We have designed the boards and we found that the Common mode 0.9v coming from ADC is not compatible to the differential driver(LMH5401) used.
LMH5401 needs min 1.4V as common mode and we have resistor divider path to generate the voltage required.
1. We Can Generate 1.4V and connect to VOCM pin of Differential Driver
2.Do we need to disable the VCM control using 0x102 and apply 1.4V on the VCM pin of ADC ? or we can leave this pin floating with 0.1uF decoupling cap.
You do not need to disable the AD9213 VCM pin, and you do not need to apply 1.4V to it. You can leave it floating, connected only to the 0.1uF capacitor.
How will you get the common mode voltage down close to 0.9V at the AD9253 analog inputs?
Thank you for the quick reply.
-- I cannot set the common mode close to 0.9V at AD9253 analog inputs.
-- As per the Figure 56 in the datasheet AD9253 can accept wide range of Common mode from 0.5 to 1.3
So I want to set 1.4V common mode and my swing is 1.6Vp-p i,e ADC inputs will be swinging from 1.4-0.4 and 1.4+0.4.
Is my understanding correct ?
Your understanding of the voltage swing is correct.
VCM = 1.4V is of course higher than 1.3V, but as you have observed your peak voltage with respect to ground will be 1.4V + 0.4V, which will not be higher than the supply voltage, assuming you are using 1.8V.
I hope it works out for you.
Yes we are using 1.8V and it doesn't exceed the supply value.
So don't we need to apply 1.4V on ADC VCM pin to indicate that we are feeding 1.4v common mode signal at ADC inputs ?
Correct, you do not need to apply 1.4V on the ADC VCM pin.