Question on analog in scaling with sample freq
Designing an adau1701 product. Hopefully an easy one here: Want to support 44.1, 48, and 96K sample rates (using an external MCLK source), with the same input sensitivity. The spec says to scale ADC_RES and ADCx resistor by frequency. But each scale by the same factor, therefore as frequency changes the input scaling should be the same? Can someone offer some wizdom here. Looks to me no change is needed. Design for 44.1K @ 100uA. Maybe lose of a couple dB's SNR @ 96K?
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If you use the same resistors in series with the ADC inputs for different sampling rates, then your full-scale input value will differ for each different fs. For example, using the equation for the R_inputtotal calculation on page 18 of the ADAU1701 datasheet, to get a 1 Vrms full-scale input voltage, you will need the following resistor sizes (total resistance, including 2k internal resistor):
48 kHz 10 kohm
96 kHz 5 kohm
44.1 kHz 10.9 kohm
If you keep the input resistor value constant (assume 10k), then the full-scale input voltage at the different sampling rates will be:
48 kHz 1 Vrms
96 kHz 2 Vrms
44.1 kHz 0.919 Vrms
So, if you need to support different sampling rates in your design, then I would recommend that you have a way to support different input resistances. Otherwise, your design will end up with different full-scale voltage specs, depending on the sampling rate.
Maybe I am mis-understanding the spec sheet, but page 18 says you need to scale the ADC_RES resistor by frequency also. So, if you double the frequency with the same resistors, the full scale current will go from 100uA to 50uA since the ADC_RES will by 2x recommended. Therefore the input resistors will have the same full scale input since they need only half the current.
Or maybe I don't understand the switched capacitor front end well enough (most likely)? I little more meat in the spec would be nice for this section.
I don't have an adau1701 eval system (or any with analog in's) to simply try this and could be easily verified or shown incorrect (I have a 1940 based devel).
Thanks for the help and really appreciate this new forum.
Yes, the two resistors do scale with the same factors. However, note that if you keep the resistors constant, the SNR of the ADCs will decrease by about 6 dB when you switch from 48 kHz to 96 kHz, since you only have half of the full-scale input current.
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