How to test my 24bit ADC - Page 1
Forum section: electronics / beginners

author: panfilero
author_rank: Newbie
timestamp: June 12, 2014, 01:51:03 pm
content: Does anyone know a good way to test a 24bit ADC to know what kind of accuracy and resolution you are really getting? I'm working on a current sensor with a large dynamic range, sense from 1mA to 100A. Mostly just DC current (I'm thinking about AC in the back of my head though). Reading the voltage off a 1-milliohm shunt to get my current.Much Thanks!

author: retrolefty
author_rank: Super Contributor
timestamp: June 12, 2014, 02:57:08 pm
content: I would be interested in seeing what people suggest. 8,388,608 measurable steps of resolution may be swamped by noise?

author: Dago
author_rank: Frequent Contributor
timestamp: June 12, 2014, 04:24:41 pm
content: Well you basically calculate it based on the specified component tolerances and such. Monte carlo simulation for the circuit etc. might be a part of the analysis. Usually accuracies (and component tolerances) are specified over a temperature range so you need to take that in to account as well.http://en.wikipedia.org/wiki/Propagation_of_uncertaintyYour resolution will be 24bit. Your effective resolution and accuracy are a different matter... I strongly doubt you will get anywhere near sensing 1mA with a 1mOhm shunt. That means there is a 1µV difference over the shunt. It will get swamped by noise, opamp offset voltages...

author: zapta
author_rank: Super Contributor
timestamp: June 12, 2014, 04:39:11 pm
content: Quote from: panfilero on June 12, 2014, 01:51:03 pmIf my calculations are correct you have on your resistor about 6 nano volts per ADC step. I doubt that the ADC will be the bottleneck, accuracy and noise wise.

author: senso
author_rank: Frequent Contributor
timestamp: June 12, 2014, 05:14:16 pm
content: Its a DAC, but I think the same principles will apply, its a nice video:http://thesignalpath.com/blogs/2012/12/10/tutorial-on-the-theory-design-and-characterization-of-nyquist-digital-to-analog-converters/

author: free_electron
author_rank: Super Contributor
timestamp: June 12, 2014, 05:22:23 pm
content: the outlyers ( endpoints of the curve) will always be off. no escaping that as you run against the rails ( ground and vref)for this kind of adc all you can do is look for distortion. that is going to tell you if you have missing codes.Send in a spectrally pure sinewave , capture output run fft and look for spikes in the spectrum. the fewer spikes the 'cleaner' the adc is.

author: Andreas
author_rank: Super Contributor
timestamp: June 12, 2014, 08:02:23 pm
content: Quote from: panfilero on June 12, 2014, 01:51:03 pmHello,Dont blame it on the ADC:The 1% of the resistor and his associated thermocouple voltages will be your limitation.The next limit will be your voltage reference.With best regardsAndreas

author: zapta
author_rank: Super Contributor
timestamp: June 12, 2014, 08:17:52 pm
content: Quote from: Andreas on June 12, 2014, 08:02:23 pmAnd the offset and noise from the amplifier.

author: panfilero
author_rank: Newbie
timestamp: June 13, 2014, 03:14:37 pm
content: So people are talking about the noise or error on other components, does this translate into: you need very expensive, fancy, precision voltage sources and other equipment?

author: AndyC_772
author_rank: Super Contributor
timestamp: June 13, 2014, 03:26:30 pm
content: At an absolute minimum, you need to work out the equation for the result you'll get from your ADC in terms of the current being measured and the values of the components in your circuit.Then, look up in each component's data sheet how the properties which appear in your equation vary with time, temperature, and from sample to sample. Plug into your equation the highest and lowest possible values for the relevant properties, and see what the effect is on the theoretical output from your ADC, assuming every other component is 'ideal'.That will tell you, to a good first approximation, what your circuit is sensitive to and which properties have to be optimised or controlled - and which are less critical.You should then also be able to work out the worst case error, which will happen when all the variables in the circuit stack up together in such a way that their individual contributions to the error are all in the same direction. If there are a lot of variables then, statistically speaking, it's unlikely that they'll all contribute errors in the same direction - but it's possible.You won't ever reduce those errors to zero. What you may be able to do, though, is keep the variability down and characterise it by measurement against a known standard. This is how precision test equipment is made; it's not built to just be 'right' in an absolute sense. It's built in such a way that effects such as dependency on time and temperature are made small, but more importantly, predictable. The equipment is then measured against known standards in a calibration lab, and the measurement errors are compensated for, usually by a software lookup table.

author: zapta
author_rank: Super Contributor
timestamp: June 13, 2014, 04:26:41 pm
content: Quote from: panfilero on June 13, 2014, 03:14:37 pmWith 24 bit full scale over a 100A, 0.001ohm, each ADC step is equivalent to100A*.001Ohm/(2^24) = ~6 nano volts.That's a very small voltage and it means that if you want to get real 24 bit accuracy (assuming perfect resistor and ADC), your amplifier must have very low noise and offset and very high stability. Did you design the analog circuit? Can you share the schema?

author: sfiber
author_rank: Regular Contributor
timestamp: July 07, 2014, 08:27:37 am
content: you mess around with nanovolts.Did you test it or will?

author: mimmus78
author_rank: Unknown
timestamp: July 07, 2014, 12:26:37 pm
content: And than you need the obligatory 7 1/2 digits multimeter ... better to have a 8 1/2 digits just in case

author: adi101
author_rank: Contributor
timestamp: July 08, 2014, 05:37:12 am
content: The best way is to get the evaluation board of the ADC, which is usually provided by the manufacturer for exactly this purpose, testing the performance.