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• Resolved

Hello.

I’m looking for ADC driver for ADC12D1600 input. This ADC has differential input scale from 0.6 to 1 V and common mode voltage +1.25 V.

Gain of OpAmp should be between 2 and 4. Frequency band should be from DC to 1 GHz (-3 dB).

I’m going to choose between LMH6554, THS4509. Only one thing I don't understand:

Input common mode voltage for LMH6554 is ± 1.25 V. ADC output common mode voltage +1.25V.

So in Figure:

we see that for common mode voltage more than +1 V signal distortions of OpAmp are very high. The same thing is for THS4509:

Am I right to bother of this or not?

PS Of course signal distortion more than -20 dBc is not acceptable for me.

• Hi Deus,

You are correct about the effect of output CM on FDA (Fully Differential Amplifier) distortion and the fact that you get the best linearity if you use the plots you've shown to your advantage. So, I've attached a circuit for the LMH6554 which skews the supply voltage so that output CM is exactly 1/2 way in the middle of the V+ and V- supplies. The distortion plot you've shown is with Vs= +/-2.5V supplies and that's why the sweet spot is around 0V (center of the range). So, with output CM of 1.25V (ADC requirement), if you need DC coupling throughout, you could use V+ = 3.75V, V-= -1.25V to achieve centering. With AC coupling, you have more options available. Application Note AN-2177 (SNOA565) delves into some of these possibilities:

http://www.ti.com/litv/pdf/snoa565

0763.LMH6554 SE to Diff ADC Driver with DC Offset 3_15_12.zip

Thanks,

Hooman

• In reply to Hooman Hashemi:

Thank you very much.

Im intended to use LMH6554 with skewed supply voltages and input common mode offset.