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In order to understand what a device is doing we use instrumentation to take measurements. These measurements might be displayed, recorded or used directly to control a system or subsystem. The instrument may have a dial on it, like the speedometer in your car, it might have a digital readout like the thermostat in your house or it might just have an indicator light, like Penny’s check engine light. We expect and even depend on these instruments to give us accurate information.
Precision, accuracy and repeatability (PAR)
To be on par with one another, we need to understand what is meant by precision, accuracy and repeatability. They are sometimes used interchangeably and often used incorrectly. Let’s start off with the easy one. Repeatability means that if we measure a variable 10 times (under the same conditions) we get the same answer ten times in a row. It is repeatable.
Precision is related to how finely we can make those measurements. For example, if I say Voltage A is 4.3 amps but Voltage B is 2.315 amps, I am being more precise about the magnitude of Voltage B. Precision is how coarsely or finely the measurement is made. A 12-bit A to D will make a more precise measurement then an 8-bit A to D. Precision is also related to resolution. The 12-bit A to D has a finer resolution then the 8 bit A to D (assuming they are set up for the same max value). Precision can be increased by averaging multiple samples while resolution is fixed.
Accuracy is simply how close the measurement is to reality. If Bob measure a voltage and get 2.312 volts and Jennifer measures the voltage and she gets 5.2 volts but the true voltage is 6.0 volts then Jennifer’s measurement was more accurate. It more closely reflected reality. While Bob’s measurement was more precise.
A good instrument will give you all three. I want the measurement to be correct (accurate). I want it to be precise (most of the time a resolution of 0.1mV will suffice). And I want to get the same answer every time I take the measurement (repeatability).
We use a standard to calibrate our measurement devices. These standards allow us to remove much of the error from our instruments. One of the causes of error is the natural drift of component values with time. Calibration is an adjustment to the accuracy of the device. Often resolution has to do with the way the device was manufactured. If your DVM has 3 significant digits there is not much you can do to improve it.
There are other causes of error. Noise being the largest perpetrator. Noise is any part of a signal that does not convey useful information. It can come from many sources. For example, the 60hz signal that permeates our lives because it is the frequency of the power we use shows up in all sorts of signals. The random vibration of electrons in a conductor due to heat is called Johnson noise. Unshielded electronics can transmit noise that can be received by other electronics and show up as noise. Having a circuit close enough to other electronics can cause coupling either through an electric field or the magnetic field generated by a current carrying conductor.
If you have been working in electrical engineering for any amount of time (usually 1 day is sufficient) you know that everything is a thermometer. Temperature sensitivity is something we all have to deal with and designing a circuit that is insensitive to temperature our greatest challenge.
Noise is often introduced in our measurements by improper grounding which includes improper placement of the ground. Ensuring a good ground and located at the system ground is paramount.
Heisenberg may have been onto something
The uncertainty principal is a quantum mechanical principal that states that there is a limit to the precision with which we can know things about very small particles, like electronics. So, you can’t know the position and momentum of an electron with absolute accuracy. I bring this up, tongue in cheek, because it is somewhat true in none quantum sizes as well. As soon as you start to measure a circuit you have altered that circuit and the measurement you get is not the same value that exists when you are not measuring the circuit. This is an important principle to keep in mind. It is why a volt meter has a rating of ohms per volt. That meter is loading the circuit when it is used.
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