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DEM-PCM1793 output gain

Pierre Watts
Prodigy 155 points
Other Parts Discussed in Thread: PCM1793

Hi, on the DEM-PCM1793 board, I notice the following:

The analog output buffer/instrumentation amplifier is exactly the same circuit as shown in the PCM1793 datasheet with its 1.83 gain, and the devboard manual states the full-scale analog output is 2.1Vrms, which checks out given the 3.2Vpp DAC output.

However, on the dev board, there is an additional 1k RC filter between the DAC output and the buffer. Surely this additional source impedance to the buffer should drop the analog output gain? Given that the buffer has a 3.3k feedback resistance and 1.8k source impedance, when an extra 1k is added to the source the gain should reduce from 1.83 to 1.18, and thus the output voltage should reduce from 2.1Vrms to only 1.3Vrms?

  • 0
    TI__Mastermind 27920 points
    Hi Pierre,

    This low pass filter seems to have been added to reduce out of band noise on the output. This EVM is no longer available so there may have been changes after the fact but the designer of this board is no longer around and it seems that the EVM documentation was not updated.

    Justin