Differential pair Common Mode Shift

[Richard]

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

 

[Tim Williams]

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

 

[Phil Hobbs]

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

Can you post a schematic?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[Richard]

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

--

| James E.Thompson | mens |

| Analog Innovations | et |

| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |

| San Tan Valley, AZ 85142 Skype: Contacts Only | |

| Voice480)460-2350 Fax: Available upon request | Brass Rat |

| E-mail Icon at http://www.analog-innovations.com | 1962 |



I love to cook with wine. Sometimes I even put it in the food.

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