lock-in amplifier

it.

Yes, I imagine an isolated little battery connected between source and gate, magically charging itself up. But does that inject charge out the drain, say? Must do I think.

The datasheets I have seen don't show anything much. I use a lot of LCA710 (for power switching, so probably not the best choice for this).

Actually I think it might have been a Jim Williams app note, not JL. Sorry to be so vague; really can't remember.

purchasing a used one but don't know enough about them. I know they are extremely effective at finding small signals buried in lots of noise but what are the downsides to a lock-in besides expense?

from Analog Devices and supposedly that can be used to make a very low cost lock-in amplifier. Anyone ever try that? Was it worth it for the money? I should mention a friend of mine has a couple of these AD630 IC's so I would be willing to solder something up pretty quick if it's not a waste of time.

last night and learned a lot. I found this interesting article and it looks like the principles are pretty understandable.

substituted instead. My main goal here is to learn about Lock-in's by building one myself since reading only takes you so far. What do you think? Is this a good starting point?

Make him pull out some scope test points that show checkpoints in the software. It's amazing how many programmers have no clue, not by 10:1, about how long it takes min-max for their subroutines to execute. Or how badly IRQs are chopping up their mainline code.

I used to use one cross assembler that added execution times to every line in the listing. On the 68332, I could figure about 1 us execution time per line of assembly code. I often comment execution times into my source code, for reuse guidance.

When designing products at the architectural level, one has to guess whether the selected CPU has the horsepower to do what needs to be done. Some accumulated experience with actual code execution times becomes valuable. The recent DDS-on-an-ARM thread is an example: Joerg's entire control algorithm and DDS sine output would easily run at 100 KHz on an LPC17xx, with the CPU clock cranked down to save some power.

it.

Photofets are used to discharge particle detector front-end integrators, but a little charge injection does no harm there. Fairchild says "no charge injection" on the H11F1M data sheet, which is a remarkable claim. A photofet is different from an SSR type structure, which has separate photovoltaic diodes and mosfets and tend to be slow.

Thompson is being the usual old biddy, flinging ignorant insults about things he knows nothing about.

purchasing a used one but don't know enough about them. I know they are extremely effective at finding small signals buried in lots of noise but what are the downsides to a lock-in besides expense?

chip from Analog Devices and supposedly that can be used to make a very low cost lock-in amplifier. Anyone ever try that? Was it worth it for the money? I should mention a friend of mine has a couple of these AD630 IC's so I would be willing to solder something up pretty quick if it's not a waste of time.

last night and learned a lot. I found this interesting article and it looks like the principles are pretty understandable.

substituted instead. My main goal here is to learn about Lock-in's by building one myself since reading only takes you so far. What do you think? Is this a good starting point?

software.

of

the

Good idea. I'll do that.

Cheers

Phil Hobbs

I did a bit of digging. NXP has discontinued all of their enhancement-mode dual gate FETs, including the cute ones like the BF1100, which had an interesting feedback FET mechanism to improve the linearity of AGC.

The best-looking one for sampling appears to be the BF998, which has reasonable I_D with V_G2S = 0.

Cheers

Phil Hobbs

OK, nice, thank you.

I just went on digikey and I see that is the same one I already clicked on, when I idly looked one up after you mentioned them!

Ordered some for a rainy day.

Thanks,

Hi John & Jim,

Just think a little before flaming someone else, please.

opto-fets can never have a charge injection, because the charge they use to load the gate comes from the chanel that the charge flows in. so the sum will allways be zero. I would say, this is as obvious as Kirchhoffs law can be.

Regards

Marte

Izzat so ?:-} ...Jim Thompson

a used one but don't know enough about them.  I know they are extremely effective at finding small signals buried in lots of noise but what are the downsides to a lock-in besides expense?

Analog Devices and supposedly that can be used to make a very low cost lock-in amplifier.  Anyone ever try that?  Was it worth it for the money?  I should mention a friend of mine has a couple of these AD630 IC's so I would be willing to solder something up pretty quick if it's not a waste of time.

Digital makes more sense these days, in most cases. A 16 bit ADC with a little noise dithering would be really good. Many tricks become possible.

Flaming? Really? First time I've been accused of that!

It was an honest question. Just thinking aloud.

Well, I don't think it is that obvious.

Even Jim "charge is always conserved" Thompson doesn't seem to think so :)

(And please, folks, we dont really need to go there again!)

Hi John,

Well? You know how reliable Larkin's advice is >:-}

What a question?

Where do you expect the injected charge comming from?

May be Jim woul tell his statement, too.

I don't see any source for charge-injection.

Marte

How is a channel formed? There was no channel prior. Where did it come from? ...Jim Thompson

The total photocurrent out of both leads has to be zero, averaged over an on-off cycle. (That's also true of a photodiode, of course.)

But within an on-off cycle, you could have the gate charging current distributed in many possible ways. For instance, suppose it works like a MOSFET with a photovoltaic cell in its gate. Once the FET turns off, you'd expect the remaining gate charge to distribute itself in inverse proportion to the interelectrode capacitances.

Of course if it's some clever structure, you could have all that charge recombine very rapidly, which would leave a lot less to couple via the capacitances.

Cheers

Phil Hobbs

I'm just playing devil's advocate here, since I'm unsure myself. Until a photon hits, there is _no_ channel... assuming N-channel, there's two N+ plugs in a P-type substrate. How is the close-to-the surface P-type region changed to N-type upon application of photons? And where does the electron go? ...Jim Thompson

Well you snipped it didn't you?

The voltage at the gate changes, relative to the other terminals. There is a capacitance between the gate and the other terminals. dQ=C.dV.

The photodiode? (or whatever equivalent generates the change in gate voltage).

There is no gate in an opto-FET. Just a channel created by photon impacts... and maybe that's the secret sauce... CONTINUOUS photon impact is required to keep the FET on??

The pragmatist in me says, if opto-FET's were so wonderful, why am I not seeing them implemented as marvelous analog switches that are sweeping the world with their stupendous performance ?:-) ...Jim Thompson

For a switch, you don't care too much about that, I don't think, because a low-resistance channel will form pretty quickly. It's when the channel starts to disappear that injection becomes a problem, because it gets left on the hold capacitor to screw up your measurement.

If all they are is a bog-standard photoconductor, i.e. a piece of nearly intrinsic Si with an ohmic contact on each end, the turn-off would happen by local recombination. (That would make sense given the 15-us speeds of those things.) Then the amount that gets kicked out the leads really would be near zero.

Cheers

Phil Hobbs

Because they're 100 times too slow for general use, maybe.

Cheers

Phil Hobbs

Maybe that's the answer... got to pile on a stream of photons before anything happens. Or too high an ON resistance? ...Jim Thompson