Pseudodifferential versus Differential Input Configurations

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Overview

National Instruments devices in the Dynamic Signal Acquisition (DSA) product line (such as the 446x, 447x, and 449x families) and in the Simultaneous Sampling (S-series) DAQ product line (including the 6110, 6111, 6115, and 6120) have pseudodifferential analog input configurations. Some of these devices also support differential input configurations. This tutorial presents a general summary of these input types and how to select one.

Table of Contents

1. 
   
2. Understanding Common-Mode Voltage
   
3. Differential Inputs
   
4. Pseudodifferential Inputs
   
5. Choosing Channel Configurations
   
6. Measurement Device Specific Information
   
7. Summary
   
8. Related Links

Understanding Common-Mode Voltage

A discussion of input types must begin with an understanding of common-mode voltages. Common-mode voltage refers to the voltage common to both sides of a differential (or pseudodifferential) circuit pair. The differential voltage across the circuit pair is the desired signal, whereas the common voltage signal is the unwanted signal that may have been coupled into the transmission line. For example, a DC common-mode voltage can be thought of as a DC offset on the device input. Typical sources of common-mode voltage noise are 50/60 Hz signals from power lines, power supply ripple, EMF, and high-frequency switching. If the input lines are perfectly balanced, the common-mode voltage cancels out. The degree of cancellation is called the common-mode rejection ratio, or CMRR.

CMRR or common-mode rejection ratio is a measure of the ability of an instrument to reject interference from a common-mode signal, usually expressed in decibels (dB). Essentially, CMRR describes the ability of a differential input to reject noise common to both inputs. The higher the CMRR, the better the circuitry can extract differential signals in the presence of common-mode noise.

Differential Inputs

Both differential and pseudodifferential inputs provide common-mode voltage rejection while single-ended inputs do not. However, differential inputs provide both AC and DC common-mode rejection and pseudo-differential inputs provide only DC common-mode voltage rejection. For sensors requiring excitation, including microphones and accelerometers powered with IEPE, pseudo-differential inputs must be used to zero out the bias voltages and allow the sensor ground to be different than the measurement device ground. A diagram showing the differential input configuration of the NI 446x is shown below with a ground-referenced sensor.

In theory, a differential input should be able to reject all DC common-mode voltages. However, most measurement devices specify a common-mode range. This is the input range over which a circuit can handle a common-mode signal. The common-mode signal is measured as the mathematical average voltage, relative to the measurement device's ground, of the signals from a differential input. If this range is exceeded, users may damage the device. Pseudodifferential inputs are also subject to a common-mode range.

Pseudodifferential Inputs

A pseudodifferential input is much like a differential input in that it provides common-mode voltage rejection (unlike single-ended inputs). However, pseudodifferential inputs are all referred, but not directly tied, to a common ground. This connection is typically made by a relatively low value resistor to give some isolation between the sensor/signal ground and the measurement device ground. DSA measurement devices have a 50 ohm resistance between the negative signal connector, often the outer shell of a BNC, and the device ground. A diagram showing the pseudodifferential input configuration of the NI 446x is shown below with a floating sensor.

When using a pseudodifferential input, the negative input should not vary much with time. The negative input should be a reference only. Another way of describing pseudodifferential inputs is that the inputs are differential only in the sense that ground loops and DC common-mode voltage are broken. The reference signal, (negative input), is not intended to carry signals of interest but only to provide a DC reference point for the signal (positive input). Pseudodifferential inputs minimize the effects of ground potential differences (ground loops) between the signal source and the device, which leads to more accurate measurements.

Choosing Channel Configurations

Configure the channels based on how the signal source or device under test (DUT) is referenced. If the signal source is floating, use the pseudodifferential channel configuration. A floating signal source does not connect to the building ground system. Instead, the signal source has an isolated ground-reference point. Some examples of floating signal sources are outputs of transformers without grounded center taps, battery-powered devices, non-grounded accelerometers, and most instrumentation microphones. An instrument or device that has an isolated output is considered a floating signal source. It is important to provide a ground reference for a floating signal. If no ground-reference point is provided—for example, selecting differential mode with a floating microphone—the microphone outputs can drift outside of the device common-mode range.

If the signal source is ground-referenced, use either the differential or pseudodifferential channel configurations. A ground-referenced signal source connects in some way to the building ground. Therefore, it is already connected to a ground-reference point with respect to the measurement device, assuming the PXI or CompactPCI chassis and controller are plugged into the same power system. Non-isolated outputs of instruments and devices that plug into the building power system fall into this category.

Provide only one ground-reference point for each channel by properly selecting differential or pseudodifferential configuration. If you provide two ground-reference points—for example, if you select pseudodifferential mode with a grounded accelerometer—the difference in ground potential results in currents in the ground system that can cause measurement errors. The 50 ohm resistor on the signal ground is usually sufficient to reduce this current to negligible levels, but results can vary depending on the system setup. The table below summarizes the configuration for various types of signal sources.

Measurement Device Specific Information

NI 446x Devices

The NI 446x supports two terminal configurations for analog input: differential and pseudodifferential. You can configure the NI 446x input channels on a per channel basis. Therefore, you can have one channel configured for differential mode and the other channel configured for pseudodifferential mode. The NI 446x is automatically configured for differential mode when powered on or when power is removed from the device. This configuration protects the 50 ohm resistor on the signal ground.

NI 447x and 449x Devices

The NI 447x and NI 449x support only the pseudodifferential channel configuration.

Summary

Interfacing a sensor to a measurement device requires matching the sensor or signal conditioning output with the input configuration. Floating sensors, such as microphones and accelerometers, should be connected using a pseudodifferential configuration. A differential input configuration can be used in these cases but users must provide a separate connection to ground to prevent drift beyond the common-mode range of the measurement device. Ground referenced sensors can be used with either pseudodifferential or differential inputs, but differential inputs are preferred because they provide additional common-mode rejection.