Recently have been trying to make a circuit that uses a ADS8885 SAR ADC to convert low frequency signals.
Here are the requirements of our design
Our frequency band of interest is between 0.1 to 200 Hz.
We want to achieve high SNR by sampling at 250 kSPS and digitally low-pass filtering data above 200 Hz.
We want to do this with as little power consumption as possible! (total should definitely be below 10 mW)
Our signals are fully differential
We use a 5 V reference.
As a starting block, we created a circuit based on the SLAU513B design by Vinay Agarwal, but we used the ADS8885 instead of ADS8881 and we use a 5 V reference instead of a 4.5 V reference.
We shorted our inputs and started sampling noise to see the noise performance of the design. If we plot the spectrum of the data we see the following:
At high frequencies the spectrum is nice and flat (as expected), but below 1 kHz, we see an increase in noise. This is very unfortunate because this increase in noise is in our band of interest. In datasheets noise plots are never plotted on a logarithmic scale, so it is impossible to verify with the datasheet if this behaviour is to be expected.
My first question is: Where does the increase in noise below 1 kHz originate from and how can I get rid of it?
Second, the total integrated noise is less than what the datasheet tells me it should be (103.8 dB measured vs 100 dB datasheet). Although this is a 'good' result, it makes me question my design or setup ;).
My second question is: Why is my measured integrated noise almost 4 dB lower than specified in the datasheet?
In summary, I seem to have "too little" noise at high frequencies and "too much" noise at low frequencies. I'm confused.
Looking forward to get some help and advise!