National Instruments NI 9234

Figure 4. NI9234 Input Circuitry for One Channel

The NI9234 uses common-mode bias current to bias the

current-limiting diodes when IEPE current is turned off. When the

NI9234 is using grounded signal sources, this current causes an

error that is dependent on the AI– lead impedance. This error is

approximately 50 ppm of range and 15 ppm of reading per Ω of

AI– impedance. The common-mode bias current causes an error

only with grounded sources and is not an issue with floating signal

sources. For best accuracy, use a floating connection or use

low-impedance leads when connecting grounded signal sources.

2 mA IEPE on/off

AI+

AI–

Amplifier

and

Prefilter

ADC

–

AC/DC Coupling

NI 9234

Common

Mode

Bias

Current

Limiting

Diodes

50 Ω