Driven switch body. (what about greater than one bootstrap)

So I copper clad proto-ed the grayhill rotary switch into my PD thing. I can post pics.. at 1 MHz an inch of bus wire is not a big deal. Anyway with the switch body grounded the step response/ BW went to crap. (100 k ohm FB) When I floated the switch with my dremel, and I got a nice improvement in BW ~400 kHz to 800 kHz (I wasn't looking at the step response at the time.. and BW measurements are squishier than the time domain.)

Anyway then I tried driving the switch body from the input signal, I got a little improvement. (In step response.) My thinking was that the driving the switch body, would pull all the un-connceted switch pins along with it. And now I'm thinking if I drive it a bit over unity...? or is that the road to misery?

I could add a shell connected copper plane under the switch and add a circular copper fence.

George H.

Reply to
George Herold
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You have to check all the gain positions. With optical signals.

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 Thanks, 
    - Win
Reply to
Winfield Hill

Yeah well the driven switch idea is a total fail at this point. (I'm not sure why I saw an improvement when I dremeled the copper around my switch body... maybe a poor solder junction in the first circuit?) Anyway that reported result is not true. I think I can have a 5 pF switch and fast, or a 5pf switch and low noise, but not both*. And the back panel (~1"x2" Al sheet) could be changed... For the diode laser experiments the full range can be totally useful.. that's a fast + switch app. (I think?)

George H.

*Or low noise and fast but no switch.. or lower C switch... some jumper thing? (it seems crazy to invite people to re-solder your circuits...)
Reply to
George Herold

Oh or JL's idea of switched cascodes... what's the C_ce capacitance when switched off? (collector to emitter)

GH

Reply to
George Herold

It's low, but worls horribly below 1uA. Bootstrap works well over the whole range, why are you fixated on cascode?

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 Thanks, 
    - Win
Reply to
Winfield Hill

I think the cascode gives me lower noise. (But I haven't done the calculation nor any noise measurements... I find it useful to do those together as I can use the electronics to check my math :^)

I've got this hair-brained idea of looking at diode laser noise just below threshold. My guess is it probably won't work, but I need a low noise, fast PD, to have a chance.

George H.

Reply to
George Herold

A cascode needs a keep-alive current. Cascode really helps to get good noise and bandwidth results for a high-capacitance photodiode.

Reply to
John Larkin

Many of the signals I look at are small, 0-10%, changes on some background light level.. so the cascode should be fine.

What I didn't appreciate, until today.. (even though it says as much in Phil's book.) Is that with the cascode the speed of the TIA opamp is not that important anymore. You can't speed it up with a faster opamp. The time cosntant is mostly the PD capacitance times R_e (25mV/I_c)

I keep bashing my head into this 5pF of switch C. (Even with no cascode, just a bootstrap.) I've ordered a few of these faster THS4631 opamps

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Four times the BW should mean I need twice as big a compensating feedback cap. I'll see.

Reply to
George Herold

could always reduce it with a knife switch :)

What's the c of a coaxial relay?

NT

Reply to
tabbypurr

OK, f the rotary switch. (I've thought about some internal jumpers or something... but if you ask students to open up your stuff.. well be prepared.) Anyway how about little toggle switches? Maybe two or three of them? there could be lots of combinations... but some series thing would look to keep the capacitance down. I bought a bunch of little plastic toggles... let's say 1 k ohm to 10 Meg.

(I ordered some little toggle switches, I think I know which box they are in. :^)

one toggle switch is easy, what about if you have two toggle switches, two or three position, (single pole) I never know what to call three position toggles. on-open-on? The one thing nice about the rotary switch, is that at DC, 10k ohm was a 10k resistor.

George H.

George H.

Reply to
George Herold

(Oh dear, can you tell I've had too much beer? please excuse me. I'm taking tomorrow off, and it's beautiful around here. GH)

Reply to
George Herold

We use a lot of little Fujitsu FTR-B3GA relays. Capacitance is tiny and they work nicely to about 3 GHz.

George could use them to do the actual switching.

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Reply to
John Larkin

So does the bootstrap, without the bias-current penalty. We're talking lots of degradation below from 1 to 100nA.

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 Thanks, 
    - Win
Reply to
Winfield Hill

0.3pF. Use them with bootstrap. Forget cascode, get a full current range. Have your cake and eat it too.
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 Thanks, 
    - Win
Reply to
Winfield Hill

Typically about 0.2 pF contact-to-contact, and a bit more from contact to coil.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

Agreed. However, the bootstrap and cascode are sort of orthogonal, so with equally good devices you wind up several dB ahead by using both.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

I was measuring toggle switch capacitance Friday. Three position, single pole. measured with SRS RCL thing. I added about 1" of bus wire to the switches to plug 'em into the contacts... The srs contact interface is meant for production. But I got it to read a 1pF cap correctly and then two 1pFs in series...

Panel mount C&K toggles were a bit less than 0.5 pF between pins and in the center 'no contact' position about ~1pF to both. (7xxx series)

I have a little NKK plastic toggle it was about 0.35 pF pin to pin and 0.8 both, but all these numbers are good to ~10% if I'm lucky. :^)

This,

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George H.

Reply to
George Herold

Being DPDT, tricks can be played in a TIA topology.

I guess one could booststrap unused contacts, or even the coil. But I'm working at 400 Hz and hundreds of watts lately.

Reply to
jlarkin

Huh... OK let me think about it. I've got a layout with three toggles which I think... discounting stray stuff, gives me just one switch C. (~0.5 pF) for my one 'sweet spot' TIA resistor. (100 k ohm.) I'll post it. three toggles and six resistor settings.. brute force. George H.

Reply to
George Herold

Yup, I did both in a switchable 1G/50G TIA for a scanning surface voltage tool for a now-defunct company called Qcept. Plus I had to short out the 1G resistor when on the 50G range, because otherwise its Johnson noise across the 0.2 pF of the open contacts would have dominated the noise floor. :)

That's one of those times that not doing a careful error budget will bite you in the posterior.

Since the company went mammaries-topmost, here's part of the schematic:

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

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