Common mode voltage for AD9467

Hi AD9467 

Thanks for your interest in AD9467.

The common-mode voltage refers to the DC bias of the VIN+ and VIN−.

For best dynamic performance, the source impedances driving VIN+ and VIN− should be matched such that common-mode settling errors are symmetrical.

I have ever used AD7765, a delta sigma ADC with differential input. The common mode voltage is defined as (VIN+ +VIN-)/2. And I can use different DC bias for VIN+&VIN-. And it works. Can you advise why AD9467 use different common mode voltage concept from other delta sigma ADCs----a same DC bias voltage for wither VIN+ or VIN-? 

Thanks

Hi JAlipio,

Any response?

Hi AD9467 

The AD9467 uses internal input common-mode as the default configuration.

But you can also disable the internal input common-mode buffer through register address 0x2C bit 2, allowing a dc-coupled input for which the input common-mode voltage can be set externally. Otherwise, use an ac coupling capacitor in series on each of the analog input for ac coupling.

Also note that the SNR and SFDR performance of the ADC depends on the analog input common-mode voltage.

Hi JAlipio,

Thank you very much for your reply.

I would like to turn off the internal CMN because my analog input voltage is only with positive voltage. If using the same CMN, the differential input range will be halfed and the resolution is lost.

Thank you for your reminding the CMN vs. SFDR curve.

By using the external CMN, I will use different DC bias(1.44V & 2.88V respectively) for VIN+ and VIN- to achieve the differential input voltage as +/-1.25V. Although they are different, the CMN should be equal to (VIN+ + VIN-)/2=2.15V, which can have best SNR & SFDR. See below for VIN+ & VIN- tansfer function.

Transfer function:

VIN+=1.44+gain*Vinput; VIN-=2.88-gain*Vinput

Pls correct me if I am wrong.

BTW, I do not know how to change my user name "AD9467".

Thanks

John

Hi JAlipio,

My transfer function is based on CMN equal to (VIN+ + VIN-)/2. When using different DC bias, the CMN can still be 2.15V. But if CMN is referred to the DC bias voltage, then per figure 40 of the datasheet, the SNT&SFDR cannot be accepted. Pls confirm me for AD9467 if CMN refers to DC bias voltage or (VIN+ + VIN-)/2.

I checked lots of application note and per these application note, CMN refers to (VIN+ + VIN-)/2, not the DC bias voltage added to VIN+ & VIN-.

Appreciated your reply.

Thanks

John

Hi JAlipio,

My transfer function is based on CMN equal to (VIN+ + VIN-)/2. When using different DC bias, the CMN can still be 2.15V. But if CMN is referred to the DC bias voltage, then per figure 40 of the datasheet, the SNT&SFDR cannot be accepted. Pls confirm me for AD9467 if CMN refers to DC bias voltage or (VIN+ + VIN-)/2.

I checked lots of application note and per these application note, CMN refers to (VIN+ + VIN-)/2, not the DC bias voltage added to VIN+ & VIN-.

Appreciated your reply.

Thanks

John

Hi JAlipio,

Any response?