slew rate filter capacitors (post DAC opamp)

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Dimitree

Well-known member
Joined
Jul 26, 2011
Messages
131
I'm implementing a I-V converter like this schematic (taken from the AD1955 datasheet) to be used for a PCM1794 DAC, and using an OPA1678 instead of AD797.
I've read on the Douglas Self book (where a similar schematic is proposed, for the very PCM1794, but with NE5532 opamps) that C10 and C11 are there to "keep down the slew rate required at the outputs of the I–V converters".
I apologise if I'm using the wrong terms, since this concept is not clear to me.
For the PCM1794+NE5532, the book proposes 2200pF for the feedback capacitor and 820R for the feedback resistor.
It is clear to me that the value of the feedback resistor determines the output voltage, and the value is chosen based on the specs of the DAC and the desired output voltage (AD1955 has 8.64mA pk-pk FS output, and PCM1794 has 7.8mA)

but I don't understand if the capacitor is chosen based on the opamp, the DAC, the feedback resistor, or all of them.
Also, on the AD1955 there is a 100R resistor in series with the capacitor.

While I could simply copy the D.Self schematic, I'd like to understand more how that capacitor is selected, so if I need to raise of lower the output voltage, or if I need to change the opamp, I'm able to calculate the right value

thank you

Screenshot 2022-05-17 at 15.55.39.png
 
C10 and C11 are there to "keep down the slew rate required at the outputs of the I–V converters".

I think it is more likely that they are to ensure stability even though there is a capacitor on the inverting input (which adds phase shift in conjunction with the feedback resistor, which will cause oscillation if the gain is not rolled off quickly enough at high frequencies).

I don't understand if the capacitor is chosen based on...

Primarily the op-amp open loop gain curve, the capacitance of the DAC output, and the discrete capacitors added for filtering on the input.
These are some good TI white paper references for background:

Compensate Transimpedance Amplifiers Intuitively
Design Considerations for a Transimpedance Amplifier
Transimpedance Considerations for High-Speed Amplifiers

Those have a lot of reference to photodiodes, which is a common application for transimpedance amplifiers.
This is a little more directly relevant:
Design for a Wideband, Differential Transimpedance DAC Output
 
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