Dear Larry,
Why does Rabbit not support A/D converters with more than 12 bits resolution?
Chris. T
Hello Chris,
Thank you for the question. In answering your question I am going to discuss accuracy and resolution of A/D converters.
To start off, here are definitions of the two terms as found at one website:
- Resolution: The fineness of detail that can be distinguished. This is a measure of precision.
- Accuracy: The ability of a measurement to match the actual value of the quantity being measured.
Understanding the difference between these two terms, both of which can be measured in “bits,” is extremely important in accomplishing whatever measurements you have to make in your system.
Let me show the difference in these terms by using some examples:
- Suppose you have a 5 digit DVM which is reading 1.2345V. This is a relatively high resolution reading. It is quite precise. However, if the actual voltage is 3.0000V it is not very accurate!
- Now, suppose you have a series of readings that measure 1.23V +/- 0.05V and the actual voltage is 1.23V. The readings are accurate but may not have the resolution/precision required by the application.
At Rabbit, most of our products that have an A/D converter use a device that has 11 bit (single ended) or 12 bit (differential) resolution (ADS7870). With an input voltage range of -20V to +20V the resolution is very close to 10mv. This does NOT mean that you will have 10mv accuracy. It does mean that the absolute best you can achieve will be 10mv accuracy. The actual accuracy of the system is dependent on several noise sources (assuming the A/D has been calibrated!):
- The SBC itself
- The noise generated by the device being measured
- The noise picked up on the wires between the signal source and the A/D.
In digital systems it is not unusual for noise to be 10 mv or more. I measured about 6 mv p-p noise on the BL4S200 board with no program running. Our Engineering team was careful to design the board in order to achieve a fairly low noise level. This required the use of a multilayer board where the top and bottom layers are essentially ground planes. Note that if the range of the input is set to -10V to +10V, the resolution becomes 5 mv – a value which is smaller than the noise.
One way to reduce noise from reaching the A/D converter is to have a low pass filter on the inputs. Most of our SBCs have a low pass filter on each input but the cutoff frequency is in the order of about 100 kHz. This will eliminate only very high frequency noise. The reason we have a relatively high cutoff frequency is because we have no idea what frequencies our customers are going to measure. If your application is for relatively low frequencies, you can implement your own low pass filter external to the SBC.
We have received a number of inquiries as to why we do not have higher resolution A/D converters on our SBCs. In general, our SBCs are used in industrial control (or similar) applications. These applications do not usually need more than 12 bits resolution. To implement higher resolutions would require that we spend a lot more time and expense - read that as higher cost to our customers - to develop circuits that are not required for the majority of our customers.
If your application really does need higher resolution, one solution would be to develop a small board that has a higher resolution A/D (maybe a 14 bit ADS7871) and put that board close to the signal. You would still need to run the SPI communications wires between the Rabbit and the A/D board. You will also have to be very careful with the design of the board so that it is capable of 14 bit accuracy. You will want to keep the digital signals away from the analog inputs; perhaps you might even need some amplification and/or guard rings. A multi-layer board with ground planes will also be helpful.
In general, the higher the required accuracy and resolution, the more care that needs to be exercised in the circuitry.
- Larry C.
Larry Cicchinelli is Rabbit’s Technical Support Manager. He has 30 years of embedded experience, and is considered one of the foremost authorities on Rabbit products. Larry and his staff offer comprehensive technical support to Rabbit customers.
Submit your questions for Larry via email at AskLarry@rabbit.com
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