Looking for Fast, Transparent Analog Switching

[PRR]

> The frequency resolution is important for high-resonance situations

Yes.

> any way to get a capacitor that switches "in" to be near the same potential

Connect them with low-value resistors. But that screws your frequency response. Unless these resistors ARE the response shapers.

A 2-pole low/band/high-pass filter is 2 R 2 C and 4 opamps. If you fix the C and switch the R you don't have big stored charge. The straight-ahead approach has neither end of the Rs grounded, though it may be so near ground it isn't really "floating" and an FET or 4051 may be a fine switch. There is a variant where DACs are used ad digitally controlled analog attenuators, which make a fixed integrator resistor act "variable" (or if current-output you don't need a fixed resistor). Many ways to swing cats; in this case I'm thinking you swing the Rs not the Cs.

For an ancient observation: the H-P 200AB sine oscillator uses a huge mechanical variable capacitor. When you sling the knob rapidly the stored-charge throws the system into a wobble. No switching (over 1:10 range) but that ~~1,000pFd going to 100pFd or back throws 900pFd of charge around the loop. You switch Rs for range-change; these kick but not near as much.

> I think it's worth playing around with. I've always learned the most from situations like this

Yes. Most good ideas are bad ideas beaten into something else. Wrestling with the bad idea gives you a clue where to beat, and where to take a different path.

If you needed it to Work Real Soon, I'd advise a dumb old analog VCF fed from a DAC. It works. It does have noise issues. We could use somethig else.

 

[jdbakker]

[quote author="skrasms"]Is there any way to get a capacitor that switches "in" to be near the same potential as the capacitor that is being switched "out"? I'm still working on even a poor solution here. [/quote]
Maybe something like this:

If your switches are fast enough compared to the slew rate of your signal, this is relatively clickless; most common chips like the (74HC)4053 should be plenty fast for audio work. An added advantage is that both switch poles are at the same potential, which reduces the effect of off-state parasitics. For a floating cap either duplicate the entire setup or use a differential amp.

Note that I haven't simmed let alone built this. The placement of the resistors needs more pondering and/or experimenting, as switching them in and out may well affect op-amp loop stability. The other obvious downside is that most switch chips have their comfort zone (lowest Ron, lowest dRon/dV) at ground.

JDB.

 

[sahib]

I have just had a quick look at the Nais-Matsushita relay catalogue.

The signal relays range have turn-on time between 4-10mS and release time of the same. Maximum operating speeds vary between 60-500cps. Other important parameters are the inital contact resistance which vary 20-100 miliohm and contact-contact, normally open and normally close contact capacitance of 3-5pF.

I don't have very much time just now but if we establish some parameters here then I can have in depth look at home tonight.

But you can anticipate the troubles that you will be having. To start with switching 1024 relays?