ADS1262: INL measurement method

Sato-san,

I don't normally support the ADS1262, but I'm going to jump in and answer some of the questions about making an INL measurement. I haven't used the ADS1262, but I have made many INL measurements on many precision ADCs. INL is a very difficult measurement to make because it requires a precision measurement over a long period of time to take the INL data. I'll discuss what you may need for the experiment, and then I'll give an example of the measurement. This example will be for a different ADC, but the measurement should be the same.

Basic Setup

Running an offset calibration and gain calibration isn't really necessary because they don't affect the INL measurement. But it won't hurt the measurement. And instead of running with 0.1V steps, I would start with 0.5V steps just to see that the measurement is working.

You'll need some precision equipment to make this measurement. I have used the Data Precision 8200 as the input sources because they are generally low noise and consistent output. However, I think that the characterization team now uses DACs to drive the inputs independently. The inputs are set so that the ADC input common-mode is always mid-supply. If your supply is 5V, then the input common-mode voltage is set at 2.5V to avoid any CMRR error. Also, if the input range is ±5V, use inputs that are not quite full-scale because over-ranging the inputs will cause large errors. I would use ±4.95V, so that the inputs go from AINP=4.975V, AINN=0.025V (+4.95V) to AINP=0.025, AINN=4.975V (-4.95V). Then use 21 equally spaced points to set the different inputs. When you make these measurements, you should make sure the analog inputs and reference have settled to their final value. You should make sure that the reference doesn't change (or if the reference does change, the value is always measured back).

For measurement, you need a precision DMM that has better linearity than the ADC. If the DMM has a worse linearity, then you'll only see the DMM's linearity error. We often use the Agilent/Keysight 3458A DMM for its <1ppm non-linearity. We generally use two DMMs. One measures the ADC inputs (AINP-AINN) and the other measures the reference input. When we use these DMMs, the auto-scaling must be turned off (so that it doesn't jump to different input scales) and there should be some line cycle averaging (In the past, I've used 100 line cycle averages). There should be enough ADC measurements to average so that the noise is reduced well below the INL error level.

On the board, you should use good capacitors for input and reference filtering (preferably C0G). If there is a dielectric absorption with these capacitors, there may be added error. A good layout is required and EMI interference will add to noise.

Example measurement:

A few years ago, I made a measurement of the INL for the ADS1248 using the EVM. The input range is only ±2.048V, so I made measurements with a 21-point INL measurement with ±2.0V. I set up the ADS1248EVM-PDK and run the GUI software ADCPro. In my setup, I only had one Data Precision 8200 and one Agilent 3458A. Normally, I'd want two of each, but I did enough results with this measurement. Again, this isn't the ADS1262, but it should be a similar measurement.

I set the measurement to AIN0 and AIN1. I've taken the 8200 and tied those to the inputs AIN0 and AIN1. In parallel with the 8200 are two 1k resistors in series. The common point of the two resistors are tied to AIN2. I use this to set up the DC common mode point of the 8200 measurement. This is done by turning on VBIAS to set AIN2 at 2.5V. Note that this common mode point is a tiny bit noisy and might contribute some small amount of noise, but it doesn't look like it in the measurement. The ADS1262 has this same VBIAS feature on AINCOM.

For this measurement I wait a few minutes for the reference to settle. I think that this can be avoided with better components with less dielectric absorption, but the EVM is a pretty basic setup. I first measure the reference at the beginning of the setup and at the end of the setup with the 3458A. In this case, I'm hoping that I don't have too much reference drift (based on the measurements, I don't think I have much, but I do think this does contribute some amount of error). It certainly would have been better to measure this for each measurement, but again, I only have one 3458A.

While I'm making the measurements, I use the 3458A to measure the input. I set the 8200 to 2.00000V, Remember that with the two resistors, and AIN2 tied to 2.5V, I've set the input common mode to 2.5V. I let the input settle for about 30 seconds for each measurement. I take 256 samples at 20SPS and record the average code and the 3548A reading.

For each reading I've set the 8200 to .25V lower, wait for 30 seconds, and take 256 samples at 20SPS. This goes for inputs from 2V to -2V, each time the common mode voltage is set by VBIAS to 2.5V.

Below is the data. The reference voltage readout was about 2.047991 at the beginning, it did change a bit to the end of the reading to about 2.048001 which does make some difference, but you can look at this for yourself.

Code ADC readout 3458A reading
8193493.13 2.000356 1.99992
7169278.1 1.750304 1.749924
6145145.06 1.500273 1.499945
5120947.91 1.250226 1.249946
4096826.18 1.000197 0.9999629
3072648.91 0.750155 0.7499653
2048517.38 0.500124 0.4999832
1024271.64 0.250065 0.2499845
-1.42 0.000000 0.000003
-1024268.37 -0.250064 -0.2499809
-2048493.34 -0.500118 -0.4999802
-3072622.43 -0.750149 -0.7499621
-4096809.81 -1.000193 -0.9999598
-5120921.13 -1.250219 -1.249943
-6145174.42 -1.500280 -1.499958
-7169311.06 -1.750312 -1.749938
-8193499.26 -2.000357 -1.999935

I entered this into an excel spreadsheet that I'll attach to this post. I'll reference the columns in the spreadsheet for the explanation below.

Column 1 of the spreadsheet is the reference voltage which I kept constant at 2.047991. Again, this may contribute some error, but I can only rectify that with a second 3458A measurement.

Column 2 is the ADC code that comes out. This is the average readout of each value you see in the above columns.

Column 3 is the ADC reading. This is calculated from the ADC code multiplied by the reference value, divided by 2^23.

Column 4 is the 3458A measurement. We assume this to be absolute correct measurement with a perfect non-linearity.

Column 5 takes the ADC reading and scales it linearly so that it matches with the 3458A measurement in gain error.

Column 6 then subtracts the offset from the scaled ADC reading in column 5 from the 3458A measurement in column 4.

Column 7 is the error from the scaled, offset adjusted ADC measurement in 6 to the 3458A measurement. Here it is recorded in Volts.

Column 8 is the INL error, divided by the full scale and converted to parts per million.

Below these columns is a graph showing the INL plotted versus the input voltage. It is similar in magnitude and shape to the INL listed in the datasheet. However, there still can be sources of error. Long cables, used in either the input drive or 3458A measurement can have extra capacitance or inductance (or even pick up extra noise), that can affect the measurement. The reference drift may cause some extra error as the measurements are taken. This may have larger effect on the endpoint calculations. The 3458A measurement may not be ideal because it is far away from the input pins of the DUT. Here, the data shows the INL is a little higher than typical and I think this is probably the result of some reference drift.

Anyway this example should be enough for your customer to make a quick test. Below is the excel file.

ADS1248EVM_INL.xlsx

Joseph Wu