Following up interesting discussion here with John Birckhead (@JohnBirckhead), I decided to ask separate question on the issue.
Here's my circuit
The circuit includes UDA1334BTS, DAC powered from +3.3 V, connected using its typical application diagram (see page 16, fig. 8) and AD8512 op-amp, powered from ±12 V. Initially we were troubleshooting the problem of my circuit not working with two new op-amp chips, with chips looking like defective or damaged.
John proposed that while circuit looks good, there's a problem when it is being turned off.
Capacitors C66 and C57, which are of 47 uF value, discharge too slowly through DAC output and 220k resistors of respective channel; when system is turned off, ±12 V supply is removed very quickly, but potential capacitors are charged to remains for next several seconds, violating condition explained in datasheet that
[AD8512] have internal protective circuitry that allows voltages as high as 0.7 V beyond the supplies to be applied at the input of either terminal without causing damage.
Let's look what is going on in my circuit:
We can clearly see that power goes off exponentially (red), at the same time op-amp's input is about -1.2 V.
However further datasheet states that
For higher input voltages, a series resistor is necessary to limit the input current. The resistor value can be determined from the formula (Vin - Vs)/Rs ≤ 5 mA.
I am confused here - what matters - over-voltage, or over-current? In my circuit there's series resistor of 100 Ohm, when power if completely gone current flowing through resistor (approximately) ( -1.2 V - 0 ) / 100 = 12 mA, higher than required by datasheet.
John has proposed putting negative voltage limiting diodes (manually drawn connections on the circuit diagram), these diodes would make caps discharging faster.
I want to know your opinion - is there any other way to mitigate this effect? I think I did not see many circuit diagrams having diodes before op-amps.
Probably it is a matter of tuning DAC's typical application circuit, C66 and C57 values (e.g. from 47 uF down to 1 uF), decreasing values of R29 to R44 (e.g. from 220k to 10k), and increasing value of series resistor (e.g. from 100 Ohm to 220 Ohm)?
As a side note, I have old computer with mixer (NJM4558-based)
and it also exhibits these issues when powering on or off, but, as far as I can see, voltage on its op-amp input goes down relatively the same rate as power voltage. In my case input voltage is too long outside of allowed range relative to power.