Lock-in amplifiers

Sounds pretty simple: a good opamp and an analog switch to flip the gain from +1 to -1, then a lowpass filter. At 10s of Hz, charge injection shouldn't be a big deal. The resistors will need to be very good to hold 1 ppm gain. VERY good.

1 ppm of 10 mV is of course 10 nV, so I hope you can amplify ahead of the synchronous detector stage.

John

It's gotta be sinusoidal excitation, which complicates things.

Yes, but I don't think tranformers will be stable enough. Best regards, Spehro Pefhany

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Sinusoidal excitation is not a problem as you could a phase sensitive demodulator to recover your signal. As I see it your biggest problem is building a very low noise narrow bandwidth stable preamplifier to get your signal up to a respectful level for use with your phase sensitive demodulator.

Howard

If it's not a total secret, what exactly do you want to achieve with this? Maybe there are other options.

I had that once at a client. Insurmountable noise issue, just couldn't get low enough. Then I asked whether we can modulate that and work it off in an RF band that's rarely used. Long silence. Then "Dang! That's it!".

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I'm thinking the demodulation is going to have to be done in the digital domain, and it's a lot of bits wide.

Best regards, Spehro Pefhany

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Yikes! you'll have to use a linear multiplier at ppm precision, or square up the sime to ppm precision.

Charge a lot for this one.

Not many things will.

Maybe you could digitize with an 18-bit SAR converter and process digitally. I assume you've gotta diditize eventually.

John

Spehro will have to, or modulate the signal onto something right there. Handling 10nV over anything lengthy won't work, except maybe in the arctic sea with no thunderstorms within a 1000 mile radius. I just had such a pleasure and we had to shield the dickens out of it.

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Do you really mean stability? Stability over what?

or resolution?

or accuracy?

Mark

TO GET THIS STABILITY YOU NEED A TEMP CONTROLED FLUKE METER AND A LAB AT 68 DEGREES F. GOOD LUCK.

Time- some hours to a day or two, TBD. Most everything else can be backed out.

Gack!

Best regards, Spehro Pefhany

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Firstly, your input (preamp, wideband amp...) has to be nondistorting at the low frequency (which will cause thermal fluctuations that generally CANNOT be ignored). A transconductance amp (output is a current source into pseudo-ground) is preferable here, as the output bias currents of a voltage-output op amp can be troublesome.

For a uA741 op amp, chip temperature is a bigger noise source than input offset voltage at about 4 kOhm input resistance... the change in temperature modulates the input currents that much, for low frequency signals.

Secondly, you have to use a mixer for your filter, and for the kind of accuracy you want, the dynamic range of an ADC is not enough. Mix down before converting to digital data, then use digital signal processing to deal with rejecting out-of-band signal. Transconductance type mixers should work out well, even an MC1496 can do this.

Your reference sinewave has to be as accurate as possible, of course. I'm not sure how one makes a high-purity 10 Hz sinewave, but triangle and square wave generators are easy enough. The feedback scheme on a Wien bridge oscillator gets tricky at low frequency (maybe you can use sample/hold for determining the amplitude, but it takes a good phase determination to pick the gate closing time).

I'm thinking look-up table and DAC sampling at some tens (or low hundreds) of kHz.

I could probably use an analog oscillator but I'd have to measure the excitation signal with another ADC. Best regards, Spehro Pefhany

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Like, with an iPod? Should work, if the subsonic range isn't filtered. I really like having test-CD tracks transferred to a little MP3 player; it doesn't have as many knobs as a wave generator, but it's accurate and convenient.