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Old 20th August 2012, 02:13 PM   #1
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Default Schematic for Microvoltmeter / add-on for Oscilloscope

Hi !

Anyone can suggest schematic (or may be finished products), which could function as micro-voltmeter (with sensitivity starting from 10-3o uV) or even better, add-on for oscilloscope with frequency response 10 Hz - 300 KHz?

Thanks in advance for any suggestion(s).
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Old 20th August 2012, 02:19 PM   #2
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30uV? That's no sinecure! When I needed one I bought one of those B&K measurement amplifiers on eBay.
Came with all kinds of filters and peak, average, RMS etc detectors as well.
30uV fsd on the most sensitive scale, with wide band noise less than 10% of that.

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Old 20th August 2012, 06:06 PM   #3
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At this amplitude level you probably want differential inputs and adjustable bandwidth-limiting filters. Some commercial products:

Tektronix AM502
Tektronix 7A22 (if you have a 7000 series scope)
Princeton Applied Research (PAR, also EG-G) 113
Stanford Research Systems (SRS) SR560

I have the first three models, all work very well. I'm mostly using the 7A22 as the scope readout conveniently gives the correct deflection factor.

You can quite easily build a simple differential preamp based on an IC mic preamp chip, e.g. a THAT1510. If you need super low noise and can live with just a single-ended input, check out my low noise preamp (A low noise laboratory-grade measurement preamplifier and SG-Acoustics · Samuel Groner · Publications).

Samuel

Last edited by Samuel Groner; 20th August 2012 at 06:09 PM.
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Old 21st August 2012, 05:47 AM   #4
Frex is offline Frex  France
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Post ERMSDCV2

Hello LinuksGuru,

Maybe you can take a look HERE and HERE.

It's a PCB design for DC..1MHz RMS/DC converter, with high performance low noise
amplifier. It allow to measure noise down to µV, and periodic signal down to 100nV using scope averaging. The bandwidth of LNA is 10Hz-100kHz, that seem near of your needs.
Regards.

Frex
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Old 22nd August 2012, 07:49 PM   #5
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Quote:
Originally Posted by Samuel Groner View Post
You can quite easily build a simple differential preamp based on an IC mic preamp chip, e.g. a THAT1510. If you need super low noise and can live with just a single-ended input, check out my low noise preamp (A low noise laboratory-grade measurement preamplifier and SG-Acoustics · Samuel Groner · Publications).

Samuel
Hi, Samuel !

Thanks a lot for feedback!
I have a few of questions:

1) What is minimum measurable input signal level?

2) What is minimum input signal level which then could be examined on oscilloscope (so noise and distortions of pre-amp will be small comparable to DUT)?

3) Is this article published in the last (#3) issue of Linear Audio?

4) I saw on your web site (very professional and informative btw) comparison of different film capacitors technologies. What methodolgy/Equipment you have used to measure so low level of distortions produced by passive components?

Thanks !
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Old 23rd August 2012, 09:12 AM   #6
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Quote:
1) What is minimum measurable input signal level?

2) What is minimum input signal level which then could be examined on oscilloscope (so noise and distortions of pre-amp will be small comparable to DUT)?
You have not given any details of your application--without context it's hard to define a "minimum measurable signal". What does the signal look like? What parameter of it do you need to measure?

Viewed on a scope without bandlimiting filters, the output noise of the preamp is about 2 mVpp. Referred to the input (60 dB or 1000x gain) this makes 2 uVpp. With a 100 kHz bandwidth-limiting filter this improves to 600 uVpp (RTI 600 nVpp). These are extremely low figures, and in most cases the noise of the source, or interference, will dominate.

Quote:
3) Is this article published in the last (#3) issue of Linear Audio?
Yes, as noted on my webpage.

Quote:
4) I saw on your web site (very professional and informative btw) comparison of different film capacitors technologies. What methodolgy/equipment you have used to measure so low level of distortions produced by passive components?
Well, distortion of some passives (e.g. polyester film) is not that subtle and can be easily measured with a good audio analyzer (I use an AP SYS-2722). More refined techniques involve indirect measurement by comparison with a "known good" part (e.g. a capacitor which is synthesized by several larger capacitors in series; in such an arrangement each capacitor sees much lower AC voltage drop, which drastically reduces its distortion). The classic method is the use of a bridge setup, but there are others.

Samuel
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Old 23rd August 2012, 10:46 AM   #7
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Quote:
Originally Posted by Samuel Groner View Post
You have not given any details of your application--without context it's hard to define a "minimum measurable signal". What does the signal look like? What parameter of it do you need to measure?

Viewed on a scope without bandlimiting filters, the output noise of the preamp is about 2 mVpp. Referred to the input (60 dB or 1000x gain) this makes 2 uVpp. With a 100 kHz bandwidth-limiting filter this improves to 600 uVpp (RTI 600 nVpp). These are extremely low figures, and in most cases the noise of the source, or interference, will dominate.
Hi, Samuel,

Basically it is for measurement noise and low-level oscillation (if any) of power amps, pre-amps, and possibly, DACs. I'm going to build DAC with ESS Sabre,
ESS and following op-amp are extremely low-noise devices, so measuring noise is quite a challenging task.
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Old 24th August 2012, 07:25 PM   #8
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Quote:
Basically it is for measurement noise and low-level oscillation.
These are completely different things. For the latter you need a BW well into the upper MHz region. Most oscillations are detectable without any additional preamp, and I'm not sure how much a preamp helps for those which aren't--in such a high measurement bandwidth noise quickly limits things. Probably a 10x preamp is of some use.

To measure noise my low noise preamp is very suitable; with a noise contribution of 0.4 nV/rtHz you will hardly find a situation where it contributes significantly to the measurement. But you need band-limiting filters after it to define the measurement bandwidth (unless you're using FFTs).

The design by Frex has those built in; however, the input resistance of this preamplifier is very low (100r IIRC) such that it is difficult to apply in many circumstances. And its noise contribution is higher.

Samuel
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Old 2nd September 2012, 11:27 AM   #9
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Hi, Samuel,

What is the upper frequency limit of your pre-amp?
On your sire it is stated 100 KHz, on Linear Audio web site - 1 MHz.
I don't care about small non-linearity.

Once I had nasty oscillation several mV at 200 - 300 KHz region, it was quite difficult to track it with digital oscilloscope, so upper bandwidth limit sometimes may be important.
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Old 2nd September 2012, 12:23 PM   #10
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I will second Sam on the 7A22N -- (although I use a 5A22n on a 5223 scope). You can find the manual for the 7A22N on the "Boat Anchor Manual Archive" http://bama.edebris.com/manuals/tek

It's not too difficult to build an amplifier with existing IC's like the AD797 or LT1028 -- just power it off a pair of 9V alkalines.
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