Measurement Computing USB-1208FS User Manual page 16

USB-1208FS User's Guide
Battery voltage should read as expected when the hardware is configured for single-ended mode.
Single-ended measurements using differential channels
To perform a single-ended measurement using differential channels, connect the signal to "CHn IN HI" input,
and ground the associated "CHn IN LO" input.
Differential configuration
When all of the analog input channels are configured for differential input mode, four analog channels are
available. In differential mode, the input signal is measured with respect to the low input.
The input signal is delivered through three wires:

The wire carrying the signal to be measured connects to CH0 IN HI, CH1 IN HI, CH2 IN HI, or
CH3 IN HI.

The wire carrying the reference signal connects to CH0 IN LO, CH1 IN LO, CH2 IN LO, or
CH3 IN LO.

The third wire connects to GND.
A low-noise precision programmable gain amplifier (PGA) is available on differential channels to provide gains
of up to 20 and a dynamic range of up to 12-bits. Differential mode input voltage ranges are ±20 V, ±10 V,
±5 V, ±4 V, ±2.5 V, ±2.0 V, ±1.25 V, and ±1.0 V.
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 6).
Figure 5. Single-ended measurement connection
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Functional Details