Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

 

 

AI+

 

 

 

 

 

Bias

 

 

 

 

 

Resistors

 

 

 

 

 

(see text)

 

 

 

Floating

 

+

 

 

 

Vs

 

Instrumentation

 

Signal

 

 

+

Amplifier

 

 

Source

 

 

 

 

 

–

 

 

 

 

 

 

 

 

 

 

 

PGIA

+

 

 

 

AI–

 

 

 

 

 

 

Measured

 

 

–

Vm

 

 

 

 

 

Voltage

 

 

 

 

–

 

 

 

 

 

 

Bias

 

 

 

 

 

Current

 

 

 

 

 

Return

 

 

 

 

 

Paths

 

 

 

 

 

 

Input Multiplexers

 

 

 

 

 

AI SENSE

 

 

 

AI GND

I/O Connector

M Series Device Configured in DIFF Mode

Figure 5-3.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 kΩ to

1 MΩ). 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

© National Instruments Corporation

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NI USB-621x User Manual