AD1938 with asymmetric input

Hello,

i was wondering if it is possible to use the input of the AD1938 with an asymmetric input and - for cost condition- with very few input component. The idea is to use only ADC-IN-Pos of the codec and load ADC-IN-Neg to ground. Of course, I've got 1/2 of the input range in that case but it's not the problem.

25e9028b1bc3d97efd927b563eabaa9a.tiff

I try it and it's working but i've got a voltage drop on the input of the ADC i cannot explain. in this case R2=R3. Without ADC-IN-P, the divider bridge works fine. with ADC-IN-P, i've got only 80% of the expected value on R2. Am I doing completely wrong ?

thanks for your advice.

  • After reading the datasheet, it was obvious, the AD1938 has 14kohm input and with R2, the divider is different. it can work but it's better with an operational amplifier in front to adjust the impedance.

  • 0
    •  Analog Employees 
    on Dec 4, 2015 11:14 PM

    Hello Norman,

    This is not the right way to do this. The ADC pins are sitting up at the common mode voltage so this is upsetting that which will cause level issues and DC offsets.

    There are a few ways of doing this.

    1) this is the easiest. Just tie the "N" pin to the CM pin on the codec. The Common mode pin is well decoupled so it will be quiet. The only possible issue is crosstalk. The CM voltage source is rather high impedance and if you have other loud signals on another channel they may crosstalk a little. The decoupling caps help to prevent it so you may be alright.

    2) Tie the "N" pin to ground through a coupling cap. The issue here is that there may be noise on the ground but it also requires a cap.

    3) Do like number two and tie through a coupling cap to ground but this time tie it to ground as close to the source as possible. This will take advantage of the differential input and give you less chance of picking up hum due to ground differences from the source vs the codec. The down side of this way is there is a better chance that there will be ground noise. It also may pick up some RF and then that will be aliased in the ADC and may cause problems. So if you do try this you may want to include the RC filter right at the input of the ADC like is shown in the datasheet. No guarantees that it will be enough.

    So I think the simplest is to tie it to the CM pin and move on.

    Yes, you will not be able to drive the ADC to full scale. -6dBFS is the best you will get.

    Thanks,

    Dave T