Measurement Computing USB-1408FS User Manual
Page 13
USB-1408FS User's Guide
Functional Details
13
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 6).
Figure 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 −3V
to
−11V. Voltages between −10 V and −3 V are resolved, but those below -10 V are clipped (see Figure 7).
Figure 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
±20V 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. The following table shows some possible inputs and the expected results.
Sample inputs and differential results
CHHI
CHLO
Result
−20 V
0 V
Invalid
−15 V
+5 V
Invalid
−10 V
0 V
−10 V
−10 V
+10 V
−20 V
0 V
+10 V
−10 V
0 V
+20 V
−20 V
+10 V
−10 V
+20 V
+10 V
0 V
+10 V
+15 V
−5 V
+20 V
+20 V
0
+20 V
For more information on analog signal connections
For more information on single-ended and differential inputs, refer to the Guide to Signal Connections (this
document is available on our web site a