Single-ended and differential voltage measurement: choosing which method to apply — Part 2

The difference between single-ended and differential voltage signal measurement is a subject that is not always fully understood. The focus of this article is to try to make the difference clearer and provide tips on achieving the best results.

Voltage is a difference in electric potential between two points and is a measure of the force for current flow in a conductor or circuit. You can choose to measure voltage single-ended, or you can measure voltage differentially. Part 1 of this article reviewed single-ended voltage measurement.

Differential input voltage measurement

Like the single-ended input discussed in Part 1, a differential input also measures the voltage difference between two points, but in this instance, one of the points is not necessarily a fixed common reference point or return. The key differentiator here is that both leads connect to variable potentials and a third connection breaks out the circuit common. That is, a differential input will actually have at least three input connections: signal high or positive, signal low or negative, and a third connection for the signal return or common (return is usually shared by other differential input channels of the same circuit and its connection may be optionally made internal to the circuit if not provided on a third terminal).

The differential input offers more freedom when connecting to earth grounded or non-grounded (isolated) signal sources than the single-ended input, because its signal connections may be offset and separate from signal return, which may or may not also connect to earth ground. However, an earth ground path to return must still be made, and you should be careful not to provide more than one earth ground connection to return to avoid creating a ground loop circuit that could offset or interfere with your measurement.

Figure 1: Simplified differential input connections. For a larger image click here.

In Figure 1a, each channel pair is differentially multiplexed to an instrumentation amplifier. A third lead carries the input circuit common or return, which may or may not also be connected to earth ground. Figure 1b shows a differential output connected to a differential input with the third leads (the I/O return leads) connected together. A connection to earth ground at return will help to keep the signals from floating and is also needed for protection purposes, but making two connections to earth ground should be avoided — as described in Part 1, ground loops can have detrimental effects on your measurement. Unfortunately, for most applications, outputs will not connect to differential inputs this simply and Figure 1b is perhaps an ideal case that rarely happens in industry. That is, differential inputs usually connect to single-ended sources having only two signal leads with no separate return lead as shown in Figure 2.

Differential input myths

You may have read something like: a differential voltage is floating, in that it has no reference to ground. This definition is partially correct, but misleading. The term ‘floating’ should not be used to describe differential input measurement. The differential potentials must have an indirect or direct reference to the input measurement return lead. This is because no differential input potential can truly float, nor is it good practice to allow an instrument’s inputs to float. In fact, no meaningful measurement can even be made if either of the differential input potentials is floating.

Another common misconception regarding multiple differential input pairs is that they are somehow isolated, because they can be offset from one another. Differential inputs are not isolated — instead this refers to the degree to which they may be offset from each other and from the circuit return. As already stated, multiple differential inputs of the same circuit actually share a common signal reference point in a required extra connection (usually return) and none of the differential input pairs can be converted without a direct or indirect reference to that point.

Figure 2: Differential inputs most often connect to non-grounded or grounded single-ended sources. For a larger image click here.

In other words, do not leave the third connection (return) floating: the I/O signals must be referenced to it as shown in Figure 2.

Making that third connection

Differential outputs with three wires will wire directly to differential inputs with three wires without confusion. But problems arise when the third return connection is not made. When a third connection cannot be easily identified, it is likely that the circuit is making an indirect connection to its return for you. Be aware that a sensor or instrument with a differential output (three wires) can be wired single-ended by connecting its signal low lead (OUT-) to the signal common reference or return. And a differential input that connects its input low (IN-) signal directly to its common reference or return is essentially wired for single-ended input. For some of our differential instruments, the following instruction may be present in the manual:

IMPORTANT: If your input source is not already grounded, connect IN- to return and connect input return to earth ground.

This only addresses what can be done to connect a floating or isolated single-ended output signal to a single differential input and will actually make the differential input single-ended by connecting IN- to signal return. Applied to all the differential inputs, it prevents the separate input channels from measuring offset from return and each other.

Unfortunately in practice, the output signal you wish to measure differentially is often not differential at all and may only have two leads (the differential input is instead connected to a single-ended output that may be floating). Connecting differential IN- to return yields a meaningful measurement, and additionally connecting return or common to earth ground ensures the signal will never float and is needed for protection purposes (connecting it to earth ground is not required to convert the signal).

To avoid confusion when you try to discern a differential from a single-ended input by looking for the third connection to return, please note that some differential products will not include a third wire connection to differential input return. Rather its connection to the differential input is hidden from the user and an indirect reference to return is made internally. For example, some differential circuits may separately multiplex the ±channel inputs to a differential A/D converter while simultaneously measuring a CJC reference (for thermal measurement) or a voltage reference with respect to return to get a stable measurement. These circuits will usually include small input bias leakage paths to return at each input lead via filtering that helps keep the inputs from being free-floating when not connected to the A/D converter — acting similar to using weak pull-downs to return as illustrated in Figure 2a. Because the input pairs connect to the A/D converter separately, they may still be offset from each other.

Common-mode rejection

Noise in any circuit will generally permeate the circuit entirely and appear on both I/O leads of differential inputs making it common mode. Even an earth grounded sensor can induce noise on both differential leads from a less than ideal connection to earth ground.

A good instrumentation amplifier is designed to amplify the differential normal-mode signal and reject the common-mode signal (the noise) common to both inputs. The common-mode rejection ratio (CMRR) is a measure of the instrumentation amplifier’s ability to reject common-mode noise (look for 100 dB or better). Think of CMRR as equivalent to the multiple of gain applied to the differential signal relative to gain applied to the common-mode noise A CMRR of 100 dB is equivalent to 100,000:1 signal-to-noise ratio. This means that the common-mode noise that can appear at the output is reduced to one part in 100,000, or less than 1 lsb of a 16-bit signal (1 lsb of 16 bits=1/65535). Unfortunately, CMR normally starts to roll off above 100 kHz and may not be effective for very high frequency common-mode noise.

To maximise the positive effect of common-mode rejection with differential inputs, it is best to wire a signal source to the input using a wiring scheme that helps to couple the noise equally into both leads (that helps to make the noise common-mode). This is best accomplished with shielded twisted-pair wiring as shown in Figure 3.

Figure 3: Simplified differential input connections to non-isolated sources using twisted pair cable. For a larger image click here.

As shown in Figure 3a, connect the shield of the twisted-pair cable to the negative lead and to the earth ground at the earth grounded single-ended source. At the differential input end of the cable, connect the shield to the third lead of the differential input (its return lead) and to earth ground. Note that return uses the shield to reference differential input return to the earth grounded output signal and the shield current is very small. While it’s normally not accepted to allow two separate connections to earth ground, in this instance, connecting earth ground to the cable shield at both ends has been shown to significantly reduce noise levels and is the recommended approach of many instrumentation amplifier manufacturers (this can also lower radiated emissions by the circuit).

Best performance is obtained using isolated sources with no direct-connection to earth ground as shown in Figure 3b.

Common-mode range

A differential input will only measure the voltage between two individual points within its common-mode range. The common-mode voltage range is defined as the maximum allowable voltage swing on each input with respect to the signal return of the measurement circuit (the third connection). This parameter will be very important to your application as it defines your window of possible signal measurement.

Figure 4: Common-mode range window of measurement. For a larger image click here.

It is helpful to think of a differential input as having a window of voltage measurement defined within a larger window of measurement corresponding to its common-mode range as illustrated in Figure 4. Think of the differential measurement as a smaller window inside the larger common-mode range window and completely overlapping it. You are restricted to measuring voltage between any two points inside the larger window. If either of your measurement points result in potentials outside of the larger range window, measurement error or failure will occur and you may permanently damage the input circuit.

You can usually find the common-mode range of your input by referring to the specifications, or optionally the schematic. The outer boundaries of the differential input window are partly dependent on the voltage rails to the input amplifier as illustrated in Figure 4B. You could increase the effective common-mode range by connecting your input voltage using a voltage divider, but it may still be limited by the part itself and input protection circuitry. If the input is an A/D converter, then the common-mode range will be limited by the magnitude of its voltage reference, or ±reference for a bipolar A/D converter. You can refer to these specifications to ballpark your input common-mode range, but always make sure the input potential at each input with respect to return does not go outside of this range.

Causes of failure

It is very easy to exceed the common-mode limit of many devices with modern applications, unless the differential input device has been specifically designed for high common-mode voltage levels. This is partly because most modern instruments have trended to the use of lower voltage rails, like 3.3 V or 2.5 V. The reason some of these devices are still able to convert input voltages well above their voltage rails is because they typically employ voltage dividers at their front-end, followed by high-gain amplifiers or high-resolution converters (allowing them to divide inputs down to lower voltages).

Instruments with differential current inputs can be problematic in this regard, because they do not employ voltage dividers in their front-end circuits. Rather, they often use current shunt resistors, like 50 Ω or 100 Ω, to convert small DC currents to voltage at the input of their amplifier or A/D converter. For example, using 50 Ω shunts and a 20 mA full-scale current (0.020*50 = 1 V), then you realise that you cannot connect more than two or three channels in series before the input common-mode range window limit is reached for an instrument front-ended with a low voltage rail like 2.5 V or 3.3 V. If the input is really an A/D converter, perhaps with a 1.235 V reference, then only one channel could be converted, as two in series would force one input potential outside the 1.235 V full-scale limit of conversion.

Forgetting to connect earth ground to differential return can also cause damage. In most cases the circuit will continue to measure properly with or without adding an earth ground connection on the return, but high impedance inputs like those of instrumentation amplifiers can be made to float in the presence of high levels of EMI, and inputs could be driven to levels outside their common-mode range (they have very low bias currents as a result of their high input impedance). Connecting return to earth ground helps to ensure the input will not float outside of its input range in the presence of high EMI. But equally important, adding earth ground gives the input circuit a low impedance path for shunting transient energy or fault voltages safely to earth ground away from the sensitive input components.

Image: ©stock.adobe.com/Croc80