Differential Readings (App Note)
Differential vs. Single-Ended:
In this discussion, a voltage is the difference in electric potential between 2 points. For a single-ended voltage reading, 1 point is an analog input terminal, while the other point is the common ground (GND) of the LabJack. For a differential voltage reading, the 2 points are 2 analog input terminals.
Differential vs. Bipolar:
Note that differential is not the same as bipolar, and they do not necessarily have anything to do with each other (but sometimes do). Bipolar refers to a voltage that can be positive or negative. We use the term bipolar or true bipolar to describe a point that can be greater than or less than ground. We use the term pseudo-bipolar to describe a voltage where the positive point can be greater than or less than the negative point (thus the difference is positive or negative), but neither point can be less than ground.
Why use differential?
Reasons #1 & #2 are key reasons for differential measurements. Reason #3 is good in theory for certain situations, but most of the time single-ended measurement performs as well as differential.
1. The signal is differential and the negative cannot be connected to GND: For example, consider a DAQ monitoring a typical Wheatstone bridge circuit that is excited by 4V/GND from the DAQ and is outputting a 2 mV signal, which means that signal+ is about 2.001V and signal- is about 1.999V. You cannot connect signal- to GND, because that would short out 1 leg of the bridge, so you must connect signal+ and signal- both to analog inputs and do 2 single-ended measurements or 1 differential measurement. If the bridge was excited by a floating source (not referred to the DAQ device), you can define the common ground wherever you want so you could connect signal- to GND and just do a single-ended measurement of signal+.
2. Measuring a small difference between 2 large voltages: For example, consider a DAQ with a simple 1% accuracy spec. This DAQ is monitoring a typical Wheatstone bridge circuit that is excited by 4V/GND from the DAQ and is outputting a 2 mV signal, which means that signal+ is about 2.001V and signal- is about 1.999V. You could take single-ended readings of signal+ and signal-, and subtract them in software to find the difference, but the 1% error of each single-ended measurement is about 20 mV which is very large compared to the 2 mV difference you are trying to measure. A direct differential measurement of the 2 mV difference with the same 1% error has just 0.02 mV of error.
3. Rejection of common-mode noise: Say you have signal+ and signal- coming from a floating AA battery through a long 2-wire cable, and you expect the long cable to pick up a lot of AC noise where the induced noise is the same on both wires. Since the battery is floating, you could connect the wires to 2 differential analog inputs on the DAQ device, and then add a high-value resistor from the negative analog input to GND to provide a path for bias currents. Since the noise at any point in time is the same on both wires, it will get subtracted out by a differential measurement. Alternatively you could connect signal+ to an analog input and signal- to GND, for a single-ended measurement. In theory, the wire connected to ground can't have noise because it is connected to ground, so you just have noise on the positive wire which all shows up in your measurement, but in reality it does not usually work exactly that way and you don't see 100% of the noise.
Differential inputs must have a reference:
The most common mistake when using differential inputs is connecting 2 signals that have no reference to ground. Do an Internet search for "instrumentation-amplifier floating-inputs" or just see the "Floating Voltages" section of the following article about instrumentation amplifiers (in-amps):
Consider an obviously floating voltage source such as a thermocouple or AA battery. If you simply connect the positive and negative leads to 2 analog inputs on a U6, or to IN+ and IN- on an LJTick-InAmp, there is no ground path for the bias currents that must flow in/out of the inputs. The voltage source will try to properly hold the voltage difference between the leads, but the voltage of each lead compared to ground will likely be near one of the power rails and the common-mode voltage will not be valid. A common solution is a resistor from the negative terminal to ground, which can be quite large if desired.
Another example is a bridge circuit excited by an external supply which is isolated from the U6 or LJTick-InAmp. In this case the negative from the supply should be connected to GND (a series resistor can be considered if you don't want a direct connection between the supply ground and GND).
The common-mode voltage must be in range:
Another common mistake is connecting voltages that are referenced to ground, but where the voltages compared to ground are not in the valid range.
For example, the LJTick-InAmp uses a pair of AD623 instrumention amplifiers from Analog Devices with power rails at VS (~5 volts) and GND (0 volts). Figures 22 and 23 of the AD623 datasheet show the common-mode range. Note that the maximum under any condition is about 3.5 volts and the minimum is about -0.3 volts. Signals with a common-mode voltage outside -0.3 to +3.5 volts will definitely not work, and for signals inside that range we recommend looking at the LJTIA signal range tables or online calculator from Appendix A of the LJTick-InAmp Datasheet.
Say you have a 12 volt battery system where the battery negative is connected to LabJack/LJTIA GND. You want to measure the current the battery is providing to some load, so you put a high-side shunt between the positive battery terminal and the load. The shunt is providing a 100 mV signal, so the voltage compared to ground on each side of the shunt is 12.0 and 11.9 volts, and thus the common-mode voltage is 11.95 volts. This is definitely too high for the LJTIA. However, if a low-side shunt is used instead between the negative battery terminal and the load, the common-mode voltage is only 0.05 volts and the LJTIA is fine.
For signal ranges on the U6, see Section 2.6.5 of the U6 User's Guide.
Why don't I worry about ground when I measure voltages with a simple DMM?
So why can you just take the 2 leads from a simple battery-powered DMM and measure the voltage across a battery or thermocouple, regardless of what grounds might or might not be connected? Because the DMM is isolated and is actually taking a single-ended reading. The black lead is ground for the DMM, but since it is isolated that ground has no meaning to the battery or thermocouple, and wherever the black lead connects is defined as ground for the DMM.
How about a fancier DMM with 2 channels, and 2 pairs of red/black leads, powered by AC mains? First the input channels are isolated (optically or galvanically) from AC mains, so there is no common ground there, and the input channels are also isolated from each other, so the black leads are ground for each channel but not the same. Each black lead defines ground for each channel.
Common amplifier types:
Operational Amplifier (op-amp): Single-ended input and output.
Instrumentation Amplifier (in-amp): Differential input and single-ended output.
Difference Amplifier (diff-amp): Differential input and output.
- Analog Signals
- Noise and Resolution
- Resolution and Accuracy
- Differential Readings
- Floating/Unconnected Analog Inputs
- Measuring Current (4-20mA)
- Measuring Resistance
- Temperature Sensors
- Digital Signals
- Controlling Relays
- I2C, SPI, 1-Wire Examples
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