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Any reason I shouldn't use a TL431 as a virtual ground?
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nuno:

--- Quote from: bd139 on November 15, 2018, 07:56:55 pm ---(...)
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On a side note, you should put a link to your datasheet in your forum signature :D
iMo:

--- Quote from: Hero999 on November 15, 2018, 06:37:30 pm ---I think a bare TL431 with a large smoothing capacitor, will be better than adding an LM358.

I've just done an LTSpice simulation showing the step response of the LM358 when the power supply voltage is changed from 9V to 12V and the output current from +2mA to -2mA and back again. Both scenarios generate pretty big transients on the output. I wouldn't trust the SPICE model though: do some bench tests.

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The Spice models must not be exact, but I can hardly imagine a 100ns pulse edge in a system OP is elaborating (an another 358 generating some 150Hz sine I/Q stuff), however. LM358 itself is an 1MHz at unity gain opamp..
Zero999:

--- Quote from: imo on November 15, 2018, 09:34:48 pm ---
--- Quote from: Hero999 on November 15, 2018, 06:37:30 pm ---I think a bare TL431 with a large smoothing capacitor, will be better than adding an LM358.

I've just done an LTSpice simulation showing the step response of the LM358 when the power supply voltage is changed from 9V to 12V and the output current from +2mA to -2mA and back again. Both scenarios generate pretty big transients on the output. I wouldn't trust the SPICE model though: do some bench tests.

--- End quote ---
The Spice models must not be exact, but I can hardly imagine a 100ns pulse edge in a system OP is elaborating (an another 358 generating some 150Hz sine I/Q stuff), however. LM358 itself is an 1MHz at unity gain opamp..


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I don't know what power supply the OP is using, but a mechanical switch to change from 9V to 12V can create some surprisingly high frequency components. In real life I wouldn't expect a 100ns step response, but a bunch of pulses, as the switch bounces, with ringing causing some over and undershoot of the set supply voltage. The LM358's bandwidth makes its PSRR lower, at higher frequencies and therefore prone to interference when the power supply voltage is rapidly changed. I agree, about SPICE models not representing reality, hence my recommendation to bench test it.


--- Quote from: bd139 on November 15, 2018, 07:56:55 pm ---
Edit: regarding step response, the supply is very heavily decoupled so there isn’t a massive ramp up. I will test this on the bench.

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How about when the power source is switched from 9V to 12V or will the never happen when the circuit is running?

What about a plain old 3.3V zener diode? It may not have so good regulation, but it will be faster than the TL431?

By the way, I don't have a problem with the TL431. I just think there are probably more efficient solutions.
bd139:
It's a DC input jack which switches from a PP3 battery to internal supply. There is a diode or gate between the internal battery and the supply. This feeds a 10uF tantalum and there is local decoupling per amplifier. There is no requirement for it to survive this change but it would be nice to understand the situation in future in case I build something that does need to survive it.

Like I said this is a one off. The main objective here is to get the project completed versus make it totally ideal. I was just making sure originally that I hadn't done something utterly stupid.

Incidentally I have used an LM385-2.5 in it now as I killed my last TL431. This has a 47k resistor so I'm eating about 100uA on that and the buffer opamp is free because it's left over from the 9 other opamps in the design.

When I said 50nV earlier I meant 50uV  :palm: half asleep.

I'll post the completed unit and schematics when I get some time to finish it. It's nothing special or exciting merely a sales tool for something :D
Zero999:
The LM385-2.5 is a superior solution to the LM431.

No you're certainly not doing anything stupid. You know asking asking a simple question here will result in the idea being heavily scrutinised from every angle. :)
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