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

[Zero999]

This is going to have a DC socket on it. It needs to be able to run off battery or 12V DC.

If I set the rail at 2.5v approx with a TL431/whatever, the supply voltage change will be in the order of 50nV. If I bias it with a voltage divider I need to accept at that time an instant change of virtual ground from 4.5V to 6V. There are two integrators this isn't going to make very happy.

If the LM358 is buffering it, then I think that's optimistic. The voltage change on the output of the LM358 will be much greater than 50nV, given a step change of supply voltage from 9V to 12V.

The +V rail will still have a poor PSRR, but it's impossible to know whether it's an issue without seeing the bigger picture. Please post a schematic.

[Zero999]

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.

[T3sl4co1l]

You gotta do



or



if you want to put capacitance on there, to reduce the high frequency impedance (i.e. reduce voltage ripple for a change in output current).

(Which, if you note the TL431 is basically an open-collector op-amp, these are identical.)

Tim

[nuno]

TL431 must be used with a resistor (or whatever equivalent), the resistor will source current.

The 431 can only sink the current, the 78L05 only source..
358 buffer with 2x10k at its +inp could be the simplest solution..

[bd139]

You gotta do



or



if you want to put capacitance on there, to reduce the high frequency impedance (i.e. reduce voltage ripple for a change in output current).

(Which, if you note the TL431 is basically an open-collector op-amp, these are identical.)

Tim

This is what they do in the HP546xx scope internal references. Totally agree and thanks for the tips.

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.

[nuno]

(...)

On a side note, you should put a link to your datasheet in your forum signature

[iMo]

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.

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]

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.

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..

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.

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.

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  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

[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.

[bd139]

Yes indeed. The more questions you ask the more questions are asked of you and the more you learn so it’s a win win all around.

[iMo]

My naive simulation - Vbat pulsing 8-9V, 1us edges, 100Hz freq.
500ohm||100uF(alum) simulate an "load".

[bd139]

Interesting. Just waiting on a new fan for my AWG today and I will use that as a crappy source impedance power supply and see what happens

[iMo]

And the variant N2:

[StillTrying]

It's a DC input jack which switches from a PP3 battery to internal supply.

Oh dear. Keep an eye on all of the different GNDs.

[iMo]

And finally four variants with 10mA current sources as "the loads" switched on/off at 100Hz, 1us edges, the upper and lower current source shifted by 2.5ms (such it simulates 4 load situations - 0/0, 0/10, 10/0, 10/10mA). Vbat is 9V, an ideal source.

[Zero999]

And the variant N2:

Why not use a single 82R, rather than 100R and 500R in parallel?

Using two capacitors, one to the positive and another to the negative rail makes matters worse, as far as supply rejection is concerned. The capacitors bypass dominate the LM358's output stage and the TL431, forming a capacitive potential divider. Try it again without C2 and C3. The LM358 might also oscillate when its output is connected to such a large capacitance, which is something the SPICE model might not show.

[iMo]

And the variant N2:

Why not use a single 82R, rather than 100R and 500R in parallel?

For clarity. 100R and 1K have been chosen randomly.

Using two capacitors, one to the positive and another to the negative rail makes matters worse, as far as supply rejection is concerned..

Here you are:

[Zero999]

And the variant N2:

Why not use a single 82R, rather than 100R and 500R in parallel?

For clarity. 100R and 1K have been chosen randomly.

Using two capacitors, one to the positive and another to the negative rail makes matters worse, as far as supply rejection is concerned..

Here you are:

I didn't say remove both the capacitors going to positive and negative. Reread the post.