With data acquisition systems like Microlink you are able to connect your analogue signals (temperature, strain, vibration, etc) in either single-ended or differential mode. 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 Microlink. The measurement is the difference between the signal and the ground or earth at the Microlink. This method relies on
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. Using single-ended inputs has the advantage of giving twice as many inputs as differential.
With differential inputs, two signal wires run from each signal source to the Microlink. 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 Microlink'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 Microlink has a socket labelled 0V. Run a wire from, say, the - wire to this OV socket, either directly or via a resistor. (If your signal source is itself grounded don't make a connection to the Microlink's 0V 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 Microlink 0V input. Differential inputs, therefore, give you the option of either mode.
The Microlink 75x USB Series, Microlink 800 Ethernet Series and the Microlink 3000 Frame Based Series all provide differential inputs. You can buy Microlink products on-line from the Windmill Software data acquisition catalogue.