Post Go back to editing

Category: Datasheet/Specs

Hello to every one, I hope you doing well.

I need some help about AD1937 datasheet. While reading the datasheet of this IC, I came across common-mode voltage topic and now I need your help to understand it better. Common mode voltage is discussed on page 15 of the IC 1937 datasheet. What is the purpose of this voltage? Should our input signal have a common mode voltage? Is it necessary that our input voltage has DC offset? In the following, it is mentioned that this voltage can be used to bias external op-amps, and as mentioned, this voltage is around 1.5 volts. Does the data sheet mean that we can use this voltage to bias vcc of op amps with this voltage? I think this voltage is a little low to bias the op amp. In the parameter table of this IC, it is written that the input signal of this IC can be 2.05 V RMS, in other words, its peak voltage can be 3 V. Now is this 3 volts considering ground as crossing zero? Or the zero crossing point of our signal will be considered 1.5 volts and our peak voltage can increase to 4.5 volts?

• 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