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Old 23rd December 2004, 11:01 PM   #11
Bricolo is offline Bricolo  France
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If I don't use differential probing, can I tie one input to ground?
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Old 23rd December 2004, 11:04 PM   #12
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AD525? I can't find anything about this one
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Old 23rd December 2004, 11:39 PM   #13
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AD524? Sorry!
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Old 24th December 2004, 08:35 PM   #14
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You could cascade a couple of op amps in series with less gain each.

I use one opa27 to boost a -60dB signal from CDP for my spectrum analyzer.

Works fine for me.
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Old 27th December 2004, 09:34 AM   #15
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Bricolo, first you must specify how low the noise signal is that you want to show.

Let's say you want to see 10 uV, then the preamp must have 1 uV input noise or less.

If you take an opamp with less nosie than 4 nV/Hz you will have a input noise 0.6 uV or so.

A simple opamp like NE5534 may very well fit your needs but if you need even lowever noise maybe an LT1115, LT1028 or AD797 will do the job.

Connect the opamp in non-inverting mode and make sure the the feedback has pretty low values, less than 100 ohms from inverting input down to ground. Feed the opamp with batteries, 2 x 9 volts or more with good decoupling. Don't förget to place the whole thing is metal box.

Since it's a measuring gear I would have both input and output coupling caps. You must probably also have DC-gain 1 or use a DC-servo. Only a few opamps will manage DC gain of 1000.

I really recommend that you check the datasheets of LT1028/LT1115/AD797

Other useful opamps could be OPA627 or more preferable OPA637. They haven't got extremely noise noise but pretty low.

OP27 or more preferable OPA37.

Don't forget to limit the very low frequency nosie below 20 Hz.

I must warn you that it's pretty hard to see uV without interference of hum and other interferences.
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Old 27th December 2004, 12:41 PM   #16
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I haven't sent the boards out yet, but the SSM2019 works beautifully as a differential amplifier for testing, I measured the THD at 0.0008% to 0.0014% -- running it from a Lambda laboratory supply. I had intended to use the SSM2017 which Walt used in the second of the series of articles on low-noise regulators in Audio Electronics circa 1995, but couldn't find the chip. The circuit can be made simpler by omitting the supply and just using a pair of 9V batteries:
Click the image to open in full size.

gain is selectable, 20, 40 and 60dB -- there are two series of resistors so that the gain can be precisely set.
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Old 27th December 2004, 01:25 PM   #17
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can't I do this with a "classical" opamp? I mean, with a single ended input. Is the differential probe necessary, or much better?
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Old 27th December 2004, 01:54 PM   #18
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Quote:
Originally posted by Bricolo
can't I do this with a "classical" opamp? I mean, with a single ended input. Is the differential probe necessary, or much better?
certainly, but differential is very nice to have -- the circuit above has subsequently been modified to provide some clamp diodes on the input to protect the opamp --

btw, the SSM2019 does run a little "hot" -- and mine consumed a lot more gas than Analog Devices described on their term-sheet, there was no oscillation eating watts...i would suggest that a cementable heat sink would help the chip.
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Old 27th December 2004, 02:25 PM   #19
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Quote:
Originally posted by Bricolo
can't I do this with a "classical" opamp? I mean, with a single ended input. Is the differential probe necessary, or much better?
What are going to measure?
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Old 27th December 2004, 02:38 PM   #20
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mostly power supplies, and dac outputs
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