AnsweredAssumed Answered

AD6643 driving circuit

Question asked by cocopa on Nov 17, 2014
Latest reply on Nov 18, 2014 by cocopa


It's the first time I'm designing a board with fast ADCs and I would like some assistance.

The signal to be digitized (single ended) is coming from a 50 Ohm cable, it has pulse width about

10-15us and a repetition rate in order of Hz. Its amplitude ranges from a few mVs to 600mV

ac coupled (or in case of a negative pulse around -1mV to -600mV).

I'm planning on using the differential input configuration provided in page 22, figure 42 of the

AD6643 datasheet with ADA4930-2 @+/-5V. First of all, I think there is a typo in the circuit,

the inverting output of the op. amp shouldn't be feedback to the non-inverting input and the

non-inverting output to the inverting input?

Second, since the schematic is not detailed the FB pins of the op. amp (pins +FB and -FB)

are omitted, where should i connect them?

Last, ADA4930-2 has 2 Vocm pins, one for each amplifier but AD6643 has only one VCM pin,

I guess there is no problem connecting both Vocm pins to VCM of the ADC, right?

Any other advice and things i should consider in schematic design or layout are welcome.

Thanks in advance.