I at the moment using the LTC2364-16 ADC. I notice some linearity issues in my application for specific ranges of the ADC conversion.
I have a sine wave as my ADC input.
The linearity issues can be seen in the attached images (the results are converted to decimal, meaning 2.5V=65536)
The input window of the ADC is 0 -2.5 V. I am using the LTC6655-2.5V voltage reference to supply the two reference pins of the LTC2364. For the VDD power supply I am using an NCP512-2.5V voltage regulator and the 0VDD is supplied with 3.3V, coming from another board in the system. I have used the stated (in the LTC2364 Datasheet) capacitors to bypass the two reference pins and the supply pins to ground.
In the past I have supplied the reference pins with 3.3V giving an ADC input window 0-3.3V, but the result was exactly the same and I was missing codes in the exact same spots (i.e.: 49300-50000 codes).
I have also connect directly a function generator to the ADC input and the result remain the same. So the problem is not caused by the input circuit.
At the beginning I assumed that it was due to noise in the reference pins. That was the reason I put the LTC6655-2.5V, but no difference.
The fact also that the conversion does not stuck to a specific code, but there is a fluctuation as can be seen in the second attached image, is not helping me to draw any conclusion of what causing the problem.
I am not sure if you have come across that kind of problems in the past,
or if you can suggest me what else I can check.
Best Answer
I had exactly the same problem 35 years ago, when a 25ks/s 12 bit ADC was serious money. Yours may not be due to the same problem, but it's a source of potential problems worth ruling out.
I had crosstalk from some of the data output lines to the analogue input, due to a poor layout. The ADC ground shared a ground badly between the data latches and the input sample and hold. The symptom was that for a slowly changing input signal, the output code 'stuck' at particular codes.
Crawl all over your input and output lines, checking for capacitive coupling, and their ground return paths checking for common ground coupling.
A simpler and more trustworthy test source than a function generator is a large capacitor, shunted by a resistor, connected directly to the ADC input and its respective ground or -input. After charging, the discharge will give a smooth ramp, with no extraneous currents to anywhere else in the circuit.