Measuring Circuits That Are Not Powered By The Main Output - Agilent Technologies 66111A User Manual

3 - Installation
NOTE:
The DVM is not designed to measure voltages that are greater than +25 Vdc or less than
−4.5 Vdc with respect to the negative terminal of the main output. The following sections
discuss restrictions that apply when using the DVM to measure voltages on circuits that are
not powered by the main output, or that are floating with respect to the main output.

Measuring Circuits that are Not Powered by the Main Output

To obtain correct voltage measurements, keep the common mode voltage within the specified limits.
Common mode voltage is defined as the voltage between either DVM input terminal and the negative
terminal of the main output (output 1). The common mode voltage range is from −4.5 Vdc to +25 Vdc.
Attempting to measure voltages outside this range may result in incorrect readings due to clipping by the
internal DVM measurement circuits.
NOTE:
Do not confuse the common mode voltage with the DVM voltage readback. The DVM
voltage readback is a differential measurement from one input lead to the other input lead.
This quantity may be as high as ±25 Vdc, depending on the orientation of the input leads.
Because the measurement circuits of the DVM are internally referenced to the minus terminal of the
main output, you must observe the following restrictions in order to guarantee accurate DVM
measurements (refer to figure 3-8).
Agilent 66309D
Agilent 66311D
DVM INPUT
OUTPUT
Minus
terminal
Figure 3-8. Measuring Circuits Not Powered by the Main Output
40
Test Fixture (for illustration only)
lead resistance
+ load
current
LOAD
V
− −
+
lead resistance
Node # V Common Mode
1
R1
DVM
12V
2
R2
36V
DVM
12V
DC
3
R3
12V
4
R4
2V
5
R5
6V
DVM
2V
DC
6
R6
DVM
2V
7
1 36 V + V
2 24 V + V
3 12 V + V
4 V
5 - 2 V + V
6 - 4 V + V
7 - 6 V + V
NOTE: The DVM common
mode voltage range is from
-4.5Vdc to +25Vdc.
voltages outside this range will
result in erroneous readings.