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Differential pair Common Mode Shift

I have a differential pair feeding to an ADC via a 50 ohm termination resistor. The common mode voltage is .75V higher than the mid span voltage of the ADC input.

What is the best way to shift the CM voltage down and still maintain the 50 impedance?

Thanks

Reply to
Richard
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Full differential op-amp. Also provides a level of isolation between ADC and whatever nasties might happen upon your pair (ESD, etc.). If you only need RF, a transformer would do nicely.

Tim

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Reply to
Tim Williams

Can you post a schematic?

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

"best" depends on a lot of factors.

Do you need to be DC coupled?

Do you need to maintain DC offsets precisely?

Do you need to maintain full signal strength?

Is the circuit price sensitive? Board space sensitive?

What bandwidth do you need to maintain?

"Best" answers could range from putting some capacitive coupling in there, to using level-shifting attenuators, to full-on high-speed op-amp circuits. It all depends.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

And... Is it differential to the ADC? Sounds like it. If so, easily solved, add a common mode resistor above the differential loads.

Though we need more info from the OP, that could restrict the dynamic range. ...Jim Thompson

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Reply to
Jim Thompson

Thank you all for the help. Since I have posted this we have switched ADCs and now the common mode voltage is close enough to the mid span of the ADC.

Thanks anyway, I appreciate it.

R
Reply to
Richard

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