3 - Installation

40

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).

A
g
ilent 66309D
A
g
ilent 66311D
OUTPUT
+
−
36V
DC
6V
DC
DVM INPUT
DVM
DVM
DVM
DVM
R1
12V
lead resistance
LOAD
lead resistance
R2
12V
R5
2V
R6
2V
Test Fixture
(
for illustration onl
y)
+
−
load
current
R3
12V
R4
2V
V
1
2
3
4
5
6
7
Node # V Common Mode
1
2
3
4
5
6
7
36 V + V
24 V + V
12 V + V
V
- 2 V + V
- 4 V + V
- 6 V + V
Minus
terminal
NOTE: The DVM common
mode voltage range is from
-4.5Vdc to +25Vdc.
voltages outside this range will
result in erroneous readings.

Figure 3-8. Measuring Circuits Not Powered by the Main Output