AnsweredAssumed Answered

Interface suggestion for AD9649

Question asked by Hinestine on Nov 18, 2014
Latest reply on Nov 18, 2014 by David.Buchanan

I have a customer that has asked questions about interfacing the OPA857 to the AD9649. The application uses an APD connected to the OPA857 ( TIA ). Here's their questions with my comments. I generated 2 options ( I'll attach schematics if I can - sorry this is my first time, not sure what i cvan and can't do here )

>>>>>>>>>>>>>>>>>>> customer question start


1) I am using a TI transimpedance amp, the OPA857 and I want to drive the input of the AD9649.  The OPA857 has some unusual characteristics.  The "OUT" is really single ended and OUTN is a reference set at 5/9 * VDD (=1.83V for 3.3V).  The AD9649 is powered by 1.8V (AVDD).  What I don't see in the AD9649 data sheet is the max voltages the inputs can swing to with a 1.8V supply, but on page 6 it specifies the test conditions as having a 2Vp-p differential input, clearly exceeding the power supply.  Can you point out a spec? table 1 of datasheet specifies the analog input span of 2v p-p with a 1.0v reference, guess he didn’t RTFM….


Though the OPA857 is not truly differential, it does provide some of the benefits of differential when using both OUT and OUTN. Not so sure…


2) The OPA857 with an APD driving it (APD connected to +V) will only swing negative.  I was planning on connecting the OUTN to the Vin+ and OUT to the Vin-.  Does that seem correct? I don’t think so…


3) In the configuration I mentioned above, do I lose one bit of resolution as VIN+ and VIN- would never cross? Possibly more…


4) There is a good bit of discussion in the AD9649 about using VCM to center the differential input.  What if it isn't centered (and it wouldn't be with the OPA857)? Asymmetrical rejection??


I also looked at the AD8015 TIA, but I didn't like that it required 5V and also I like the gain switching of the OPA857.


>>>>>>>>>>>>>>>>>>> customer question end

My recommended solution uses 1/2 of the OPA857 OUTN ( 1.83v/2 ) to drive the Vcm of a differential amplifier, AD8132 and convert the OPA857 OUT ( 1.83v-0v output range ) from a single ended signal to a differential output with gain/attenuation and inversion of the signal, then to the AD9649. My second option is a direct connect of the  OUTN/2 to the Vcm of the AD9649 without the buffering / conversion of a diff amp, OUT to - input, gnd to +.


Am I off base here? TI goes to a lot of trouble to generate the 1.83v, but as an old analog guy, i always try to put my Vcm in the middle of the signal range ( 1.83v - 0v ). Guidance appreciated.