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Never Buy Maxim (again)

It's pretty simple. The physics is that (in the 1D approximation), in order to move the edge of the depletion zone from z to z+dz, you have to create a sheet of charge

d sigma = rho dz

and the capacitance change is

dC = A epsilon(1/(z+dz) - 1/z) ~ -A epsilon/z**2 dz. Here epsilon is the dielectric constant of fully depleted silicon.

The E field goes as the integral of the sheet charge elements

E = 1/epsilon integral (0 to z) rho(z) dz

and V is the line integral of the E field.

V = integral (0 to z) E(z) dz

So you get z from the capacitance, and then rho from the second derivative of V with respect to z. Haven't got time to do the derivation properly, but that's more or less how it goes.

There are fine points having to do with where you take the origin of voltage (probably the contact potential of the junction) and Debye shielding, so some care is needed.

What I found was that a lot of my favourite high-speed and high-linearity PDs have a buried layer of higher doping, like an APD.

Boonton 72BDs are all the go.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs
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I like the 72Bs with the analog meter. Very retro.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

I like them too, but the analogue display doesn't do the stability of the meter justice. Once the 72BD is warmed up, it drifts somewhere around a femtofarad per hour in my lab. It's good enough to use for a capacitive gauge.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

I've been meaning to try a parallel-plate cap as a displacement transducer. Measuring nanometers ought to be easy.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

You want a differential cap to take out the thermal expansion.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Three-plate is better. Picometers -- not that hard.

Reply to
Spehro Pefhany

e

echnique.

re.

Thanks Phil... (ask for a reference and I get a mini-dissertation :^)

There were guys doing this measurement back in grad school, but I didn't pa y any attention. There's some distinction between high frequency and low frequency response too. But there's there's lots on the web, just a matte r of removing the dregs and dross.

George H.

Reply to
George Herold

Oh, the whole thing was bootstrapped right to the eyeballs, otherwise the op amp capacitance wouldn't have been that big a deal.

You can't usefully bootstrap an op amp's supplies and noninverting input at high frequency, I wouldn't think--its differential input signal would go away, so it wouldn't do anything except measure the quality of the bootstrap. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

..and, obviously, a driven guard was of little help.

Reply to
Robert Baer

Why don't you just show what a jerk you are, again, and plonk me, again? Idiot.

Reply to
krw

Me too! Plonk me too!

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin 

--
Why ask someone else to do what you could so easily do yourself. ;)
Reply to
John Fields

company in England makes an instrument using capacitance to measure displacement. Incredible resolution in the range of 20 to 50 ppm, that's like 25 nm at a distance of 0.1 inch? One application is to use it to adjust distances to focus an e-beam on a substrate and take into account the roughness of the substrate. Very interesting technique to do it. I'll have to see if I'm limited by an NDA before I can describe the methodology. I do remember to use it properly I had the PCB shop do five PCB's at a cost of $25,000 - very difficult: big cutouts, big plated holes, blind vias, buried vias etc etc plus had to be vacuum compatible. The PCB house was so proud of their work, they put a sample PCB in their lobby to show potential customers their capability at doing really 'difficult' PCBs

Reply to
RobertMacy

Queensgate Instruments, maybe? They've been making picometer-resolution piezos in the 10-100 um range for some years now, but nothing in the 2.5 mm class. (They cost a lot, so I've only admired them from afar.)

I have a 100-um class XY flexure stage that was made by Wye Creek Instruments, a garage operation run by Fred Scire and Clayton Teague of the NBS (as it then was). The piezo stacks run at 0-3 kV.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 USA 
+1 845 480 2058 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Yes, that's it. we were using their capacitor displacement sensor in the

25-50 nm range [from memory]
Reply to
RobertMacy

SED has become like a leftist organization... the bombastic egotistical ignorant claim supremacy and chastise those of us who know how semiconductors function when we point out their errors.

I _will_not_stop_ pointing out the ignorant whether they froth (or is it foam ?:-) at the mouth or not. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Larkin's right. You are a senile old geezer.

Hell, you won't even remember in the morning.

Reply to
krw

See what I mean about bombastic egotistical ignorance? All Larkin and krw can manage is name-call. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Egotistical? Not *you*!

Sometimes the truth hurts. You can plonk me anytime you want. ...that is, if you can remember how.

Reply to
krw

Jim-out-of-touch-with-reality-Thompson strikes again. John Larkin exhibits a much broader spectrum of ignorance than krw, and has been known to post circuit diagrams and LTSpice net-lists.

We may be arguing precedence between a flea and a louse, but Larkin is definitely the performing flea and krw the crawling louse.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

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