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Operation
2-9
The supplied Model 237-ALG-2 cable is designed to mate with
the INPUT connector. The other end of the Model 237-ALG-2
is terminated with three alligator clips. Input high is color coded
in red, input low is colored black, and chassis ground is color
coded in green. Keep in mind that these connections are for the
unguarded mode. In the guarded mode, red is high, black is
guard, and green is chassis ground. The COM binding post pro-
vides a connection to input low through 100
Ω
for use in the
guarded mode.
NOTE
It is recommended that zero check be en-
abled when connecting or disconnecting
input signals.
WARNING
The maximum common-mode input volt-
age (the voltage between input low and
chassis ground) is 500V peak. Exceeding
this value may create a shock hazard.
CAUTION
Connecting PREAMP OUT, COM, or 2V
ANALOG OUTPUT to earth while float-
ing the input may damage the instrument.
CAUTION
The maximum voltage between input
high and input low is 250V RMS, DC to
60Hz sine wave (10 seconds maximum
in mA ranges). Exceeding this value
may cause damage to the instrument.
2.7.3 Making voltage measurements
The Model 6512 can be used to measure voltages in the
range of ±10µV to ±200V. In principle, the instrument oper-
ates much like an ordinary DMM, but its special characteris-
tics allow it to make measurements in cases where an
ordinary DMM would be unable to perform well. In particu-
lar, the very high input resistance of 200T
Ω
(2
×
10
14
Ω
) al-
lows it to accurately measure voltage sources with high
internal resistances. In contrast, an ordinary DMM may have
an input resistance of only 10M
Ω
, resulting in inaccurate
measurements because of instrument loading.
Use the following procedure to make voltage measurements:
1. Turn on instrument power, and allow the unit to warm up
for two hours to reach rated accuracy.
2. Check to see that the voltage function is selected by
pressing the VOLTS button. Use the auto-range mode,
or select the desired range with the ranging pushbuttons.
3. To achieve specified accuracy, especially on the lower
ranges, it is recommended that you zero the instrument.
To do so, first enable zero check, and then press the
ZERO CORRECT button. Correcting zero on the lowest
range of any function will correct all ranges because of
internal scaling.
NOTE
The input circuit configuration changes
with zero check enabled. See paragraph
2.10.1 for details.
4. Connect the supplied Model 237-ALG-2 triaxial input
cable or other suitable triax cable to the rear panel IN-
PUT jack. For sources with high output resistance, the
cable should be kept as short as possible to minimize ca-
ble capacitance.
5. If response time and leakage resistance are consider-
ations, place the instrument in the guarded mode as de-
scribed in paragraph 2.7.4.
6. Connect the other end of the cable to the voltage to be
measured, as shown in Figure 2-4. Disable zero check.
INPUT HI
INPUT LO
CHASSIS
GROUND
INPUT HI
GUARD
CHASSIS
GROUND
A. UNGUARDED
(V,
Ω
GUARD OFF)
B. GUARDED
(V,
Ω
GUARD ON)
100
Ω
COM
Figure 2-3
Input connector configuration
Summary of Contents for 6512
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