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OPA1612: low limit of DAC output impedance

Brian Han1
Intellectual 530 points
Part Number: OPA1612
Other Parts Discussed in Thread: TIPD177, PCM1690

Hello All,

I have been making a circuit based on the TI reference design TIPD 177 and I have a question regarding the circuit.

Attached is my schematics and it's mostly the same as the reference design.

I'm paralleling the differential current mode outputs from the DAC to improve the numbers and the summed output impedance of the output is around 50ohms. But the manufacturer said to be careful when connecting the paralleled outputs to the I/V stage opamps because it could be hard for op amps to keep the THD low. From my experiences and readings, it's best to have low output impedance from the DAC and high input impedance from the amp. Even the Voltage noise spectral density curve in the OPA1612 datasheet figure 32 shows lower noise when the source resistance is low.
The supply voltages are +/-12V and both are capable of supplying up to 1A currents.

Is this something that I just have to try out ?

  • 0
    TI__Guru* 93105 points

    Hello Brian,

    I read through TIPD177 and it appears to cover all electrical aspects of the current output DAC and high performance audio op amp application. When it comes down to subtle differences in noise and/or distortion performance in a very high performance audio systems good engineering practices as outlined in TIPD177 must be fully applied. Once they have been thoroughly thought through and applied precise measurements are the only way to determine if the predicted performances have been achieved.   

    It is not clear what you mean by paralleling the DAC differential current mode outputs together. Since differential outputs implies the two outputs that differ in phase by 180 degrees they can't simply be tied together. It would be helpful if you could more fully describe or illustrate what you intend by paralleling the differential current mode outputs.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Intellectual 530 points

    Hello Thomas,

    Sorry if I wasn't clear in my description. By parallel, I meant tying + signals and - signals together to increase the signal level.

    It's a very rough diagram using PCM1690, and each of the ch1+, ch1-, ch2+, ch2- will be connected to the input of OPA1612 and each will have DAC output resistance of 50ohms each.

    I'm not experienced so much with the op amps, do you have a pointer to a good documents, readings that could explain the stability and noise made in op amps?

    And please don't reference the values in the attached schematics. Resistors and capacitors are not optimized to my design. I just added it to know where to be cautious.

  • 0 in reply to Brian Han1
    TI__Guru* 93105 points

    Hi Brian,

    Okay, now I see how you intend to connect the DAC outputs. I read through the PCM1690 datasheet and didn't find any mention of paralleling the DAC outputs. It does indicate in section 7.3.1 - Analog Outputs, "The PCM1690 includes eight DACs, each with individual pairs of differential voltage outputs pins." Remember voltage outputs are low impedance outputs and I do not known how connecting the like DAC voltage outputs in parallel will affect performance, noise and distortion compared to a single DAC output driving the OPA1612 inputs. I suppose that is what you will learn when you try the connection and make the measurements.

    TI has a very extensive on-line training series called TI Precision Labs - Op amps, that covers the many aspects of the op amps and their performance including noise and stability. I think you will find the series very helpful and informative. Here's the link:

    https://training.ti.com/ti-precision-labs-op-amps

    Regards, Thomas

    Precision Amplifiers Applications Engineering