UMDAS 0802DA User's Guide

Functional Details

In differential mode, the following two requirements must be met for linear operation:

!Any analog input must remain in the −10V to +20V range with respect to ground at all times.

!The maximum differential voltage on any given analog input pair must remain within the selected voltage range.

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 V pp 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 = −8V. 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

CHHI 0V

Measured Signal

-4V

8V Differential

+/-8V

+4V

CHLO

-4V

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).

+15V

Measured Signal

CHHI +11V

8V Differential

+/-8V

 

CHLO +11V

+7V

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 −3V to −11V. Voltages between −10 V and −3 V are resolved, but those below -10 V are clipped (see Figure 3-7).

-3V

 

Measured Signal

 

CHHI -7V

 

 

3V

 

-11V

 

 

8V Differential

+/-7V

-3V CHLO -7V

-11V

Figure 3-7. Differential voltage example: common mode voltage of -7 V

3-5