AD1937 ADC input

Hello ELEC23,

The IC is running with single ended power to simplify the design of the part and also the design of PCBs. The ADC can only go as low as ground and as high as its input voltage which is 3.3V. That is in a perfect world, is reality the converter is not able to go all the way to ground or all the way to AVDD but that is not super important to this discussion. 

So you want to have the converter sample audio. Audio goes positive and negative and you want the digital output to output a positive or negative digital number based on the audio coming in. If the audio is referenced to ground then when the audio goes negative there is no way for the converter to pick it up! You would only get the top part of the waveform. 

The solution is to bias up the DC level on the pin so when no signal is applied, the DC bias will be roughly AVDD/2. This we call the common mode voltage. This will produce a number that is close to zero. It is never perfect and all converters will produce a slight DC offset but it is close to zero. This is why it is a good idea to turn on the High Pass Filter in the actual ADC to get rid of this DC offset. 

So now that the pin is biased so that zero signal in will be sitting at around 1.5V, now the audio can make it go positive or negative from there. 

You actually do this with an OpAmp all the time without realizing it. You have bi-polar power rails, +/-12V for instance. So you setup a reference point in the middle that we happen to call ground. But it is 12V more positive than -12V power. 

So if you run the OpAmp on a single ended power supply you HAVE to setup a reference mid-way between the positive and negative power rails. 3.3V/2 =~1.5V.

We develop this common mode voltage internally and supply it on a pin in case you want to DC couple the OpAmps directly to the ADC input. So use the same reference that the ADC is using internally. By the way, you need to buffer it with a voltage follower. 

The other way is to cap couple the inputs so you can reference the OpAmps any way you like and have the ADC use its internal reference. I suggest you download the user guide for the evaluation board and look at the circuit we designed on that board. There is a jumper to allow you to choose DC coupled or not and you see the OpAmps are using the CM voltage as a reference. 

Here is an older post where I discuss this topic in mode detail:

(+) AD1938 CODEC Full-Scale Input Voltage (Differential) - Q&A - Audio - EngineerZone (analog.com)

Dave T

Hello ELEC23,

The IC is running with single ended power to simplify the design of the part and also the design of PCBs. The ADC can only go as low as ground and as high as its input voltage which is 3.3V. That is in a perfect world, is reality the converter is not able to go all the way to ground or all the way to AVDD but that is not super important to this discussion. 

So you want to have the converter sample audio. Audio goes positive and negative and you want the digital output to output a positive or negative digital number based on the audio coming in. If the audio is referenced to ground then when the audio goes negative there is no way for the converter to pick it up! You would only get the top part of the waveform. 

The solution is to bias up the DC level on the pin so when no signal is applied, the DC bias will be roughly AVDD/2. This we call the common mode voltage. This will produce a number that is close to zero. It is never perfect and all converters will produce a slight DC offset but it is close to zero. This is why it is a good idea to turn on the High Pass Filter in the actual ADC to get rid of this DC offset. 

So now that the pin is biased so that zero signal in will be sitting at around 1.5V, now the audio can make it go positive or negative from there. 

You actually do this with an OpAmp all the time without realizing it. You have bi-polar power rails, +/-12V for instance. So you setup a reference point in the middle that we happen to call ground. But it is 12V more positive than -12V power. 

So if you run the OpAmp on a single ended power supply you HAVE to setup a reference mid-way between the positive and negative power rails. 3.3V/2 =~1.5V.

We develop this common mode voltage internally and supply it on a pin in case you want to DC couple the OpAmps directly to the ADC input. So use the same reference that the ADC is using internally. By the way, you need to buffer it with a voltage follower. 

The other way is to cap couple the inputs so you can reference the OpAmps any way you like and have the ADC use its internal reference. I suggest you download the user guide for the evaluation board and look at the circuit we designed on that board. There is a jumper to allow you to choose DC coupled or not and you see the OpAmps are using the CM voltage as a reference. 

Here is an older post where I discuss this topic in mode detail:

(+) AD1938 CODEC Full-Scale Input Voltage (Differential) - Q&A - Audio - EngineerZone (analog.com)

Dave T