PCM1792A Output Impedance

Hi TI,

Is thera any answer to this ?. Even if it is not known, output impedance is dynamic or other, will putting a resistor in the output stage first opamp positive input have any effect on performance or reesponse ?.

Thanks and regards,

Richard.

Hi TI,

Is there any answer to this question - the Output Impedance of the PCM1792A ?. Thanks.

Regards,

Richard.

I don't know the answer to your question, but to say that the output current is 25% of peak is incorrect, the outputs are held at a constant level of -6.2mA, this translates to what would be the zero/middle point of a sine wave and everything swings about this value.

Hi Matt,

I would have thought that the current output will be zero mA for nil output - since the recommended output I/V converter is designed to provide 6.396volts for the full scale output.

I am assuming that the current flows positive (out of the PCM1792A) and negative (into the PCM1792A) for the same pin on the PCM1792A.

Hence, when there is no audio data entering the PCM1792A, there is no output and the current outputs are not sinking/sourcing any current.

Where as the NE5534 may be sinking current and i wanted to ensure that currents were balanced in both inputs of the NE5534.

Regards,

Richard.

This is incorrect, please look at the data sheet a little more closely, it clearly defines the centre current that represents the zero level of a bipolar signal as being -6.2mA. This corresponds to a constant DC offset after the I/V stage that is determined by the transimpedance set by the feedback resistor of the I/V opamp. After I/V conversion each half of the differential outputs, sit at the same DC offset, which the analogue signal swings about. One of the jobs of the following, differential-configured opamp, is to remove this DC offset. As the DC offset is common to both the +ve and  -ve sections it cancels out leaving you with a theoretically perfect zero volts DC offset. As the analogue audio signal on both the outputs is equal in magnitude but opposite in sign this adds within the differential stage and leaves you with the analogue audio signal only.

Hi Matt,

Thanks.

So if the output is a steady -6.2mA from each output pin, then using figure 36 of the datasheet we have 4.65volts output of the I/V converter.

With the 7.8mA peak to peak current for full scale output - this i calculate will result in the Negative Current output reducing to -2.3mA and the Positive Current output increasing to -10.1mA for full scale output.

So, would it be a reasonable assumption to expect that the idle output impedance is the reciprocal of the 6.2mA which is approximately 161ohms ?.

Thanks and regards,

Richard.

I do not know, but as far I thought that wasn't quite how it worked. I don't have any theory to back this up with except that I have seen wildly different output impedances for current output DACs. If what you are saying is true then the output impedance would be directly related to the output current, which doesn't appear to be the case. You'd need someone from TI to chime in.

On another note though, couldn't you simply try out what you're suggesting and see how it measures? I'd have thought that it would degrade the performance, otherwise TI would have included it in their reference design.

Hi Matt,

Thanks - on that note - TI did not state that the reference resistor on Pin 20 must be connected to ground plane using a via, rather than pcb track to pin 19 - so as to enhance performance. There is another thread on this forum where there is degradation in performance using the latter.

So i would anticipate that it may be possible to enhance performance from the reference design - but i did not want to commit to a PCB design if there is degradation. In addition, i do not have the relative test equipment - this is a hobby at the moment.

Thanks and regards,

Richard.

Yes that thread is my own and I'm still trying to resolve the issue. Altering the connection of the resistor on pin 20 did nothing to alter the performance, or rather I had it connected the right way originally and then made a PCB that had the potential for a lot of different ground connections and altering pin 20s ground connection didn't affect things too much, if at all. I am sure that it might be possible to better the measured performance of the eval board, however TI usually specs their products conservatively, with a you will get this performance if the circuit is implemented correctly, and most of the time you will actually exceed the specd performanced. I've noticed this on a number of their products, funny that the PCM1792/4 is giving me so many issues. This is also a hobby for myself, measuring equipment doesn't need to be anything more then a high performance sound card.