Never Buy Maxim (again)

John Larkin · 2013-10-13T19:34:32+00:00

Maxim makes an opamp, MAX44280, that has a specified input capacitance of 0.4 pF. That would be great in some apps where Cin causes noise-gain peaking. I figured I'd give Maxim another chance, after being s*****d by them in the past. So I went online and filled out their sample request. The process makes no sense, website runaround, but eventually I got it done. Two days later I got an email to the effect that the sample request was waiting for a product-line manager approval. I responded and asked why, and was told that it was standard internal procedure and would take only another 24 hours. OK, a week later I got two samples, packaged in a short tube and wrapped in a ball of sticky masking tape. These opamps, like most Maxim parts, have nonstandard pinout. I built a test fixture to measure Cin. With power off, I measured 1.4 pF on the non-inverting input pin. I powered it up and the opamp didn't act anything like an opamp. I checked everything three times and figured the amp was blown, so I tried to replace it. It's an SC70, really tiny, and my tweezers slipped and I pogoed it into the carpet, never to be seen again. The sales engineer emailed me to see how the samples were coming along, and I told him the situation. He promised to get me 10 more ASAP. Then he called back with a technical guy. I explained my setup. I'm using +-2.5 volt supplies, and the technical guy said that this is a single-supply opamp and might not be qualified for dual-supply operation. He said he'd contact the designers or something. I asked him how they defined and measured the 0.4 pF Cin, and he said he'd find out. They were due to call me last Monday. So far, no more parts, no call. -- John Larkin Highland Technology Inc jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT,...

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Phil Hobbs 12 years ago

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

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John Larkin 12 years ago

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

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Phil Hobbs 12 years ago

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

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John Larkin 12 years ago

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

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Phil Hobbs 12 years ago

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

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Spehro Pefhany 12 years ago

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

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George Herold 12 years ago

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.

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Phil Hobbs 12 years ago

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

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Robert Baer 12 years ago

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

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krw 12 years ago

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

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John Larkin 12 years ago

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

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John Fields 12 years ago

Why ask someone else to do what you could so easily do yourself. ;)

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RobertMacy 12 years ago

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

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Phil Hobbs 12 years ago

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

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RobertMacy 12 years ago

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

25-50 nm range [from memory]

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Jim Thompson 12 years ago

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

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krw 12 years ago

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

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

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Jim Thompson 12 years ago

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

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krw 12 years ago

Egotistical? Not *you*!

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

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Bill Sloman 12 years ago

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

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Never Buy Maxim (again)

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