Switching power to an opamp

Question

I'm trying to switch off an opamp using the high-side MOSFET switch (Q4, Q5).

The issue is that after OpAmp_Vcc_Switch goes low, the voltage on the +VCC pin doesn't go exactly to 0, but stays at 1.8V, leaving the opamp operating.

Why is this happening and how can I fix it?

(The opamp I'm actually using is LT1490A, Q5 is SSM3J338R)

Here is the rating table for Q5:

Answer 1

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For the op-amp LT1490 in your text

You won't damage the op-amp by exceeding the supply at the inputs, so probably there is just a small current flowing. You should be able to just drive the DAC outputs low. If there is still a tiny current it might just be leakage through the p-channel MOSFET, though that should be very small (nA). If that is bothering you, drive the DAC outputs low and use a high value resistor across the power supply (maybe 20% of the Iq of the op-amp).

Edit: Because the 10V maximum gate voltage of the load switch (mentioned in the text, not the schematic) is being exceeded it's possible that Q5 is heating (due to the internal zener diode used on some MOSFETs to protect the gate) when on, and thus has much higher than normal leakage after being turned off.

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Below applies to the op-amp in your Schematic

You are exceeding the absolute maximum ratings- which state that input voltage cannot exceed the supply voltage rail at all. Your feedback resistors are reasonably high value, so differential voltage is probably not a problem but you may have damaged the input transistors by exceeding the supply voltage at the inputs. Adding a BAT54C dual small signal Schottky to the inputs might be a good idea.

Answer 2

It's entirely possible that we're looking at an xy problem here. So start by telling us what you want to (probably drop the output of this section to zero on command), rather than how to deal with your implementation.

If you want to zero the output, and the input to the next stage is a high impedance, then you could, for each signal, put 2 1k resistors in series on the output, then use an n-type to drop the junction of the two to zero. Like this

^{simulate this circuit – Schematic created using CircuitLab}