ADC INPUT

Question

Hi to everyone; 
 I was checking UG-045 Ev board user guide. in page 9 of that document there is a design in ADC section. They use from a jumper to bypass a capacitor and I think its for AC coupling of signal. Now my questions is that (I know that my question may seem like a beginner ) our ADC in codec is a single power supply one and so it cant support negative voltage,But in this schematic when we don't close JP12, that capacitor will block DC part of signal and our signal will be above and under of ground level and we will have negative voltage in our signal that ADC can not read it, so question is that why designer use that DC blocking capacitor in this schematic?As I know none of the AD193x series can not read negative voltage due their single power supply.

DaveThib · Answer

Hello ELEC23, 
 The ADC inputs are biased up to the common mode voltage. So the voltage on the ADC side of those caps will go up and down from the CM voltage and as long as the signal is not too high, it will not go negative. The ADC will clip before the signal reaches ground so if it does go negative it will be very much clipped. 
 
 Note that the OpAmps are biased up to a Vref voltage. If the Vref is the CM voltage of the ADC then the caps are not required. If a different Vref is used then they are required. So it all depends on the power supply of the Opamps and the signal levels expected and what the gain is of the input stage. 
 In this case the OpAMps are powered by +/-12V. So the best VREF will be ground so the clipping will be symmetrical and you can get maximum output level. 
 However, you might not want this. You might have smaller level signals coming in and you may not care about the positive side clipping earlier than the negative because you will never get there. Instead, you might want to omit the caps and run the amp directly to the ADC. In that case then the Vref needs to be the CM voltage. 
 In this eval board you actually can choose this Vref. 
 
 You can choose Ground, CM or the DAC FILTR. 
 This eval board allows for lots of experimentation and options. 
 Dave T

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ADC INPUT