Keithley 191 Service manual
Instruction Manual
Model 191
Digital Multimeter
01979, Keithley Instruments, Inc.
Cleveland, Ohio, U.S.A.
TABLE OF CONTENTS
6-1
6-l
6-1
h-l
6-l
6-l
6-2
6.2
ii
MODEL 191 DMM
GENERAL INFORMATION
SECTION 1. GENERAL INFORMATION
l-l.
INTRODUCTION.
1-2.
The Keithley Model 191 is a 5-l/2 digit,
200,000-count,
meter with dc volts and ohms ranges standard. It
provides highly accurate, stable, low noise and
fast-responding readings from 1uV to 1200 volts dc
on 5 voltage ranges,
ments from 1 milliohm to 20 lrlegohms on 6 resistance
ranges. The 191 is capable of 0.0005% resolution
and luVlln!G sensitivity. In addition, if you
purchased the Model
will provide readings from 1OuV to
4 ra"gES. This option may alsn be purchased later.
and field installed.
manual-ranging bench digital multi-
and 2 and 4 terminal measut-e-
1910 AC
Voltage Option, your "MM
lOO(1 volts ac on
1-3. Your OMM also has features and advantages that
might not be readily apparent. Soae of these aw:
. 5-l/2 digit LEO display with appropriate de-
cimal point - 0.5 inch digits permit monitorinq
nx?asurements fron1 across tllc r""!".
l
Pushbutton NULL eliminates potenti"mrtPr
zeroing, corrects for lead resistance in ?-wire
ohms, bucks out thermal EMF's in 1ow IPYPI dc
nleasurements 3°C permits you t" medsuv? deviations from a set value. The NULi~ light indicates that the function is dctivc for "p~rat"rsaietv and to lessen the? ChdrlCC Of I.IP‘3S,II'P,~IPnt
FIGURE
l-l.
Model
191
Digital Multimeter
I-1
GENERAL INFORMATIOP
MODEL
191
DMM
Continued.
l-3.
A Micro-processor
provides:
A Combination of single slope and charge balance
AD conversion - for faster response and better
linearity.
Automatic non-linear digital filtering - for
faster response
display.
A reduction in the number of parts while maintaining high accuracy and speed of measurement simplifies high accuracy measurements and calibration of the instrument and provides higher
mean time between failures.
-1EEEEE error message indicates improper
uses of the instrument - prevents erroneous readings and reduces possibility of injury to the
user or damage to the instrument.
Each range has:
Automatic polarity operation minus sign displayed, positive implied.
Effective input overload protection.
Overrange indication - polarity and overrange
digit displayed.
Decimal point positioned by range pushbutton.
Automatic Z/4 wire ohms operation - saves
time and simplifies Z-wire or 4-wire ohms
measurements.
A full line of optional accessories that
exterd the measurement capability of your llcdel
191.
IGOO High voltage Probe allows your Dill1 to
measure fron 12OOV to 40kV dc.
1901 Plug-l" Current Adapter allows your D,,,, to
read dc current from lvA/digit to 2000mA.
the AC Voltage option, it reads from lOti/digit
to 2000nA ac.
1682 High Frequerxy (RF) Probe allows your Dills to
RedSUre fr0n 0.25v to 3ov t-lx ac wer a frequency
t-awe of 100kHr to 1OOMHr.
without the AC Voltage Option.
1685 Clamp-On AC Current Probe (when used with AC
Voltage Option) allws your Dllll to measure fron
zero to 200A rns ac.
Some of these accessory ncde1s are:
based design that
and reduced noise on
It can be used
the
Hi th
1651
50.Ampere Current shunt allows your D!IM to
measure fron O-50A dc, ard with AC Voltaqe Option
fron IOA to 50A ms ac.
NOTE
Refer to Section 4 for more detailed information
on these accessories.
l-4. WARRANTY INFORMATION.
1-5. The Warranty is given on the inside front cover
of this Instruction Manual. If there is a need to
exercise
Representative in your area to determine the proper
action to be taken. Keithley maintains service
facilities in the United Kingdom and West Germany,
as well as in the United States. Check the inside
front cover of this Manual for addresses.
l-6.
l-7.
which occur after printing of the Instruction Man-
ual will be explained on a Change Notice sheet
attached to the inside back cover.
1-8.
1-9. Safety symbols used in this manual are as
follows:
The b
this Manual.
CAUTION statements with regard to proper use and
handling of the instrument.
instrument may occur if these precautions are
ignored.
This\ ~
this Manual.
on the instrument which are potential shock
hazards.
statements with regard to proper use and handling
of the instrument.
result if these precautions are ignored.
l-10.
I-11.
given in Table 1-I.
the warranty, contact the Keithley
CHANGE NOTICES.
Improvements or changes to the instrument
SAFETY SYMBOLS.
IMPORTANT
symbol can be found in various places in
Carefully read the associated
Oamage to the
svmbol can be found in various places in
This symbol indicates those areas
Carefully read the associated WARNING
Serious personal injury may
SPECIFICATIONS
Detailed specifications for the Model 191 are
l-2
MODEL
191
DMM
GENERAL INFORMATION
TABLE l-l
SPECIFICATIONS
OPERATING INSTRUCTIONS
P1006
MODEL
AC OPl
YODEL
191
DMM
e
LINE/FUSE
(F101)
LINE'VOLTAGE
SELECT SWITCH
(S102)
2-o
FIGURE 2-1.
Location of Line Fuse and Line Voltage Select Switch.
MODEL 191 DMM
~OPERATING INSTRUCTIONS
SECTION 2. OPERATION.
INTRODUCTION.
Z-l.
2-2.
incoming
operation of the Model
2-3.
2-4.
mechanically and electrically before shipment. Upon
receiving the Model 191,
shipping container and check for any obvious damage
which may have occured during transit.
damages to the shipping agent.
original
required.
Model 191 orders:
2-5.
2-6.
on how to connect the Model 191 to your available ac
This section provides information needed for
inspection, preparation for use, and
191 and
UNPACKING AN0 INSPECTION,
The Model 191 was carefully inspected, both
packaging materials if reshipment is
The following items are shipped with all
a. Model 191 OMM.
b. A Copy of this Manual.
Installed or separate optional accessories, as
C.
ordered.
PREPARATION FOR USE.
The Model 191 is shipped ready-for-use on the
line voltage marked on its rear panel. Instructions
line power are contained in Paragraph Z-7 Line
Power.
its accessorieS.
unpack all items front the
Report any
Retain and use the
NOTE
The line voltage sct/tinq of the instrument
is marked on the &r panel. The following
procedure can be uqed to either confirm the
factory setting, or 40 set up the instrument for
operation on another ivoltage range.
voltage range is chdnged, the box next to the
selected line voltaTe should be appropriately
marked as an externalI reminder of the setting.
Use a water soluable barking pen.
Line Voltage Selec~tion.
2-9.
Z-10. Set up the Modejl 191 to operate on your
available ac line voltage as follows:
Turn the DMM botltom side up and loosen the
a.
four screws in the boqtom cover. These sci-ews are
held captive hy rubber O-rings.
Hold the top an?
h.
prevent their separatjon and turn the DMb1 over to
normal position. Rem+, the top cover.
Set switch
c.
line fuse, as indicated in Table 2-1. for your
available input line ~ voltage.
shown in Figure
d. Reinstall the top cover.
8102
2-l.
bottom cover
aind install the proper rated
if the line
together t"
These items are
2-7. LINE POWER
2-R.
cord which mates with a 3rd wire earth grounded
receptacle.
voltage ranges of 60 or 50 Hertz ac power.
voltage ranges are 105 to 125 volts and 210 to 250
volts.
positioning an internal slide switch and installing
the appropriate fuse for that range.
your available ac power inaccordance with the
The Model 191 is provided with a 3-wire line
The instrument will operate on 3
Standard
Either of these ranges may be selected by
An optional
line voltage range of 90 to 110 volts is available
by special order.
different transformer.
following procedures:
Instruments with this range use a
Connect the Model 191 to
TABLE 4-l.
Line Voltagq Selection
*Requires special~factory installed
transformer.
2-l
OPERATING INSTRUCTIONS MODEL
191
DMM
NULL
FUNCTION*
FIGURE 2-2.
POLARITY (NEGATIVE Is INDICATED, POSITIVE Is IMPLIED
rblHEN MINUS (-) DISPLAY IS OFF.)
Rear View Showing Line Cord.
/
2-2
1 SET POWER ON
(I.3
*SEE TEXT, PARAGRAPH 2-15.
I
@j SELECT RANGE
4 CONNECT SOURCE
0'
FIGURE 2-3. Operating Controls.
I", 3 ,.
OPERATING INSTRUCTIONS MODEL
2-2
Z-11.
z-12. The Model 191 is provided with a 3-wire line
cord, shown in Figure 2-2, which mates with third-
wire grounded receptacles. Connect the instrument
to ac line power as follows:
2-13. OPERATING INSTRUCTIONS
2-14.
Model 191 [IMM are outlined below, and Condensed
Operating instructions are provided on the bottom
cover of the instrument. These instructions should
only be used after becoming completely familiar with
the operation of the Model
"SC?.
performance and safest operation will be obtained by
using the individual instructions provided in this
section which describe how t" make specific function
mea*"reme"ts.
OMM a5 f0110w5:
Connecting Line Power.
WARNING
\,
Ground the instrument through a properly earthgrounded receptacle before operation. Failure to
ground the instrument can result in sevet-e injury
or death in the event of short circuit or
malfunction. In addition, connect only t" the
line voltage selected. Application of incorrect
voltage can damage the instrument.
Plug the power c"rd into a properly grounded
a.
outlet of a source having the selected line
voltage.
Operate the Model
b.
Paragraph Z-13.
The basic operating instructions for the
Until this familiarity ha5 been achieved, best
Refer to Figure 2-3 and operate the
/&CAUTION
no not exceed the Maximum Inputs limits given in
Table 2-2.
Turn on the power by depressing the ON/OFF
a.
pushbutton.
it is useable immediately, but a 1 hour warmup is
required to obtain rated accuracy.
additional hour may be required from temperature
extrelne5.
Select the function with the ACV, IDCV or n
b.
pushbuttons.
Select the range by depressing the appropriate
c.
pushbutton,
Connect the source to the INPUT terminals and
d.
make the ,n,easurement.
Section 4 should he (used as required.
If the instrument is within 18-2R°C,
191
as described in
191
through day-to-day
up to 1
Accessories described in
Summary of M
UNCTION
2-15. NULL FUNCTION.
Z-16.
and functions. It is
based function. The dr
function is selected.
depressed with an "n-5
that reading is subtrx
1ngs. The nulling pr"
of two numhers, and ha5
or function selected.
primarily designed to
compen*ation for te5t
emf's generated in circa
terminals,
measure variation5 db"'
example,
the display, and variat
made by switching to t
This,is possible hecaur
is 100,000 (counts) in
sign is active for n (a
2-17. it is important
reduce5 the dynamic re
stance, if
voltages greater than
A,0 c"nverter (200,OOI
range would occur at *
readings less than -1
less than +lV) because
ing of
dynamic range of the
Figure 2-4.
Dynamic Range and the
exceeded and thus, bot
of the meaS"i-erne"t.
dynamic range can be e
The NULL functi
the null f
+1.00000 VDC
t1.00000
-199,999 coun
TAB
RANGE
ZOOmV, 2
zov-1200
ALL
ALL
VL
In ocv
imum Inputs.
MAXIMUM INPUT
7OOV Continuous;
12OOV for 1 minute
Rh3Xllll"lll.
IZOOV Continuous
25OV rms; 360V peak
IOOOV rnis sine or dc
2 x 107 v .Hz
is operable "n all ranger
switch selectdblr software
nciator is lighted when the
?en the NULL pushbutton is
1~ reading on tlw display,
d from all subsequent read55 is merely d subtraction
"thing to do with the range
For this wds"n, althouyh
,vide convenient pushbutton
lad re5istance dnd thermal
s connected to the DMO INPill
:tion cd" also be used t"
or below a 5et valur. 101.
put coutd be used to null
ns ahovc 1O.OOOWO could be
2OMn range and n function.
the numher heing subtracted
th instances, and the minus
ACV) in the NULL mode.
o note that the use of NULL
e of meas"rerne"t.
is the nullpd value. input
I would still overload the
counts), even though "ver-
10.000 counts displayed, and
would cause overrange (2V
f the maximum display read-
This reduction in the
~surement is illustrated in
unction, both the Display
nput dynamic range cd" he
:dn limit the dynamic range
ACV and n, only the input
!eded.
191
For in-
DMM
2-3
OPERATING INSTRUCTIONS
199,999
DISPLAY DYNAMIC RANGE
A
/ NULL \
VA&UE
199,999
MODEL
191
MEASUREMENT
DYNAMIC RANGE
IN NULL MODE
(SHADED)
DMM
\
v
INPUT DYNAMIC RANGE
FIGURE 2-4.
Effect of NULL function on Dynamic
Range of DCV Measurement.
Z-18.
described in DC Voltage and n Measurement
Procedures.
2-19.
Z-20. Overrange is indicated by the minus sign
along with the over-range digit and the appropriate
decimal point. All of the remaining less significant digits are blanked. Example:
Overrange is indicated whenever the dynamic range of
DCV measurement is exceeded. With the NULL function
off,
described in Paragraph Z-17, the dynamic range of
the measurement is reduced by an amount determined
by the size and polarity of the nulled signal when
the instrument is in the null mode.
The Use of NULL as pushbutton 'zero" is
OVERRANGE INDICATION.
(-I--.---).
this occurs above
*199,999 CO”“tS. As
/
2-23. DC VOLTAGE MEASUREMENT.
2-24.
microvolt/digit to
displayed reading is 139999. Overrange is indicated
by (-)I-----, except on 1200 volt range. On the
1200 volt range,
maximum allowable input voltage. Maximum allowable
input: 12OOV for
continuous on the 200mV and 2V ranges; 1200 volts
continuous on the ZOV-1200 volt ranges. Use the
Model
The Model 191 reads dc voltages from I
1200 volts. The maximum
the display can read beyond the
I
minute maximum,
191
to measure dc voltage as follows:
CAUTION
A
Do not exceed the maximum allowable input voltage
limits. Instrument damage may occur.
700
volts
2-21.
2-22.
function is selected. These selections are:
2-4
ERROR INDICATION.
-1EEEEE is displayed when an improper range -
ACV function - when AC option is not installed.
20Mn range - with ACV or DCV function selected.
ACV function with 2000, 200mV range selected.
a.
Turn on power with the ON/OFF pushbutton and
depress the DCV pushbutton.
b. Select the desired range from the five ranges
available.
range pushbutton.
by the
The decimal point is positioned by the
The 1200 VDC range is selected
1000
pushbutton.
MODEL
c.
off) unless measurements are to be made as
deviations from a preset value.
d.
INPUT HI and LO binding posts. The binding posts
accept wires, spade lugs or banana plugs for ease
of connecting the circuit to be measured. LOW
thermal cabling and connections are recommended
for measurements on the 200mV range.
e.
displayed digits, polarity sign and decimal point
locations.
in volts.
f.
NULL function to obtain rated accuracy. Zeroing
is necessary to compensate for thermal EMF's
generated by the connections to the circuit to be
measured.
microvolts or several tens of microvolts. set
zero as follows:
191
DMM
Ensure that the NULL pushbutton is out (light
Connect the signal to be measured between the
For the top four ranges, merely observe the
The top four ranges are direct-reading
For the 200mV range, ZERO must set with the
These voltages may be only a few
1) Set
2) Disconnect the test leads at the circuit to
be measured and short them.
3) Depress the NULL pushbutton.
4) Reconnect the test lead and make the
measurement by applying the signal and reading
millivolts on the display.
Model
191 to
200mV
range.
g. The optional Mode
be used with the Modt
up to 40 Kilovolts, a
Paragraph Z-30.
2-25.
Z-26.
millioha/diqit to 20
rarqes.
4.wire oh,:6 operation.
sense leads are conmec
natically dew 4-termi
not connected, the me
For 46terminal meax
digit) can be obtaine
log as the maximum 1~
2-3 are mt exceeded.
measurements on the 20
the NULL furrtion to o
RESISTANCE (n)
The Model
The Model 191
191 I
tlodcl 191 to measure 7
&CA"
MAXIMUM ALLOWABLE I
36OV peak, 25UV rms
voltage.
a. Turn on power and
h. Connect the circ
INPUT terminals and I
the six ranges avail
positioned by the ran
Instrument
IOPERATING INSTRUCTIONS
1600
High voltage Probe can
191 to measure
reduced accuracy. Refer to
JASUREMENT
IN ~nedsures resistdrre fron 1
eqohms.
prwides d~tondt>c 2-wit-e or
This nedns that if the ohms
ed, the rneasurcment IS autoIal if the sense leads are
iureiaent is done 2.terminal.
eme nts
on the top five rarqes as
ION
'UT VOLTAGE (all ranges):
Do not exceed maximum
amage may occur.
depress n pushbutton.
lit to be measured to the
,lect the desired range from
ble.
e pushhutton.
SEC Table 2-3 for
rated
The decimal point is
dc voltages
dCC"l'dCY (tl
TABLE 2-3
Resistance Ranges
RANGE
SETTING
200 n***
2k n
20k n
200k n
2000k "
20M n
MAXIM""
READING
199.999
1.99999
19.9999
199.999
1999.99
MAXIMUM OUTPUTS'
I(Shorted) V(Openl
-4mA -4OOmV 7n
-4mA -4V 22"
-4OOuA
-4OuA
-4uA -4v 7000
1g.gggg t-““-
OVERRANGE: MAXIMUM
-l---_
* HI binding post (red) is negative.
** Maximum resistance per lead for additional
l
** Zero must be set by NULL to obtain rated accuracy.
1
digit en-or.
.,
-4v
-4v
-4v
ALL RANGES
-r
ALLOWABLE
360Vpeak,
MBX.
2200n
INPUT:
4-w,
700
22on
;*
250Vrms
2-5
OPERATING INSTRUCTIONS
MODEL 191 DMM
For 4-terminal measurement c"nnect the sense
c.
leads t" the circuit t" be measured and to the n
SENSE terminals on the 191. This arrangement
eliminates the err"r due to the voltage drop
across the current-carrying leads.
d. Ensure that the NULL pushbutton is out (light
off) unless measurements are to be made as
deviations from a preset value.
c. For the top five ranges of Z-wire or 4-wire
measurements, merely observe the displayed digits
and decimal point to make the measurement.
f. For d Z-wire or 4-wire ohms imeasurement on the
200n ra"qE,
function to obtain rated accuracy.
necessary to colrlpfnsate for test Icad resistance
On Z-wire & Thermal Emfs on 2 & 4-wire.
as follows:
1) IDisconnect the test leads at the circuit to
be rmeasured, and short them.
2) ocpress NU,.L p"shb"tt"n.
3) iReconnect the test leads and <make the
IIIEaSIItm!x!"t.
g. "iodf Test.
diode testinq.
resistance Of J
approximately 190n. (Hiqh Terminal is Negative)
Z-27.
OPTION).
2-28.
reads ac voltages from 10 microvoltsldigit to 1000
lCK0 rmust be set with the NULL
Zeroing is
set LPr"
Thr 2Kn range is rccomxnded for
On this range the forward oil
silicon diode
Ac VOLTAGE MEASUREMENT (WITH 1910 AC
With the Model 1910 option, the Model 191
will read
volts. The instrument is avera9e responding and
displays the root !:ledo square
with a frequency of 50Hr t" 100kllr. Accuracy is
specified for 1000 counts and above. The maximum
reading is 199999. Overrange is indicated by (-)
i-----, except on 1000 volt range. On the 1000 volt
range,
allowable input voltage. Maximum allowable input:
IOOOV r‘ms or dc; 2 x lO'V.tir. "se the Model 19,
to "leasure dC voltage as follows:
a. Turn on power with ON/OFF pushbutton and
depress the ACV pushbutton.
h.
available. The decinlal point is positioned hy the
range pushbutton.
C.
off) ""less measurements are to
deviations front a preset value.
the display can read beyond the lmaximum
/!&CAUTION
Do not exceed maximunl allowable input voltage.
Instrument damage may occur.
Select the desired range from the 4 ranges
insure that the NULL pushbutton is out (light
NOTE
Do not use NULL to zero Lhe range.
residual zero reading is normal (approx. 20tiuV)
If NIJLI. is used to zero this offset. reailirlgs in
specified accuracy range wili be 1"~ by the
offset amount.
d. Connect the signal to be measured between the
INPUT Ill and LO binding posts.
accept wres,
of connecting the circuit to be measured. Observe
the displayed digits and decimal point.
e. The Model 1682 RF Probe can he used with the
Model 191 to measure 0.25V to 30V rms ac signals
with a frequency IOOkHr to IOOMHr (and above at
reduced accuracy). Refer to Paragraph 2-36.
spade lugs or banana plugs for ease
value Of a 51~ W~VC
he Imade as
A Sllldll
The hinding posts
2-6
MODEL
191
DMM
SECTION 3 PERFORMANCE VERIFICATION.
3-1.
GENERAL.
Performance verification may be performed upon
3-2.
receipt of the instrument to ensure that no damaqc
or nisadjustment has occurred during transit. Verification may also be performed whenever there is
question of the instrument's accuracy, and following
calibration, if desired.
NOTE
For instruments that are still under warranty
(LESS than 12 month5 since date of shipment),
if the instrument's performance falls outside
specifications at
Keithley rcpresentativc or the factory iomediate1y.
3-3. RECOMMENDED TEST EQUIPMENT.
3-4. Kecommended test equipment for performance
verification is listed in Table 3-1. Alternate test
equipment nay be used.
the alternate test equipment is not at least 3 tines
better than the instrument specifications, addition-
al allowance must be made in the readings obtained.
Some of the equipment listed in Table 3-l is not 3
times better than the 191 specifications because
such equipment is not readily available. In these
instances, the verification procedures indicate the
equipment manufacturer's specified uncertainty, and
include the uncertainty in determining the allowable
rcadiny for the Model 191.
any point,
However, if the dccuracy of
contact your
PERFORMANCE
VERIFICATION
3-5. ENVIRONMENTAL COND1TIONS.
All neasurenents slhouid be made dt a" dnbirnt
3-6.
tenpcraturc within the rpnge of 18' to 28°C (65" to
82Or). and a relative hu#idity of lcsr thdri 80%.
3-1. PERFORMANCE VERIFI$ATION PROCEDURE.
3-K.
basic accuracy of the Moidfl Ill DI,!M for dc voltnge.
resistance and dc voitagc (rrlth l.lodei 1910 AC
Voltage Option installdd) 1mcd5urcnwnts. it thP
instrument is out Of s~pecificdtion at Jny po,nt,
perfor,, a complete ca)ibration a5 d,xc,.ibv! in
Section 6,
wavva"ty, as noted above.
3-9.
Use the f"llowin(i procedures to verify the
~nlcss the :instrwwnt 15 still iindrr
NOTE
Performance verificbtion should bc pcrfonncd by
qualified pcrsonnel~ using accurdtc and rcliablr
test eq"lpncnt. ~
initial Conditions;
Recommended Test Equipment For Performance Vcrifica
T
ITEP
DESCRIPTION
-
A
DC Calibrator
8
AC Calibrator
C
High Voltage Amplifier
(Used with Model 745A)
"
IIccade Resistor
E
Kelvin-Varlcy Voltage
Uivider
(Used with Model 343A)
TABLE 3-l.
SPECIFICATION
190n, 1.9kn,19kil,
190k0,1.9Mn, lOMn,
t0.017,
.19v, 1.9v
with .2ppm Terminal
Linearity
tilot
1.
72OA
3-l
PERFORMANCE
VERIFICATION
MODEL
191
DMM
b. Turn on the Model 191 and allow it to wdrrl up
for one hour.
WARNING
Sonc procedures require the USC of hi9h voltage. Take cat-c to prevent contact with live
circuits which could cause electrical shock resulting in injury or death.
3-11. DC Voltage Accuracy Check ('ZOV to 1ZOOV
R‘?"%? Applied Allowable Readings at 18O to 28'C
I I
I 4
I
3-12. DC Voltage Accuracy Check (200mV and 2V
Ranges).
4. Select dc voltaqe function.
h. Connect the DC calibrator (Item A, Table 3-l)
to the instrument.
c. Select the 2flV range, and apply positive 1OV
dc to the DMM. The readin &st be within the
limits specified in Table 3-2.
Select each rcnaining range and apply r-c-
!i.
quircd voltage specified in Table 3-2.
that the reading is rrithin specifications.
f. Repeat all checks with negative voltage.
DC Voltaqc Performance Check (2OV to 12OOV Ranoc).
Voltage
I
Ranges).
2. Sclcct DCV and 200mV range.
b. Disconnect test leads at the DC calibrator
(A) and short them.
Ibutton. Verify a display indication of OO.OOOmV
+O.OOlnV flashin9.
t
I
TABLE 3-2.
99.986 to 100.014
999.86 to 1000.14
9.9986 to 10.0014
Depress the Model 191 NULL
Verify
c. Connect the DC calibrator (A), Kelvin-Varley
Voltage Divider (E) and Model 191 as shown in
Fi9urc 3-1.
.0190000 output.
Temporarily disconnect the test leads from
i,
the DC calibrator (A) and short them.
Model 191 PlULL button for a display indication of
00.000 ~OO.OOLmV flashing.
c. Reconnect the DC calibrator (A) and set to an
output Of +10.00000V.
f. Verify that the Model 191 reading is between
t189.978 to t190.022nV. Note that the allowable
reading includes a 6 digit allowance for the
uncertainty of the DC calibrator (A) and Voltage
Divider (E).
~9~. Kopfat step d thru f with negative voltage.
h. Select the 2V range and rcleasc the NULL
button. Set the Kelvin-Varlcy Voltage Divider
(E) to .I90000 output.
1. Verify that the Model 191 reading is between
t1.89981 and t1.90019V. Note that the allowable
reading includes 24 digits for DC calibrator (A)
uncertainty.
i. llfpeat step i with negative voltage.
3-13. AC Voltage Accuracy Check (With Model 1910
I
1
AC Voltage Option Installed).
A. Select ac voltage function.
b. Connect the AC calibrator (Item R, Table 3-1)
to the DMM. Set the calibrator frequency to IkHr.
2. Set the DMM to the 2V range and apply IV ac
to the DMM. The reading must be within the
limits specified in Table 3-3.
Select the 20 and 200 volt ranges and apply
d.
the required voltages as specified in Table 3-3.
Verify that the readings are within specifi-
Set the voltage divider (E) for
DepESS
3-2
DC
Calibrator
(A)
+
OUT
FIGURE 3-l.
1.0
HIGH,
Kelvin -
Varley
LOW
Voltage
LOW
Divider
(El
Test Circuit For 2OCmV And 2V Accuracy Check.
MODEL
191
MODEL
191
DMM
PERFORMANCE
VERIFICATION
To check the 1000 volt range, connect the
2.
High Voltage Amplifier (Item C, Table 3-l) to the
output of the AC calibrator per the manufacturer's instructions. Connect the amplifier
output to the Model 191 INPUT terminals. Set the
AC calibrator for amplifier output of 1000.00
volts at lktlz. Verify that the DMM reading is
within the specified limits in Table 3-3.
TABLE 3-3.
AC Voltage Accuracy Check
Range Applied
Voltage
Allowable Readings
at 18’= to 28’C
at
2:;
2oov 1oo.oov
1ooov
10 1 .ooov .ooov
1ooo.ov
9.9868 .99868 to to 10.0132V 1.00132V
99.068 to 100.132V
998.00 to 1002.00v
m
2ov 1o.ooov
at 20ktlz
2ov 1o.ooov
at 100kHr
9.9868 to 10.0132V
9.9868 to 10.0132V
2ov 1o.ooov 9.90 to 10.10
L
f. To check accuracy /at 5OHz. ZOktlz, and lOOkliz,
select the 20 volt ;ranqe, apply the voltage
specified in-Table 3-13 at 5011z, then repeat at
20kHz and 100kHz.
are within the specifiied limits.
3-14.
Resistance (n) Accjuracy Check.
5. Select resistancejfunction by depressing the
R pushbutton.
0. SeleCt
C.
3-I) to the DMM.
L!.
sate for lead resist+xe by depressing the IiuLL
for a display indicition of 00.000
flashing.
Set the decade rjsistar to 1900. Verify that
J2.
the reading for the! 200n range is within the
limits specified in Tbblc 3-4.
f. Select the Zkr! rpngc.
Set the decade rcpistor to LC~O and reset the
9.
NULL.
h. Set the decade rbsistor to 1.900kn.
that the reading is bithin the limits spc!clfiCd
in Table 3-4.
L. COntiNE by uSin the NULL to eliminate lead
resistance on each ra'ngc and measure the next rcsistance as specifiei in Table 3-4. Test each
item in the table andi verify that each wading is
within specificationsi.
200n rc3ngfi
Connect the decape resistor (Itw 0. Table
Set the decade rjsistor to zero and comp~n-
Veinfy that the DMkl readings
~
* 00.301
vwify
TABLE 3-4.
Resistance Accuracy Check.
RANGE RESISTANCE
zoon 190n
2kn
20kn 19.00 kn
200kn
2000kn
20Mn
* Manufacturer's specified uncertainty of the decade resistor (D)
This uncertainty has been added to the specified accuracy of the
obtain the allowable reading.
1.900 kn
190.00
1900.0
kn
kn
10.000 nn
ALLOUABLE READING AT 18' to 28'C
189.955 to 190.045ri
1.89956 to 1.90044 kn
18.9956 to 19.0044
189.956 to 190.044
1899.22 to 1900.78
kn -p
kn 219 diqits
kn
9.9908 to 10.0092 Mn
~ *
,
?I9 digits
219 digits
+ 9 digits
?(19
digits
410
digits
3-3
MODEL 191 DMM
ACCESSORIES
SECTION 4.
4-1. GENERAL.
This section describes the various accessories
4-2.
and options available for use with the Model 191
DMM.
4-3. LINE POWER OPTION.
The Model 191 can be powered by a line voltage
4-4.
of 90 llOV, 50 - 60Hz with the special factory
installed transformer option.
available by ordering a Model
- 60Hr).
4-5~ MODEL
4-6.
has a
on the DMM corresponds to 1 kilovolt.
TO operate:
Volt range.
1600 to the INPUT terminals.
gator clip on the Model 1600 to source low. Connect
the probe tip to source high.
Specifications: Voltage Rang
40,000 volts DC.
Input Resistance:
1000
Division Ratio:
Ratio Accuracy
+1.5% at Z!ikV, decreasing to
fZ.fl% at 20kV and 30kV
f3.0% at 1OkV and 4OkV, and
+4.0% at 1kV.
Ratio Stability:
Heating Effects: Self-heating due to application of
high voltage for period in excess of 1 minute will
cause a maximum of 0.2% additional error at 40kV
(error is less at lower voltage).
4-7 . MODEL 1651 50-AMPERE SHUNT
4-8. The Model
be made from 0 to 50 amperes DC and from
50 amperes AC with AC Voltage option.
O.OOlohm
current will correspond to 50 millivolts.
To operate: Connect separate CUTrent leads (not furnished) between
the source and the Model 1651 hex-head bolts.
leads that are rated up to 50 ampere capacity.
Connect the voltage leads (furnished) between the
Model 1651 screw terminals and the DMM INPUT
1600
HIGH VOLTAGE PROBE.
The Model 1600 extends the DMM to 40kV. It
1OOO:I
division ratio which means that 1 volt
Set the DMM to OCV and 200
Connect the banana plug on the Model
imegohms.
1OOO:l.
tO.Ol%
1651
kl%
4 terminal shunt.
per "C;
allows current measurenlents to
This option is
191 DMM (90 IlOV, 50
Connect the alli-
eO.l%
per year.
10 t0
It is a
A fifty ampere
Use
ACCESSORIES
terminals.
DCV and 200 millivolt iange.
on DC 200mV.
4-9.
4-10.
leads 1.2~1 (48 inches) 1 ng, terminatedY p
with banana plug and spri,ng-action clip-on
probe.
4-11. MODEL 1682 RF
4.12. The Model 1682 ex
voltage response of the
lOOkliz to lOOMHz,
To Operate:
and 200 Volt range.
to the DMM INPUT termina
Specifications:
Voltage Range: 0.25 to
Transfer Accuracy:
peak responding calibr ted in rms of a
sinewave.
Input Impedance:
Maximum Allowable Input:1 30V pms AC. 2OOV UC.
Accessories Supplied: shraight tip. hook tip,
ground clip, hi adapt+, banana plug adapter.
4-13.
4-14.
long with 12 screw-in tips - 2 banana
plugs, 2 spade lugs. 2 alligator clips ,I\
with boots, 2 needle ti
and 4 heavy duty tip plugs.
4-15.
4-16.
vinyl case with a fitte
insert with room for the Servic
Manual and small access+ies.
4-17.
4.18.
clamping onto a single conductor.
the current path is unngessary.
detects current by sensilng magnetic field produced
by current.
To Operate: Set the OM
volt
Set the DMlj to ACV and 2V range or
MODEL
1681
CLIP-04 TEST LEA0 SET
The Model
NODEL 1683 UNIVERbAL TEST LEAD KIT.
Two test leads, 11.2m (48 inches)
Model 1684 Carrying Case
The Model 1684 is a hard
MODEL 1685 CLAMP-ON AC CURRENT PROBE.
The Model 1685 "edsure~ AC current by
range.
1681 CO tdins two
Set the DMM~to DCV
Connect t/he Model 1685 to the DMM
!
"SC NULL to zero
PRO
Con ect the Model 1682
5.
1
0 volts rms.
to.5 B, 1OOkHz to 1OoMtlZ
1
4 megop shunted by 3pF.
s with chucks
v
foam
1
Interruption of
The Model 1685
to ACV and 20
4
/I"
f! J
A
\
/
i
4-l
ACCESSORIES
MODEL
191
DMM
INPUT terminals. The DMM will display 0.1
volts per ampere.
Specifications:
Range: 2, 20
and 200
amperes r-m.
Accuracy: i-4% of range at
60Hz. ST% of range at 50Hz.
Temperature Coefficient:
200 ampere range.
range.
Maximum Allowable Current: 300 amperes rms.
Maximum Conductor Voltage: 600 volts rms.
Conversion Ratio: 0.1 volt rms per ampere.
4-19.
4-20.
with overall dimensions 5-I/4 inches (133mm) high
and
MODEL
1010
SINGLE RACK MOUNTING KIT.
The Model
19
inches (4l33mm) wide.
1010
?O.O5%/"C an the 20 and
*0.3%/Y on the 2 ampere
is a single rack mounting kit
4-25.
4-26. The Model 1901 allows your DMM to read dc
current from 1nAldigit to 2000mA. With the
Voltage Option it reads from lOnA/digit to 2000mA.
The Model 1901 plugs into the INPUT terminals of the
191.
(full scale input voltage burden) is 200mV. Shunt
resistors are connected so as to eliminate contact
resistance errors. Use the Model 191 200mV dc range
and 2V ac range, for dc current and ac cut-rent
respectively. Input voltage burden can be reduced
by selecting the lowest shunt that provides the
necessary resolution.
MODEL
1901
CURRENT ADAPTER.
1910
Maximum allowable continuous voltage drop
AC
4-21. MODEL 1017 DUAL RACK MOUNTING KIT.
4-22.
with overall dimensions 5-l/4 inches (133mm) high
and
4-23. MODEL 1641 KELVIN TEST LEAD SET.
q-24.
making 4-terminal measurements. The test leads
pair) are 1.2m (48 inches) long twin-lead cables.
Each cable is terminated by a twin-banana plug and a
spring-clip Kelvin contact. Plug twin banana plug
into DMM horizontally (HI to HI and LO to LO).
The Model 1017 is a single/dual mounting kit
19
inches (483mm) wide.
The Model
1641
test leads are for use in
(1
4-27.
4.28. The Model 1910 (not shown) is a factory or
field installable option which allows your DMM to
read ac volts from IOuV/digit to 1OOOV.
1910 is internally installed in the Model
is important to note that field installation or
removal/replacement of the Model
recalibration of ac voltage.
1910 are given in Table 1-l and ac voltage
measurements are described in Paragraph 2-27.
4-29. MODEL
4-30.
calibration cover and an Instruction/Service
Manual for the Model 191 DMM. The calibration
cover is installed in place of the normal 191
top cover during calibration.
to reach normal internal operating temperature
and has openings that are marked to facilitate
making the calibration adjustment.
MODEL
The
1913
1910
AC VOLTAGE OPTION.
1913
CALIBRATION COVER KIT.
(not shown) contains a
The Model
191. It
1910
requires
Specifications for the
It
allows the 191
4-2
MODEL 191 DMM
SECTION 5. THEORY OF OPERATION
GENERAL
5-l.
5-2. This section contains circuit descriptions for
the Model 191 DMM and the Model
option.
description of overall instrument operation, followed by descriptions of individual functional circuit blocks. To facilitate understanding, the descriptions arc keyed to accompanying simplified block
and schematic diagrams. Detailed schematics. of the
Model 191 and Model 1910 are provided in Section
1.
5-3.
5-4.
count, bench DMM with 5 dc voltage and 6 resistance
ranges standard. It has luV and Inn sensitivity, and
The information is arranged to provide d
OVERALL FUNCTIONAL DESCRIPTION
The Model 191 is a 5.112 digit, ~200,000
1910
AC Voltage
THEORY OF OPERATION
0.00052 resolution. Whe
tion is installed, ac
1000 volts can be "leas"
of the Model 191 is
converter which uses b"
slope conversion techni
control of the microcc
high conversion speeds
some of the Imajor bcnef
Other benefits and func
to the use of the micr(
the number of componer
filtering; pushbutton n
signal; and automatic 2
will bc described 1n
sectlo".
,.“, “.-..J
MODEL 1910
AC VOLTAGE
DCV ATTENUATOR/n
REF. RESISTORS
& RAFIGE SWITCHING q
ACV
O--I- n
ACV
ACV
f2
COE'VERTEI
VZERO
VSIG
A/D
VREF
57+
V/F
PULSES,
cnw.0111
PICRO-
COMPUTER
DISPLAY
FIGURE 5-l.
Simplified Signal Flow Block Diagram, Mod.
41
191
cl
DMM.
5-1
THEORY OF OPERATION
MODEL
191
DMM
(b) Timing.
loome‘
--I t-
Inteqratian_Ph_asej Vsig 1 Vzero I Vsig
Reading Updates
FIGURE 5-2.
Block Diagram of DC Voltage Measurements.
200msec
Charge-Balance
I Vref I Vsig I
in
vrero ----
Figure 5-1 provides a simplified signal flow
5-5.
block diagram of the Model 191.
tioned,
around the A/D converter, operating under the control of the nicrocomputer.
designed to handle input signals up to t2 Vdc, and
up to four separate signals (i.e., VLEKO, VSIG, Vn
and VREF).
the function switches control which of the siqnals
that can be applied to the A/D convcrtcr, as well
as the signal conditioning path of the input signal
applied to the input terminals. The microcomputer,
through the A/D Control lines, controls the sfquence and timing of signals applied to the A/D
converter.
second integration period for the charge balance
phase of conucrsion, up to 1 millisecond for single
slope conversion and the necessary delays to allow
an input signal to Settle and to perform nathema-
5-2
operation of the Model 191 is centered
It can be seen from the diagram that
Timin includes a precise 100 milli-
As previusly men-
The A/D converter is
tical calculations and housekeeping chores.
microcomputer also provides the automatic zero and
automatic calibration corrections to eliminate zero
and gain errors frown the signal to be displayed.
This is done mathematically and requires that more
than just VSIG be converted. Three signals are required for dc voltage mesurements, and four signals
for ohms and JC volts. Lath signal required for
the wzas~~rement is applied to the A/D converter and
the resulting digitized value is stored in ~memory.
The microcomputer muses the stored values to calcu-
late the reading and sends it to the display. In
this way, the microcamputcr corrects for zero and
gain errors, arId the displayed reading is the digitized value of the input signal within the specified accuracy of the
instrument.
The
MODEL
191
DMM : THEORY OF OPERATION
5-6.
DC Voltage Measurement.
5-7.
5-2, the input signal either goes directly to the
A/D or is connected across a decade attenuator with
a total resistance of 10 megohms. The attenuation
of the dc input signal is determined by the range
selected.
signals to the A/D converter arc rcquircd for dc
volts operation (i.e.,
signal is presented to the AID input and (measured
for 100 milliseconds (See A/D Converter discussion). Each digitized value is stored in memory and
then used to calculate a reading by the formula:
In dc volts operation, as shown in Figure
As previously mentioned, three input
"SIG, "ZERO and VREF). Each
(a) BLOCK DIAGRAM
-4oomv/-4V
Voltage
SO"rCe
r- --i
eference Resistor Decade
It can be seen that
tracted from both the
then the ratio is ti
needed because the r
milllivolts on the loti
5-R.
gram, VSIG is medsure
and VREF are alternati
This permits the disp
two integration (char!
ing that up to an addi
required (to complet
counting of the remail
tic.31 computations),
made approximately (
approximately 4 read'
As shown in th
the zero error is sub-
II and the reference, and
Multiplication by 2 is
nce is 2 volts (or 200
ic range).
ling portion of the dia'ry other time dnd VZtRO
in the other tine slots.
3 be updated after every
am?) phases. Consider11 70 milliseconds nay bc
Igle slope conversions.
in counters and mathcmaI display update cdn be
210 nilliseconds, or
;econd cd" be obtsined.
%
(b)
TIMING
9 IOOms /---
INTEGRATION PHASE 1 DELAY 1
READING UPDATE
FIGURE 5-3.
Block Diagram of Resistance Measureme:
VSIG 1 VLEKO DELAY
cl
'0 VKEt
_.
4
n%.
5-3
THEORY OF OPERATION
MODEL 191 DMM
5-9. Resistance Measurement.
5.10. In ohms operation, as shown in Figure 5-3,
the ohms voltage source is connected as an input to
the A,0 converter and to one end of the reference
resistor decade. The resistance reference resistors
are the same resistors that are use for dc volts
attenuation, but unlike dc volts, where only the
ratios affect accuracy, the absolute characteris-
tics of the resistors determine accuracy of the
ohms measurement. The value of the ohms reference
resistor (Itn) is determined by the range selected.
An ohms source voltage of -400 millivolts is used
on the 200~2 range, and -4V is used for all other n
ranges.
signals to the A/D converter are required. Each
signal is measured for 100 milliseconds and its
diyitirec value is stored in memory. The microcomputer then calculates a reading using the formula:
It can be seen that "$2 - VKEF is the voltage across
Kn (Ifi X rtn) and that "SIG VZEKO is the volt-
age acrossRX ('n X i(X). therefore:
For resistance measurements, four input
nD,SP = vslc - VZEKO
vn VREF
includes
added to
,"easurt3"ents,
RX and the effect of lead resistance can be
calculated as shown.
this discussion why the stated accuracy for the 200
n range requires that the effect of lead resistance
be cancelled with the NULL pushbutton for both
Z-terminal and &terminal measurements.
the rESlSta"Cf of RI
the unkown (RX). For &terminal
the n SENSE leads are connected to
It can be readily seen from
and 'I4
Thus, the ohms reading depends only an the value of
the ohms rcfcrfncf resistor ("n).
5-11.
diagram, there are two 100 millisecond delays and
four 100 millisecond integration p?riods needed to
gather the information for calculating a reading
with the above formula.
times, as described for DC Measurements, are considered it might appear that approxilnately 2
seconds would be necessary for three readings.
IloWeYfr, in actual operation, approximately three
valid readings per second can be obtained by calcu-
lating a new reading after each 300 milliseconds,
using the new data and the stored data from the
previous 300 milliseconds.
5-12. Up to this point in the discussion, the
Effect of lead
n~easurement has not been considered. As shown in
Figure
displayed ohms reading in both Z-terminal or 4terminal meas"rements.
resistances of the test leads have been designated
3s '31 @4. If n
terminals are not connected to Rx, the sensing
occurs at the HI
rfsistors
AS shown on the tilning portion of the
When the additional delay
resistance an the resistance
5-4,
lead resistance can
For this explanation,
SENSE HI
KS
LO INPUT terminals through
and
the displayed
affect the
and LO
reading
FIGURE 5-4. Affect of Lead Resistance in
Ohms Measurements.
5-4
MODEL
191
DMM
THEORY OF OPERATION
FIGURE 5-5. Block Diagram of AC Voltage Measure+nts.
5-13. AC Voltage Measurement.
5-14.
5-5, the Model
between the input and the A/D converter. The
converts the ac input voltage to a dc voltage between zero and -2 volts. On other than the 2V
range, the input signal is divided by 10,
1000 - depending on the range selected. For ac
voltage measurements, four input signals to the A/D
converter are required. Each signal is measured for
Where VSIG is the
line) is its dc offset, VREF is the 2V reference
and Vn is signal ground. The 2 is required because
of the 2V reference.
phase measurement, with the input signal measured
only once during the measurement, the maximum con-
version rate for ac volts is two valid readings/
second.
5-15.
5-16.
is given in Figure 5-6, and its waveform is shown
in Figure 5-7. In operation, the microcomputer
provides time division multiplexing of the input
signals by controlling switches S1 through 54.
The sequence and timing of the switches is dependent on the function selected (DC Volts, Ohms, or
AC volts).
input MUX switches to achieve the necessary high
off resistance, low leakage current and low thermal
In ac volts operation, as shown in Figure
1910
AC Voltage Option is placed
1910
100, or
100 milliseconds and its digitized value is stored
,n memory.
reading using the formula:
The microcolnputer then calculates a
V,, = L?VSIG - VZEKO).
(VKEF
1910
A/D Converter.
A simplified schematic of the A/D converter
Copper leaded JFETs are used for the
vn)
output, VZERO (AUTOZEKO
Since ac volts is d four
characteristics.
ing, high Z alnplificrlthat looks dt each input siqnal with either a xi or. x10 gain. A gain of xi0 IS
used on the 200mVDC~ and 2000 ranges, all otner
ranges
applied to the Tra sconductance Ainpiifier. lhis
amplifier provides t o
input voltage to d cufrent, which goes to the antegrater when requested, and provides a" offset CUTrent so that its hi alar
verted to unipolar ou
5-17.
operates first in a
then in a single sl pc
second interval was selected to look dt each input
as the best compramis,e to achieve good line rcjcction (50 and 60tlr) dbd relatively fast convex-soon
speed. A CB phase is ibegun when INPUT !II>AKLC goes
1OW.
period that allows thb signal to settle after tur-nlng on the appropriate input MUX switch. The delay
is software generat d and is dcpendcnt on the
function selected,
INPUT DISABLE is released, Iin IS connected to
the integrator, and
flops then act as a!comparator, providing timing
and control. After V
U106A. 91 goes high at the next positive going
clock edge.
wing)
the ikt$raraPd)rqs "'bb ""fs gre~~~:cct:ha/C"21:~
maXlGl""\,
negative.
d$is also low at
(h low. At the next pegative clock edge
I ater) ,
by 42 going low agaio. What has happened to this
point is that lC8 wbs turned on for one clock
“SE xl.
The A/II conve ter, as shown in iigurc 5-I.
This occurs al the completion of d dcldy
and th"
1~0 is turted off and D] is enabled
TheI input Uuffcr is a non-~nveit-
The output of the input buffer IS
functions. It converts the
1
input
voltdqes
put cur-rents.
barge balance (CU) phase. and
(8s) phase. A 100 mills-
/
5 given is Table 5-1. llben
,"
o ramps positive. The D flip
d
exceeds the " threshold of
At theI next clock edge (negative
P
13 v.
s time which sets and holds
thil
imediately rmp5
are con-
(1 cycle
5-5
THEORY OF OPERATION
MODEL 191 DMM
El--
l/O LINES FROM UC.
*
+v
~RATDR -
U106A
D,
CK
’ CL
50OkHZ
CLOCK
INPUT
QI
U106B
-02 Q2 CK -
QZ--
r’-
cl
1
V-F
&iSE
OUTPUT
SINGLE-SLOPE
FIGURE 5-6.
cycle (2 microseconds) and then turned off. The
earliest it can be turned on again is one clock
cycle later.
counter is incremented by an inverted V-F PULSE
from 92. It cdn be seen that the flip flops
divide the clock frequency by two, limiting the
maximum number of charge balance integrations and
output co"nts to one half of the clock frequency.
And, since 50,000 clock cycles occur in the precise
100
millisecond charge balance period, the maximum
number of times that (12 can go high and be
counted is 25,000.
5-18. At the end of the charge balance phase, the
output of the integrator is resting at some posi-
tive voltage. The single-slope comparator output is
also positive and it will not switch until the integrator output crosses zero. The comparator output
is ANDed with a one millisecond pulse in the digital section to produce SINGLE SLOPE ENABLE.
allows ISS to flaw into the integrator. A 1MHz
clock is counted from the time SINGLE SLOPE ENABLE
went high until the single-slope comparator changes
state (vo crosses zero). When this occurs,
Each time 1,"~ is turned on, a
Simplified A/D Schematic.
This
ISS is shut off and the counting is stopped.
The amount of charge delivered by ISS in one
microsecond (IMHz period) is equal to I/256 of the
charge delivered by ICB in two microseconds.
The microcomputer multiplies the CL3 counts by 256
and adds the SS counts. to it to obtain the corn-
posite count ((6.4 million maximum).
TABLE 5-I
Settling Delays, SC") on to
Turn On Of Integrator.
JFET Delays (msecs)
SWITCH DC"
81
30
52 1
53 1
s4 x
AC" n
30 100
1 1
1 100
1 1
5-6
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