Current limiting stability problems

[Remondepemon]

@void: What is your experence using a switched regulator for your DAC/ADC tasks? Will it be clean enough or is it better to set the switcher around 7V and after the switcher an good old 7805?

@dannyf: Yes, I was thinking about this, but do you have any schematics that proves that it will work.

I'm surprised that nobody can tell me how to use a transformer with a center tap to get two separate voltages. If I have 2x 9V, then I'd like to power my power supply output from the two coils (18V) and my digital part from the first coil only. My question is how to work out the diodes to get two voltages from the one transformer.

Regards,
Remon

[David Hess]

@void: What is your experence using a switched regulator for your DAC/ADC tasks? Will it be clean enough or is it better to set the switcher around 7V and after the switcher an good old 7805?

Keep in mind that linear regulators have decreasing rejection at higher frequencies so they are not a panacea for switching regulator noise.  Simple RLC filtering is better for high frequency noise.  Also switching regulators (and sometimes digital logic) may spew noise into other circuits via magnetic and capacitive coupling which no filtering or regulation will solve.  Sampling converters are especially vulnerable to high frequency noise that integrating converters will reject.

I am not sure if you were asking about using a regulator's output as a reference for a converter but that almost always turns out badly except if the converter is ratiometric in which case reference variation is rejected but the noise issue with sampling converters remains.

I'm surprised that nobody can tell me how to use a transformer with a center tap to get two separate voltages. If I have 2x 9V, then I'd like to power my power supply output from the two coils (18V) and my digital part from the first coil only. My question is how to work out the diodes to get two voltages from the one transformer.

This is trivial.  Normally a center tapped output would be used with 2 diodes to make a full wave output with the center tap grounded.  If 4 diodes are used in a bridge with a center tap grounded, then +/- supplies are available.  Since the transformer winding is floating, ground the - supply and then the + supply becomes +2V and the center tap becomes +V.

High power linear supplies may use the same configuration with SCRs replacing diodes to create a switched 2V and V outputs for different voltage ranges.

[dannyf]

do you have any schematics that proves that it will work.

No. But I would encourage you to think of a center-tap'd transformer as a transformer with two identical windings in serial. Thus, the output voltage can be either 1V or 2V, depending on if 1 winding is in or two windings are.

You can report back if it works.

[void_error]

@void: What is your experence using a switched regulator for your DAC/ADC tasks? Will it be clean enough or is it better to set the switcher around 7V and after the switcher an good old 7805?

I'm using going to use an external voltage reference for the DAC/ADC. There will be some additional filtering at the DAC output to keep things nice and smooth. For the ADC I'll probably do a rolling average over a few samples.