LMH3401EVM: Applying a DC shift with a differential amplifier

Brandon Buckley

Part Number: LMH3401EVM
Other Parts Discussed in Thread: LMH5401

Hello,

I am interested in amplifying a single ended DC coupled RF signal, converting to a differential DC coupled RF signal with a differential amplifier. The DC component of the input signal will be relatively small (<100 mV).

We are wondering whether it would be feasible to shift the DC voltage of the output RF signal by applying a different DC voltage to the unused input power of the differential amplifier as such:

Specifically, the VDC will be different from the DC component of the RF input, so as to shift the DC component of the output signal.

I can foresee two issues with this design: 1) non-zero common mode voltage, 2) RF impedance mismatch on the negative input terminal.

Is there a proper way to design this? Or is this type of design fundamentally infeasible?

Thank you.

over 4 years ago

0 Hasan Babiker31 over 4 years ago

TI__Genius 15555 points

Hello Brandon,

Your image wasn't properly attached.

Best,

Hasan Babiker

0 Brandon Buckley over 4 years ago in reply to Hasan Babiker31

Prodigy 135 points

Hello Hasan,

Thank you. Hopefully this worked.

-Brandon

0 Michael Steffes over 4 years ago in reply to Brandon Buckley

Guru 48395 points

So Brandon, the FDA operates as two loops - for more flexibility you should look at the LMH5401 FDA,

The output CM voltage is controlled by the CM input voltage independentally of the signal inputs. You scheme looks like it is intended to to reset the output differential offset voltage which is fine, the VDC source needs to have a matched impedance broadband wise to what the + input sees looking back,

As for impedance matching, running a single ended input has an interesting advantage with an FDA in that you can get an active input impedance match - much lower noise, I originally published all that in these articles, but it applies to all FDA's,

https://www.edn.com/wideband-matched-input-impedance-with-ultra-low-noise-using-the-active-match-capability-of-a-new-type-of-amplifier-part-1-of-2/

https://www.edn.com/wideband-matched-input-z-using-active-match-and-a-new-type-of-amplifier-part-two-of-two/

0 Michael Steffes over 4 years ago in reply to Michael Steffes

Guru 48395 points

So Brandon, this discussion is also going on with Alex Wargo, http://e2e.ti.com/support/amplifiers/f/14/t/909546

to continue

a good model of your source would be very useful - you say RF with DC offset - is the source DC coupled and what does its impedance look like across frequency? If you must buffer the source and the DC offset voltage, best to use the same type of buffer - but that might degrade your noise to use a buffer in the signal path

What gain and output DC CM voltage are desired - I am guessing this is headed towards an ADC - usually we put an RLC filter after the FDA to the ADC.

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LMH3401EVM: Applying a DC shift with a differential amplifier