Using Differential or Single-Ended Inputs to connect your Signals?
When monitoring analogue signals such as temperature, strain or vibration, you often have a choice of single-ended or differential connections. What is the difference between the two and which should you use?
With single-ended inputs you connect one wire from each signal source to the data acquisition interface. The measurement is the difference between the signal and the ground or earth at the interface. (The interface might be a card that plugs into your PC, or an external unit connected to your PC.) This method relies on
- the signal source being grounded (earthed), and
- the signal source's ground and the interface's ground having the same value.
Differences in Ground Levels
We think of the ground as a constant 0V, but in reality the ground, or earth, is at a different level in different places. The closer together the places, the more likely the ground level will be the same. Make a connection between two grounds and the difference in levels can drive large currents, known as earth or ground loops. This can lead to errors when using single-ended inputs.
Single-ended inputs are sensitive to noise errors. Noise (unwanted signal contamination) is added because signal wires act as aerials, picking up environmental electrical activity. With single-ended inputs you have no way of distinguishing between the signal and the noise. The ground and noise problems can be solved by differential inputs.
With differential inputs, two signal wires run from each signal source to the data acquisition interface. One goes to a + input and one to a - input. Two high-impedance amplifiers monitor the voltage between the input and the interface ground. The outputs of the two amplifiers are then subtracted by a third amplifier to give the difference between the + and - inputs, meaning that any voltage common to both wires is removed.
This can solve both of the problems caused by single-ended connections. It means that differences in grounds are irrelevant (as long as they aren't too large for the amplifier to handle). It also reduces noise - twisting wires together will ensure that any noise picked up will be the same for each wire.
A common problem when using differential inputs is neglecting any connection to ground. For example, battery-powered instruments and thermocouples have no connection to a building's ground. You could connect a battery, for instance, between the interface's + and - inputs. The 2 input amplifiers will try to monitor the voltages + to earth and - to ground. However, as there is no connection between the battery and ground, these voltages to ground could be any value and may be too large for the amplifier to handle.
For these "floating" signal sources you should provide a reference. The interface will generally have a socket labelled 0V, REF or GND. Run a wire from, say, the - wire to this ground socket, either directly or via a resistor. (If your signal source is itself grounded don't make a connection to the interface's ground socket.)
Amplifier Ability and Operating Range
The three amplifiers used for differential inputs are collectively known as an "instrumentation amplifier". Ideally, as previously described, any voltage common to both wires (common mode voltage) is cancelled. In practice the two input amplifiers are not perfectly matched so a fraction of the common mode voltage may appear. How closely the instrumentation amplifier approaches the ideal is expressed as the common mode rejection ratio (cmrr). This is the reciprocal of the fraction let through and is usually given in decibels. The higher the rejection ratio the better.
Another specification to look for is the common mode range. This is the maximum contamination voltage with which the amplifier can cope. If the difference in ground levels between your interface and signal source exceeds this value, your measurement will be inaccurate. (Your hardware operating range may be given as higher than the common mode range, but the operating range just guarantees that your hardware won't be damaged, not that it will work properly.)
Less Signals with Differential Inputs?
An obvious disadvantage of differential inputs is that you need twice as many wires, so you can connect only half the number of signals, compared to single-ended inputs. Should you decide that single-ended inputs are OK for you - if you have short signal wires, close together signal sources, and signals larger than around 100 mV for example - you can use differential inputs in single-ended mode. To do this short one of the signal wires (usually the - input) to the interface ground input. Differential inputs, therefore, give you the option of either mode.
Some manufacturers offer pseudo-differential inputs. These may be used when the signal sources are close together and share a common ground. Pseudo-differential is similar to single-ended but the signal source's ground is isolated from the interface's. A wire runs from this ground to the interface. By subtracting the interface ground from the signal ground, differences are removed from the measurement. However, this method is no use for reducing noise.
- Signal leads over a few metres in length?
Choose differential to reduce noise.
- Small signals under around 100 mV?
Choose differential to reduce ground and noise errors.
- Signals with different grounds to each other, as happens when signals are remote from one another?
Choose differential to remove ground errors.
- Sensors with high resistance such as strain gauges?
Choose differential to remove common mode voltage. High resistance gives greater pick-up and thus higher common mode voltage.
- Need twice as many inputs, and have none of the above problems?
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