Choosing Differential or Single-Ended Measurements for Data Acquisition Systems

In a sampled-data system (see Figure 1), the individual signals are sampled at a high speed by a switching network called a multiplexer. When you provide a single-ended or differential signal, each input channel has a SIG-HI and a SIG-LO terminal. You'll also have one or more analog common terminals. In most data systems, you'll have to choose either single-ended or differential inputs for the entire group of channels sampled rather than specify the type of input on channel at a time. Often, you will have twice as many single-ended channels as differential channels, which may encourage you to make a choice that may not be the most optimal for your application.

Although the multiplexer is an important element in any sampled data system, the concepts of single-ended and differential measurements are more easily understood if you assume the multiplexer is frozen on one channel and you ignore it. The difference between the two input choices is whether you use the SIG-LO terminal. For a single-ended input, the SIG-LO terminal is not used; you connect the signal return to the analog common. For a differential input, the SIG-LO terminal connects to the low side of the input amplifier. The input amplifier is generally an instrumentation amplifier, which is inherently differential.

A differential voltage , as it applies to an instrumentation amplifier, is the difference between the voltages of the two inputs (see figure 2). The portion of the two input voltages that the voltages share, or have in common, is called the common-mode voltage. The differential amplifier rejects the common-mode voltage, and the remaining voltage is amplified and presented on the amplifier output as a single ended voltage. The rejected voltage can be AC or DC. The maximum common-mode voltage is typically 10 V. The effectiveness of the rejection diminishes as the frequency of the common-mode voltage increases.

An instrumentation amplifier (see Figure 3) typically consists of three amplifiers and a well-matched resistor network combining the features of high input impedance and low output impedance. The two inputs are noninverting buffers that have matched gains of unity or higher followed by the actual difference amplifier that has relatively low input impedance.

One of the most common mistakes made when setting up a differential measurement is failing to understand and establish an input bias current path. For example, a user working with differential inputs may connect a 1.5 V battery to the two inputs of a differential channel and obtain an invalid reading. Adding a resistor of almost any value between either input and the analog common will generally correct the problem.

The need for a bias path on a differential input is sometimes taken care of internally by a resistor to the analog common from each side of the differential input. The resistor can be either permanent or switched (allowing you to decide whether you can use it or not). Permanent resistors can have values as high as 10 M; switched resistors can have values as low as 10 K. If the input impedance of a differential input is higher than 10 M, you have to provide an external bias current path to the analog common for any floating signal source, which is one without another DC path to the analog common, such as the previously mentioned 1.5 V battery or a thermocouple.

Consider the unity gain buffer configuration of an operational amplifier (see Figure 4), with its input and output voltages referenced to the analog common. The invisible or seldom shown operational amplifier bias rails (V+ and V-) are also connected to analog common. When you connect a signal source between the analog common and the noninverting input, a small but vital input bias current flows through the signal source. The typical instrumentation amplifier front end consists of two noninverting buffers, each of which delivers a voltage referenced to the analog common. The difference amplifier provides an output that is proportional to the difference between the two buffered voltages and is referenced to the pin of the instrumentation amplifier.

The most widely used differential input sources are Wheatstone bridge circuits, which you would frequently use with strain gauges, thermistors, and other resistive sensors. If you connect the bridge excitation source to the analog common or the instrumentation amplifier supply rails, you will establish the necessary input bias current path. You can always make a differential input single-ended by connecting the SIG-LO input to the common, but the measurement accuracy may be reduced by an inadvertently created common-mode voltage.

When you have to choose between the two types of inputs, if an equal number of channels of both types of signals is available, consider the source(s). If you have multiple voltages with a readily shared common point, single-ended inputs are often adequate. If you lack a readily shared common point, or if low level signals are involved, differential measurements are preferable.

A typical single-ended application is measuring the outputs of a multiple output power supply with a single common (see Figure 5). On the other hand, a typical differential application involves measuring voltages across a number of high current shunts with a common bus on one end (see Figure 6). If multiple signal sources have measurable voltage differences between their return lines, you should choose differential connections to acquire accurate data. Differences between common lines add to desired measurement and create errors.