Measurement Computing miniLAB-1008 - Manual (Page 19)

miniLAB 1008 User's Guide Functional Details
The input [common-mode voltage + signal] of the differential channel must be in the −10 V to +20 V range in
order to yield a useful result.
For example, you input a 4 volt peak-to-peak (Vpp) sine wave to CHHI, and apply the same sine wave 180° out
of phase to CHLO. The common mode voltage is 0 V. The differential input voltage swings from 4 V-(-4 V) =
8 V to -4 V-4 V = -8 V. Both inputs satisfy the -10 V to +20 V input range requirement, and the differential
voltage is suited for the ±10 V input range (see Figure 3-5).
+4V
-4V
+4V
-4V
0V
CHLO
CHHI
+/-8V 8V Differential
Measured Signal
Figure 3-5. Differential voltage example: common mode voltage of 0 V
If you increase the common mode voltage to 11 V, the differential remains at ±8 V. Although the [common-
mode voltage + signal] on each input now has a range of +7 V to +15 V, both inputs still satisfy the -10 V to
+20 V input requirement (see Figure 3-6).
+11V
+7V
+11V
+/-8V
+15V
8V Differential
CHLO
CHHI
Measured Signal
Figure 3-6. Differential voltage example: common mode voltage of 11 V
If you decrease the common-mode voltage to -7 V, the differential stays at ±8 V. However, the solution now
violates the input range condition of -10 V to +20 V. The voltage on each analog input now swings from -3 V to
-11 V. Voltages between -10 V and -3 V are resolved, but those below -10 V are clipped (see Figure 3-7).
-3V
-11V
-7V
+/-7V 8V Differential
-11V
-7V
-3V
3V
CHLO
CHHI
Measured Signal
Figure 3-7. Differential voltage example: common mode voltage of -7 V
Since the analog inputs are restricted to a −10 V to +20 V signal swing with respect to ground, all ranges except
±20 V can realize a linear output for any differential signal with zero common mode voltage and full scale
signal inputs. The ±20 V range is the exception. You cannot put −20 V on CHHI, and 0 V on CHLO, since this
violates the input range criteria. Table 3-3 shows some possible inputs and the expected results.
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