How to test 24Bit ADC INL in the Lab? - Page 1
Forum section: electronics / general

author: eleday
author_rank: Newbie
timestamp: March 04, 2022, 06:16:42 pm
content: HiI am currently working on a project which I need to measure the INL for 24bit ADC, I have builded two 24bit ADC boards to test their INL, one is using LTC2400, and the other is LTC2440. After reviewing a lot of pages/documents about the INL test, I know that for lower resolution ADC INL test, normally we will use a Ramp or Sine wave for the ADC INL test. But for a 24bit ADC, it seem that it's impossible. For example,LTC2400, Total sample needed for INL will be 2^24 * 10 =1677721600 (10 samples for average one voltage point) ,LTC2400 data rate is about 7Sample/s, this would take about 6657 hours.For LTC2440, its data rate can up to 3.5kS/s, but it still need about 13hours. For this LTC2440, we can do it in the Lab, but it still not possible for production line.So I am asking how does TI/ADI/LTC measure their 24bit ADC INL in the Lab?I am trying to borrow or rent a Fluke 5700 device to test my 24bit ADC INL. What I am thinking is that, use 5700 to output a Ramp signal cover the ADC input range, do more average to cancel out the noise, although it may not cover all the 2^24 points , but this is the only thing I can do now. If I want to get an accurate result, I need to test a lot points which would take a lot hours, but long time mean drift, not sure what's the best way to do it? Any suggestions/links/ideas?

author: jwet
author_rank: Frequent Contributor
timestamp: March 04, 2022, 07:24:17 pm
content: Maybe not what you want to hear, but I think I'd find a local calibration lab that will do it for you. Most labs have the equipment but are used to doing very straight forward things like calibrating DVM's. I would call a couple labs and see if you can find one that will do what you want. If you don't need certs or a cal. report, etc, they will knock some off. You could basically want to get an hour of their time for $150 or so- if their especially nice, they might let you observe. Find a lab that is hungry or new or both.

author: Andreas
author_rank: Super Contributor
timestamp: March 04, 2022, 07:59:56 pm
content: Quote from: eleday on March 04, 2022, 06:16:42 pmHello,but datasheet also says that noise is 0.3 ppm (RMS) = 10 uVpp for a single measurement. So averaging 10 measurements only reduces the noise to around 20 Bits. I need 1 minute averaging to get the noise down to 1 uVpp or 22 bits.During 6000 hours your voltage reference might also drift several 10 ppms.And to be realistic. According to AN86 since the error curve is parabolic you only need one (1 minute) measurement at the half of the reference voltage to determine INL on the LTC2400 with a accuracy of better than 1 ppm.BTW the datasheet results on noise need a low ohmic source and the switching noise of the LTC2400 input may interfere with the output of your buffer OP-Amp.Attached a hobby measurement of INL. (all AXIS are in mV. Y-Axis is the needed correction value to get the linearized value)with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2022, 08:08:43 pm
content: Testing slow high res ADCs is a real pain and naturally slow. Dependign on the speed one may not be able to do a full test at all codes. With the knowledge on how the ADC works one could likely reduce the number of points to test. A tricky part could be telling appart drift in the gain from INL. testing just 1 single slope may not work as over a multiple hour test the gain / references involvled may dirft. At least it would need a very stable temperature and good warm up / burn in before the test.So the production test can only be a kind of spot check with a few point. It is not uncommon to see quite a spread between the typical and max INL in the ADC data-sheet. The higher max. value is more easy to test and one may not need any repeat for a point and can use less points.With knowledge about the ADC one may know the type if INL to expect. This can be certain more critical regions and also an overall more smooth part (like U² or U³).With a precision ramp or test pattern they can test many ADC's in parallel and only need one precision signal. When test some 100 (or more) ADCs in parallel the Fluke 5700 may no longer look that expensive. Chances are they would use a faster, well characterized DAC and maybe some HP3458 to check. Chances are the will test way less than 16 million points in production.

author: eleday
author_rank: Newbie
timestamp: March 05, 2022, 04:31:09 pm
content: Thanks for the reply and a lot of great information.If the TI/ADI/LTC have a very stable temperature room/chamber, but the source like fluke 5700 range & resolution is still not enough to measure the whole range of the ADC some time. Like a 24bit ADC with +-2.5V range , 5700 range can be 220mV, 2.2V or 11V, but none of them can cover all the range, small range has a better resolution, but can not cover the whole range of the ADC input. Is it normal to add a voltage offset circuit to lift the range to cover the whole ADC input range? It seem this offset circuit would add some noise, you must carefully design this circuit so that it would not add much noise. Is it possible?I am going to create a ramp with 5700, this ramp would have around 3000~ 6000 test point, so the test time would be around 1 to 2 hours, then to get the ADC output code point by point(not a histogram method), for those un-test points, I will use least square fitting polynomial method to get it. And then I can get the whole 24bit ADC transfer function, so finally I can calculate the whole INL base on the ADC transfer function.

author: eleday
author_rank: Newbie
timestamp: March 05, 2022, 04:40:05 pm
content: Quote from: Andreas on March 04, 2022, 07:59:56 pmThanks for the reply, could you share the documents about this knowledge? About the averaging number relationship with noise.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 05, 2022, 06:21:13 pm
content: From standard statistics the noise goes down with the square root of the number of measurements. That is for independent measurements. With a SD ADC consecutive readings may be correlated from a more complex FIR fitler function. This would give an reduced advantage from averaging.One can add a voltage to another with relatively low noise, espeically if independent / isolated.An additional problem is that separate references for the DAC and ADC would also add noise, especially in the range with significant amplitude. Drift of the references and ADC gain (this can also be a slow process different from temperature) would also deform the curve measured. So ideally the ADC ref and DAC ref would use the same source.One may have to balance between a repeated measurement at the full scale and the settling needed. A single slow ramp may not be such a good ideal. To at least see the effect, I would consider 3-5 ramps with less points and incluing some points in all the ramps to see drift.Often the INL is a combination of more local effects and a more global background. So one may also need to measure a small range with rather high density.For a 2.5 V ADC range one may choose to use only 2.2 V from the 5700 or use the 22 V range for the over view and the 2.2 V for the details on a fine range. Ti / AD would likely have a solution more suitable for the ADC range.

author: TimFox
author_rank: Super Contributor
timestamp: March 05, 2022, 06:35:21 pm
content: Not a direct test, but one of my acquaintances used SINAD (starting from a known clean sinusoidal drive and computing the harmonic distortion from the data) as a production verification of performance on a data-acquisition system. It wasn't directly relevant to the system performance, but it would show deviations from consistent performance from unit to unit. Clean (low THD) AC generators are readily available.