| Electronics > Projects, Designs, and Technical Stuff |
| Any reason I shouldn't use a TL431 as a virtual ground? |
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| Zero999:
--- Quote from: bd139 on November 15, 2018, 02:52:28 pm ---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. --- End quote --- 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. --- Quote from: imo on November 14, 2018, 10:18:13 pm ---The 431 can only sink the current, the 78L05 only source.. 358 buffer with 2x10k at its +inp could be the simplest solution.. --- End quote --- |
| bd139:
--- Quote from: T3sl4co1l on November 15, 2018, 06:41:58 pm ---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 --- End quote --- 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. |
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