ADAU1772 How to use analog input reference pins (AINxREF)

Hello bklein,

Three use cases,...

Tied to ground through a 10uf cap

Tied to CM pin

Left floating

Left floating will work fine but it leaves a pin open terminated very weakly so it is susceptible to RFI getting in or other noise. So it is not the best solution but certainly low cost. 

Tied to CM pin is good but there is a possible problem. If you have more than one input signal then there will be some crosstalk between the channels. The CM pin voltage is driven by a fairly high impedance source so a signal in one channel can still be felt on the CM pin and the other channel can pick it up. Increasing the size of the CM cap to like 47uf can help with this and depending on the application this might good enough. If you need better separation then you can use an Op-amp in voltage follower configuration to pick up the CM voltage and drive the unused pins. If you are using other amplifiers that need to also use this reference then this may be a good solution. If it is just to tie two pins to the CM pin then it is an expensive solution. 

10uf cap to ground is a good middle ground here. It does help with the crosstalk issue and does not load down the CM voltage. There are plenty of 10uf small ceramic caps available now that makes this a reasonably low cost solution. 

In the end the decision will be governed by how many channels you have to tie and how many signals you have and the tolerance for crosstalk in your design. 

Hope this helps.

Dave T

Hello bklein,

Three use cases,...

Tied to ground through a 10uf cap

Tied to CM pin

Left floating

Left floating will work fine but it leaves a pin open terminated very weakly so it is susceptible to RFI getting in or other noise. So it is not the best solution but certainly low cost. 

Tied to CM pin is good but there is a possible problem. If you have more than one input signal then there will be some crosstalk between the channels. The CM pin voltage is driven by a fairly high impedance source so a signal in one channel can still be felt on the CM pin and the other channel can pick it up. Increasing the size of the CM cap to like 47uf can help with this and depending on the application this might good enough. If you need better separation then you can use an Op-amp in voltage follower configuration to pick up the CM voltage and drive the unused pins. If you are using other amplifiers that need to also use this reference then this may be a good solution. If it is just to tie two pins to the CM pin then it is an expensive solution. 

10uf cap to ground is a good middle ground here. It does help with the crosstalk issue and does not load down the CM voltage. There are plenty of 10uf small ceramic caps available now that makes this a reasonably low cost solution. 

In the end the decision will be governed by how many channels you have to tie and how many signals you have and the tolerance for crosstalk in your design. 

Hope this helps.

Dave T