Originally Posted by damiangt3
I'm hoping to make a start on the I/V and I have a question on the selection of resistors/capacitors on the -ve feedback loop. I can see on Jens' design he's using 470pf and 1.27k, on ESS application note 100pf/272 and on Mr Slim's 330pf/820. How are these determined?
The resistor is chosen first to give a desired I/V stage output voltage swing. The value is chosen according to ohm's law, R = E/I, where E is the peak output voltage and I is the maximum dac output current. There is also a DC offset that has to be added which is Vref = AVCC/2.
The cap should be NPO or C0G, and is chosen to give a particular time constant in combination with the resistor. tau = RC. That sets the first filter corner frequency.
There are some tradeoffs involved with the resistor value. If we keep it small-ish (such as 820-ohms for ES9038Q2M), the fact that there is a Vref offset voltage can used to bias the I/V amp to always run class-A. Since Vref=AVCC/2=1.65v for Sabre dacs, if we keep the I/V output swing to less than +-1.5v, the opamp output will never go below ground and will be running class-A.
On the other hand we could use a bigger resistor to get more voltage output from the first stage, if we think that would be of more value for our needs.