Measurement-computing USB-1208LS Uživatelský manuál stáhnout pdf (Strana 12)

USB-1208LS 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 5).

Figure 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 6).

Figure 6. Differential voltage example: common mode voltage of 11V

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

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Measurement-computing USB-1208LS Uživatelský manuál Strana 12