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OPA1632: Output symmetry, also VOCM drive.

Andy Peters
Genius 3945 points
Part Number: OPA1632
Other Parts Discussed in Thread: PCM4202, PCM4222, OPA1652, TINA-TI

  1. I am using the OPA1632 as an audio ADC (PCM4202) input driver, using the circuit shown in Figure 13 of the ADC data sheet, and also shown in Figure 14 of the OPA1632 data sheet. The input to the driver circuit is single ended, so the Vin- is grounded (before the 1k resistor). Simulating with both TI-TINA and LTSpice give me the same odd result -- the outputs of the circuit are asymmetric, and they are offset from the offset. See the attached image.

    The input to the circuit is a 1 kHz 10Vp (20 v p-p) sine wave. The obvious question -- is this a simulation artifact, or is this really how the circuit operates?

  2. PCM4202 data sheet shows the OPA1632 VOCM input being driven by the ADC's VCOM out buffered by an op-amp through a 1k series resistor. PCM4222 data sheet shows the OPA1632 VCOM input driven by the op-amp directly (no series resistor). The OPA1632's data sheet, figure 14, shows the op-amp directly driving the VOCM pin with no series resistor. What is the intent of the series resistor? In the image above, it is modeled with a 2.5 V voltage source buffered by an OPA1652 as a follower.  I noticed that if I include it in the simulation, the voltage at the VOCM pin drops from 2.5 V to 2.34 V. See the following image. This doesn't make sense -- what is going on here?

  • 0
    TI__Genius 15975 points

    Hi Andy,

    Sorry for the delay in reply.

    To answer your questions:

    1. I think this is really a simulation artifact rather than the circuit operating this way. I have verified in the TINA-TI model as to what you are seeing and will check with the modeling team about this.

    2. If you look in the PCM4202 datasheet for the VCOM circuit, the 1k series resistor is present to mainly isolate the buffer op-amp output from the 0.1uF cap mainly for stability. Unity gain buffers always run into stability issues while driving huge cap loads and it is always recommended to have a series isolation resistor for that purpose. Now in the PCM4222 or the OPA1632 datasheet, the 0.1uF cap is before the buffer amplifier close to the ADC VOCM output which means that the buffer output could directly drive the OPA1632 input without the need of a series 1k resistor.

    If you notice the functional block diagram in the OPA1632 datasheet, the VOCM input has two 30k resistor to the positive and negative supplies. If you apply KVL at the VOCM node of the OPA1632, you will end up with 2.34V for the VOCM.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Genius 3945 points

    Thanks for the reply!

    Regarding 1., the asymmetry, I have a few of the parts so I will build up a breadboard. The point of the exercise was to work out gain structure, so I can calibrate the analog input to what the converter outputs. Basically, to verify that the data sheet values would work as a user expected (+4 dBu analog input gives -12 dBFS or such).

    Regarding 2, I see what you are saying, and it makes sense.

    Regarding 3, I did see the resistors (intended to ensure that VOCM is at 0 V or whatever is mid-scale between the rails, so it need not be connected when that is acceptable). Doesn't this shift in output offset from the 2.5 V "typical" to 2.34 V, affect headroom? The PCM4202 full-scale input is 6.0Vpp, so the converter inputs are expected to swing from +1 V to +4 V centered on the +2.5 V. Driving from OPA1632, with the arrangement shown in the data sheet, we will swing (at full scale) from 0.84 V to 3.84 V, centered on +2.34 V, which means clipping on the bottom but not the top. This seems grossly wrong. I suppose, then, that the solution is to do what is noted in the PCM4222 data sheet, and put the filter cap at the converter filter, buffer that, and drive the OPA1632's VOCM input from the output of the buffer amp.

    And that might be why the discussion of the common-mode voltage output in the PCM4222 data sheet shows, in Fig. 39, VCOML/R with filter caps and series resistors driving the buffer which then drives OPA1632 VOCM. Perhaps a lesson learned?  Of course in Figure 50 of the PCM4222 data sheet, we see the 100 nF cap on the VOCM input, which is presumably driven by that buffer ...

    Thanks,

    -a