060 (3) .396 (11)
10-7 A 10-2 A .042 (1) 5 .210 (5) ,042 (1) .OlS (3) .312
10-6 10-2 (10)
A A .042 (‘1) 4 .168 (4) .042 (1) ,015 - .267 (6)
10-5 A 10-2 A .042 (1) 3 .126 (3) ,042 (1) ,012 - .222 (5)
10-4 A 10-2 A .042 (1) 2 .084 (2) .042 (1) .003 - ,171 (4)
:;I; ; .042.042 (1) 01 ,0420 (1) ,042,042 (1) ,003.003 -- ,129,087 (3)
(1) (0) (1) (2)
PIGoRE 5. Reading Time.
4 0971

MODEL445 OPERATION
2-3. FRONTPANELCONTROLS.
a. POWERSwitch (S1021. This switch controls the
line power to the instrument. A rear panel fuse
PlOl protects the instrument in case of malfunction
of the power supply or power switch.
b. ZEROCHECKSwitch (S1204L This switch selects
either "NORMAL"or "ZEROCHECK"modes of operation.
I" "ZEROCHECK""asitio", B relay is activated which
places a short across the feedback of the picoammeter.
Zero Controls R1238 and Rl207 ca" be used to adjust
the zero offset as necessary.
c. ZEROADJUST(R12071. This control provides
"FINE" adjustment of the zero offset. The internal
zero control Rl23S provides "COARSE"adjustment.
d. FILTER Switch (51201). This switch selects
either mininu,, damping ("OUT" position) or additional
damping ("IN" position).
A more complete diacussio"
of the "FILTER" mode is give" L" paragraphs 2-2, c
and 2-2, g.
e. RANGE- HOLD/AUTOMATICSwitch (51203). This
switch selects automatic re."e.i".z weratio" in "ADTO-
_ _
MATIC" position or manual ranging in "HOLD" position.
I,, "HOLD", the range is held at the existing range
displayed until the switch is changed or the range
is manually changed by either S1202 or the external
control lines.
f. RANGE- 10-2/DOWNSwitch (51202). This switch
is a nomentary-contact, "armally open switch for ma"-
ual range changes. This switch should be used with
Switch 51203 in the HOLDposition. When Switch S1202
is nmentarily set to "lo-2", the Picaawneter resets
the range display t" 10e2. However, each time the
switch is set to "DOWN"Lmsition. the Dicoammeter down
w to a more sensitive range: The'sequence af-
dawn ranging would be typically 10-2, lo- , 10 4, 10-5,
;riO$ith a reset after 1O-g range is reached, that
) 10-9, 10-q.
g. DISPLAY RATE (R12371. This control adjusts the
A/D Converter conversion rate from 24 readingslsec.
(MAX) to 1 reading/Z sec. (approx).
2-4. OPERATINGPROCEDURE.
a. Preliminary Pracedure.
1. Check the 117-234" Switch (SlOl) on the rear
panel for proper line voltage.
2. Check for proper rated fuse.
3. Connect the power cord, place the POWERSwitch
ON, and allow a 30-minute warmup for critical meas-
urements.
4. Adjust the ZEROContra1 (R1207) as necessary.
Zero is indicated by alternately flashing + polarity
lights.
5. Connect the source as described in paragraph
2-l.
b. Measurements. The Model 445 measures current
over a full-ran e
6 from 10 milliamperes (10m2A) to 1
"anoampere (lo- A) with resolution to 1 picoampere
(lo-12A). The display will indicate either a posi-
tive or negative input current autamatically. (A
p"sitive current is defined as a positive "conven-
tional current" applied at Input High with respect
to Input LOW).
2-5. DIGITAL DISPLAY. The display has three digits
plus one for overrange indication. The range is
displayed in scientific notation where the exponent
is automatically displayed for ranges 10-2 through
10-9 amperes, To interpret the display refer LO
Figure 6. A" overload condition will cause the dis-
play cubes to blank o"t so that "o ambiguous display
is possible. This situation is described under 2-4,
d Autoranging.
How to Interpret the Digital Display
The reading shown in Figure 6 above should be
interpreted as follows. The polarity is dis-
played by a lighted "f" or "-" indicator. The
decimal point remains fixed with respect to the
digits. I,, the example, the reading is 1.676
(three digits plus a "one" for overrange). The
range multiplier is 10-9 (the exponent is a"to-
matically selected by the autorangi" circuit).
The complete reading is +1.676 x lo- 8 amperes.
FIGURE6. Digital Display.
0971 5

OPERATION MODEL445
TABLE 2-2.
Output Information and Controls.
lode1 4401 Printer Output Cards: Provide SCDoutp"t
and external control of Model 445; may be purchased
installed or separately for field installation (no
wiring required).
Printer Outputs: SC0 pwitive o"tp"t represents
each of the four digits, exponent, range, zero,
polarity and averrange. Standard code is l-2-4-8.
"O"< +0.4 volt; "1" 5 +lO volts at up to one milli-
ampere; O=OOOO.
Print Command: Positive pulse of 14 volts from a
2200-ohm source with 1 volt per microsecond rise
time, 100 microseconds minimum pulse width.
Remote Controls:
Hold #l: Closure to ground inhibits A to D conver-
sion at that iridtant.
Hold 82: Closure to ground inhibits A to D canver-
sion after reading has been completed.
Trigger: Closure to ground initiates one conversior
when in Hold 112. Integration period starts 8.3 ms
(10 ms on SO-Hz models) after "Trigger" or release
of Hold i/Z.
Connector: SO-pin Amphenal Micro-Ribbon mounted on
Model 445. Output mating connector supplied with
4401.
NOTE
The term "Closure to Ground" or "Grounded control
line" means a short to commn directly or through
a saturated NPN transistor (VC, < +0.4V). Only
the "COMMON"as supplied at the PRINTER/CONTROL
connector should be used for closures to ground.
TABLE 2-3.
Full Scale Magnitude.
Connector
Pi" No. Output Decimal Digits
1 .l x 10" 1
2 2
x 10"
2
26 4 x 10"
4
27
8 x 10" 8
3
1 x 101 1
4 2
x 101
2
28
4 x 101
4
29 8 x 101 8
5 1 x 102 1
6
2
x
102
2
30 4
x 102 4
31 8 x
102
8
0971