Other Parts Discussed in Thread: PCM1792, OPA134, , OPA1612
Figure 37 in the datasheet for the PCM1792 shows a typical OP-AMP setup to first convert the current output of the DAC to a voltage (U1 and U2) and then to add the two differential paths together (U3).
Based on the datasheet for the OPA134 family, it would appear higher resistor values are clearly better in terms of distortion. Low pass filtering can be set at the right 3dB point by suitable selection of the capacitors. Of course, the resistor values must not be too excessive, otherwise offset issues and possibly also noise will start to become an issue.
The choice of resistor 820R for R1 and R2 is necessary to reach the output voltage swing (4,5V). There is little room for further increase here – otherwise, the large swing will itself result in increased distortion.
However, when it comes to the summing amplifier U2, why use such low values for R3 to R6? Surely these could be set to 2k2 for example, with C3 and C4 adjusted to 330pF to keep the 3dB at 220kHz.
This leads to the following questions:
1. In general: What resistor values are optimal in an ultra low distortion / ultra low noise audio application using the OPA134 family?
2. Would the 2k2 / 330pF combination around U2 not result in a lower overall distortion – at least on paper?