ADC and square wave
Greetings,
I have faced a following problem. My task is to implement a system of an impulse response measurement and I use the MLS method. The method uses pseudorandom binary sequency as an input signal for a DUT after which it is required to get a response and do mathematical processing. But my problem, as I suspect, related with pecularities of ADAU1701 ADC which I use. To check correctness of ADC work I tried to measure square wave with ADAU1701 built-in ADC. The result of measured samples is given on the attached pictures. As you can see, received signal has strange distortion at the edges like jumping voltage without direct transition from one logical level to another. If ADC cannot provide more correct samples of a square wave, usage of a binary sequence is impossible (because a sequence completely consists of similar abrupt transitions). Therefore I have following questions:
Can ADAU1701 ADC be used to measure signals with abrupt transitions like square wave or binary sequence?
If it is possible, how to improve ADC accuracy?
Perhaps, sigma-delta ADCs aren't suitable for measuring signal like these but I am not sure so I ask.
Also increasing sampling rate to 192 kHz did not help.
Audio Signal Processors
SigmaDSP Audio Processors
adau1701
Hello Forthan,
The Sigma-Delta converters have filters in them so they are not suitable to produce a perfect square wave. Keep in mind that a perfect square wave is an infinite series of odd harmonics and so if you truncate it at some point it will distort the square wave and that is seen in the edges and near the edges. Any converter will have issues with reproducing a square wave. Heck, an Op-Amp has difficulty because its frequency response is not infinite either. A DAC ( not a Sigma-Delta) is a little easier because you can send it a value of zero on one sample period and then on the next sample period send a full level signal and the DAC will attempt to produce this change which gets back to the Op-Amps. But it will do a decent job.
For an ADC is a different situation. You have to have a very low impedance source to be able to drive the sampling cap fast enough to make the change from 0 to 1 very quickly. So look into our high speed converters and ask that group about this. They should be able to help you with this problem. I only work with Sigma-Delta converters and as long as there are filters involved there will be some ringing when giving it a square wave.
Dave T
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