COM pin use
Could you discuss the use of the COM pin in the ADAU1772. I see analog ref pins tied to it in some diagrams and tied to 10uF caps to ground in other diagrams.
hardware
adau1772
Integrated/Special Purpose A/D Converters
Audio CODECs
Hello bklein,
It is the CM pin, this is Common Mode voltage reference that is used by the ADCs and DACs. It is brought out to be able to place a large cap for filtering. Since it is there it is a handy place to terminate a pin that will normally sit at the CM voltage like one of the ADC inputs. If you are not using the ADC it is best to connect it to CM pin rather than ground. If you ground it there will be current flow to ground.
Here is the explanation in the datasheet:
Dave T
Figure 75 in the datasheet shows the ref pins tied to CM, but the eval board has them tied to ground via individual 10uF caps. Very similar circuits but done differently....
Hello bklein,
Yes, there are a couple of ways to do this. I guess more than two. The best way for little risk of crosstalk and for the CM to be loaded down, would be to either use a cap to ground on each ref pin. This is a bit more expensive and takes a little space. Another way is to use an Op-Amp as a voltage follower to buffer the CM voltage from the CM pin and then use this buffered version to drive the ref pins or just use some larger caps on the CM pin and just tie the ref pins to the CM pin. This last way is the lowest cost and what is usually done.
Usually, you will not be using those ADC channels and turning off the PGA and setting the gain to minimum. This will help to not draw any current from the CM pin. This discussion is going into an entirely new set of topics for interfacing the ADCs... The Ref pin is tied to CM internally but very weakly. Some people tie it externally but still use the ADC minus input. When doing that there is some risk of crosstalk between channels and when adjusting the gain of the PGA the ref pin impedance slightly changes and can draw a little more or less current from the CM pin. This can slightly skew the CM voltage which will change the gain of all the ADCs so changing the gain on one will make a very small change in gain on other channels. Some people have wanted to be precise with the gain down to hundredths of a dB. If your application is that sensitive to gain changes then buffer the CM voltage if you are going to use it or use individual caps to ground. The impedance of the CM pin voltage source is not a low impedance so it does not take much current to throw it off.
Dave T
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