How To Measure Resistance - Keithley 160B Instruction Manual

OPERATING INSTR"CTIONS
MODEL L6OB
IMPORTANT
The Model 1608 provides ac rejection (NMRR)
of greater than 60 dB on the highesf range.
However, a large ac signal superimposed on
a dc level could cause damage if the input
CO the Model 160" exceeds 1200 volts dc +
peak UC.
d. How to Zero the Display. To accurately zero
the instrument, place a low-thermal short (such as a
piece Of clean copper wire) across the input rermin-
als, set Lhe Range Switch to the lm" range, and ad-
just the front panel Zero conerol for a flashing "i"
sign at the display.
The display should then read
t.OOO, with some flicker of the least significant
digit due to noise and A-D converter uncertainfy.
Once the zero has been set accurately, there should
be little reason to rezero the instrumenf again soon
when making measurements on the 10~1" through 1000"
ranges. The time stability of rhe zero setring is
excellent, and in applications requiring continuous
operation of the Model 160B, the zero setting will
hold for months. The front panel Zero control is
convenienf when making measurements on fhe 11"" Range.
A test set-up and cables when connected to the 160B
may cause offset voltages because of thermoelectric
effects (see section 3-9). These offset voltages
might be only a few microvolts or several tens of
microvolts. In such situations, the Zero control
can be used to buck out any initial offsefs. I.""
thermal cabling and connections should be used when-
ever possible. The instrument zero can also be check
ed, and adjusted, by shorring the input on any range
(voltage, current, or resistance).
me zero c**t*01
will have varying effects on the differenr ranges
depending on the sensitivity of the input amplifier,
but the zero control can be used in this manner.
Again, it should be emphasized, that for most meas-
"remene~ with the Model 160B, the Zero control can
be set once and then left untouched. (See also ?ec-
tion 3-6c "LO" Ohms Zeroing").
NOTE
With the input open 0" the lm" Range, the
Model 160~3 display may show a reading as
high as i.100. This 19 equ*va1ent to an
offset currene of lo.opA. If the display
shows a reading grearer than t.100, then
the internal potentiometer, R121, should
be readjusted. Typically, ehe offset cur-
rent will be less than 5p~.
e. 80" to "se Model 1601 AC/DC Probe.
TO make ac
measuremenfs, connect the Model 1601 accessory K/DC
Probe to the Model 1608 inpur terminals.
Set the
Range to desired voltage range.
Set switch on probe
to AC.
"se the probe tip plus ground clip to make
connections to circuit under eesc.
see section 5 for
complete specifications.
1-6. HOW TO MEASURE RESISTANCE. The Model 1608
measures resistance in 10 ranges: lsr, 1on, 10011,
Ikn, IOk&?, lOOk0, IM,,, lOMn, lOOK,, and 100011!2.
a. HO" to Select Range.
Function and range is
selected by rotafinf: the Ranae Swifch LO rhe BPPTO-
priate position. Three posi;ions are direct-reading
in ohms (n): three positions are direct-reading in
kilohms (kn); four positions are direct-reading in
megohms (MQ) Decimal point is selected by Range
Switch. If the input signal exceeds twice full range,
the display blanks to indicate an "overra"& condl-
ti**.
TABLE 3-3.
Resistance current Ranges
Range Max. Impressed
setting LIiF3play units current
1. n 1.999 ii I InA
10 n 19.99 n lti
1 MI1 1.999 Mn 100 "A
10 Mn 19.99 MO 10 "A
100 Mr! 199.9 Mn I"'4
1000 M* 1999 MO 0.1 "A
b. How to Determine Accuracy. The accuracy of cbe
Model 1608 is specified in terms of a nercen~ of
reading and a p&cent of range. For example, a dis-
play of 1.000 kilohID (kn.1 will haw an uncertainty
of ~0.004kQ.
C. HO" to Measure "LOW" ohms. when making meas-
urements on the ln, lOn, and 1OOn ranges, rhe Model
160B is to be zeroed on the IQ range. The zeroing
is accomplished by shorting the test leads together.
and adjusting the display for a flashing + and -
.ooon. By zeroing the instrument in this manner,
lead resistance of 200 mill.iohms (minimum) can be
compensated for. The instrument must be rezeroed on
the lil range each time a different set of iese leads
is used. Before making meaS"reme"tS right *t the in-
put rerminals (no test leads used), rhe instrumene
should be zeroed on the iii Range after placing a
short, such as a piece of copper wire, across the
binding posts. Copper leads and clips are best for
making measurements on the ln Range, because the
voltage drop across a In resistor is only Im".
CAUTION
Care should be taken when making resistance
measurements in circuits which may have volt-
ages on capacitors, etc. or "here line volt-
age is present. Although the Model 1608 is
fully protected against accidental voltages
up to 250" rms in resistance function, if
higher voltages are applied, damage may occur.
f.
How to "se Model 1682 RF Probe.
To make rf ac
measurements, connect the Model 1682 Probe to the
Model 160B input terminals.
Set the Range to desired
voltage range. Maximum allowable input is 30" *Ins
ac, 200" dc.
10
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