National Instruments Data Acquisition Device NI USB-621x User Manual

Page 62

Chapter 4

Analog Input

4-29

Figure 4-18. Differential Connections for Floating Signal Sources

with Balanced Bias Resistors

Both inputs of the NI-PGIA require a DC path to ground in order for the
NI-PGIA to work. If the source is AC coupled (capacitively coupled), the
NI-PGIA needs a resistor between the positive input and AI GND. If the
source has low-impedance, choose a resistor that is large enough not to
significantly load the source but small enough not to produce significant
input offset voltage as a result of input bias current (typically 100 k

Ω to

1 M

Ω). In this case, connect the negative input directly to AI GND. If the

source has high output impedance, balance the signal path as previously
described using the same value resistor on both the positive and negative
inputs; be aware that there is some gain error from loading down the source,
as shown in Figure 4-19.

USB-6210/6211/6212 Device Configured in DIFF Mode

PGIA

–

Floating

Signal

Source

Bias

Current

Return

Paths

AI GND

AI SENSE

Input Multiplexers

Measured

Voltage

Instrumentation

Amplifier

AI+

AI–

I/O Connector

Bias
Resistors
(see text)