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SeePeeAar , 02-04-2024, 08:46 AM

Hi, I'm currently working on an Audio-DSP board, while referring to Phil's mixed signal hardware design course on Fedevel Academy and Phil's Lab video#78.

In the FA course, Phil uses an op-amp buffer at the ADC input, citing low input impedance of the ADC as the reason (about a couple kilo ohms), which makes sense. However in video #78, an audio codec (CS4270) is used, for which the input impedance is already 300kOhms and a buffer is still used. While implementing a buffer wouldn't hurt, I want to know if it really contributes to the performance, or would the codec still have worked just fine with raw, unbuffered (but filtered) input signals?

Also a doubt unrelated to the buffer section --- I'm deciding on using the same codec (CS4270), but there's one thing I don't understand looking at the datasheet. What DC level does the ADC expect the voltage to swing around? Usually the input signal has to be biased at half of an externally provided Vref, to allow full swings but the recommended analog input design for this codec suggests AC coupling the input with a capacitor, which would mean the input signal would be about ground...is there something I'm missing or is it really how it's supposed to work?

Forgive me if it's something obvious, I'm still pretty new to this.

return0 , 02-04-2024, 08:09 PM

If you want to do things the right way, then yes. You have to use a buffer followed by a low pass filter. They ares basically used to allow the SAR ADC to settle the input voltage so that the ADC can perform the convertion. They can also be used to improve the dinamic range, or other improvements. There are plenty of app notes from top level companies about this subject. I'll give you some useful links that you can read:

A little bit of theory first:
https://www.changpuak.ch/electronics/Choosing_OPAMP_to_drive_ADC.php
https://www.embeddedrelated.com/showarticle/110.php
https://www.digikey.com/en/articles/analog-basics-part-5-tackling-difficult-input-driving-issues-for-the-sar-adc

Some App Notes pr App Reports:
https://www.ti.com/lit/ug/tiduck3a/tiduck3a.pdf?ts=1707062717916
https://www.ti.com/lit/an/sbaa314/sbaa314.pdf?ts=1707027820102
https://www.ti.com/lit/an/slaa416a/slaa416a.pdf

Example of an online calculator:
https://tools.analog.com/en/adcdriver/#adc=AD4000&cext=1.8e-10&driver=ADA4805-1&frequency=2000&gain=1&inputSettlingChart=1&multiplexed=false&nTab=1&oss=false&rext=200&rf=0&sTab=1&samplingRate=2000000&showAdcVariant=false&showDriverVariant=false&sourceType=1&tab=1&thd=200&topology=3&vSupplyNeg=-0.4&vSupplyPos=6&vref=5&wbNoise=0