Taming the ADA4817

I've seen opamps as current sources oscillate with out any series output resistors, maybe try 50 to 100 ohms. Especially if they drive a capacitive load like wire.

Cheers

Grin... that did seem pretty silly to me too... I'm thinking another power supply issue, but it's hard to tell.

George H.

Ahh yeah, one for each rail. Putting the filter before the noise source doesn't work.

George H.

I haven't looked at the schematic. I was thinking the capacitive multiplier was another circuit like the charge pump to up the voltage. I looked it up and it's a way to use an opamp to boost a capacitance. But in that case you need higher rails to run the opamp. So why not use a simpler dropping circuit?

Hopefully this is a more detailed description of my circuit since I can't post it. Mind you, after reading what you guys have said, yes there are some questionable design going on with the supplies and c-multipliers.

So I have a +5V_RAW coming into the board. That +5V_RAW then goes to a c-multiplier to make my +5V (for simplicity here, I'm just calling it +5V, but I know it's lower). That +5V then goes to a inverting charge pump to generate the -5V. And that's that.

In hindsight, it should have gone: +5V_RAW to inverting charge pump and then both -/5V go to their own c-multiplier and then I have cleaner supplies for my opamps.

I've never made a C-multiplier with an opamp... always just a bipolar transistor. It does drop ~0.6V and has an output impedance of something like the resistor/ beta (current gain). To me the circuit is more about reducing the effect of the resistance in the LP rather than increasing the capacitance.. so the name in unfortunate in that regard.

George H.

This very much sounds like motorboating.

To a GHz amplifier, 3.5 Mhz is the moral equivalent to low audio.

Joe Gwinn

You're no fun anymore. ;)

Cheers

Phil Hobbs

DC is a moving target, for sure. ;) Motorboating is an AC-coupled amp issue, though, whereas this is a TIA.

Cheers

Phil Hobbs

Use a low value resistor and a giant polymer aluminum cap in the c-multiplier.

Yup. Op amps aren't much use in high performance cap multipliers, because their supply rejection is inadequate.

Cheers

Phil Hobbs

or Palladium (Pd)

Jamie

Your explanation was great until I read "LP". What's an LP?

Sorry, Low pass... to me it looks like an RC low pass, that is "activated" by the transistor. Well that's the way I always draw it on schematics.

George H.

Well, I'll agree that resistive divider is not a good name either. (For a year or so it's refereed to as a Hobbs filter in one of my notebooks, I'm not sure that name will pass the ANSI. :^) (or whomever is in charge of naming things.)

George H.

If you had used LPF I would have known. Shortening it to just two letters seems pretty lazy... ;)

I've never seen this circuit before. Interesting.

Well, it was invented before I was.

Cheers

Phil Hobbs

cap multiplier is this: --+-------+ +----- | \ 7| [R1] \ / | ----- | | +----------+ | | [R2] === C1 | | ----+----------+-------- the load transient response is approcimately as-if C1 was multiplied by transisor's beta thus capacitance multiplier. the source transient reposonce has time constant R1||R2 times C1, so bigger is better.

It can also provide a means of active filtering.

However, in some instances you may need some protection around the transistor to ensure the terminal voltage ratings are not exceeded.