AD6643 driving circuit


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.

  • 0
    •  Analog Employees 
    on Nov 17, 2014 11:43 PM

    Hi Yiannis,

    You are correct that there is a type-o in the ADA4930 circuit on the AD6643 data sheet.  The amplifier should be connected for negative feedback (-output to +input and +output to -input).  The +FB and -FB pins are what you use to connect the feedback into the circuit.  In the example in Figure 42 on page 22 of the AD6643 datasheet you connect the 33ohm resistors up to the output pins of the ADA4930 and connect the 200ohm resistors between the input pins and the FB pins.  I've attached an example circuit for you.

    The VCM pin of the AD6643 can be used to input to both the Vocm1 and Vocm2 pins of the ADA4930-2.  Please make sure to route the VCM lines symmetrically to avoid introducing even order distortion and degrading the SFDR of the ADC.  Also, please make sure to use one 0.1uF decoupling capacitor at the VCM pin of the ADC and one 0.1uF decoupling capacitor at EACH Vocm pin of the ADA4930-2.

    You may also want to include a series resistor in each path of the VCM node to help with isolation.  You can refer to the evaluation board schematic at for further details.