Nano Volt DC amplifier

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asrock70

Full Member level 3
Looking for design, radiometric DC amplifier.
I found see Appendix.Sorry for bad scann.
Amplifier with 2x JFET Q + MC34081 in the upper left corner is 1x, 10x, 100x with offset null with (DAC).
Second amplifier is LT1124 again with 100x 10x 1x.
This work with range 1mV x10000 , 10mV x1000, 100mV x100, 1V x10, 10V x1.
It is a very accurate solution from meter about $4,000, but it is a solution for about 20 years old. Do you know of any solutions to today's modern amplifier? Primarily, I want the first amplifier with a differential pair of JFET. Ideally, the sample solutions from AD, LT, Maxim, etc. Thanks View attachment a.pdf jgruszynski Junior Member level 3 This is a tricky area. Look up the formula for thermal noise voltage. Basically these amplifiers are operating on the bleeding edge of what's theoretically (and practically) possible. And it's not even just the circuit that is tricky - how you assemble them also. A typical solder joint will generate far more "thermoelectric" EMF than the noise floor in tens of nV. This is why the unit below uses a custom connector for fixture to DUTs. This is also why they are expensive. BTW here is a "modern" design: 34420A NanoVolt, Micro-Ohm Meter | Agilent (US$ 4000)

It came out a few years ago. The basic circuit isn't that different from your old one. There are only so many ways of doing it. (See the Service Manual - page 160).

Otherwise use something like this (we still include this exact model in many of our test systems - it's old but it works)

Low Noise Voltage Preamplifier - SR560

FvM

Super Moderator
Staff member
You can expect better noise performance, offset voltage and -drift parameters with state of the art operational amplifiers than with dual FETs. Input respectively source impedance (to achieve noise matching) and input current should be specified first, however. You'll face serious difficulties to get dual FETs with specified noise parameters. Vishay has left the JFET market, Interfet is maintaining a reduced choice of the former Siliconix products.

asrock70

Full Member level 3
OK Let's talk about amplifiers in 34420A, 162 page service manual.

Firs step is discrete step with Long-Tailed Pair JFET SNJ3600X05 (obsolete, have not found DS)
followed MC34081 (It is not very good amp)
Totally do not understand how it can be measured with a resolution 100nV?
Bias current null. Bias current is nulled with DAC and Amp U314b over R305 10M (left of the U315 2x JFET),
I understand it, but as the amplifier is measured this Bias current?

mtwieg

Advanced Member level 5
Just curious, why would JFETs be the choice for the input stage? Generally they have higher noise voltage than BJTs (but lower noise currents). Are you assuming the source impedance is very high?

asrock70

Full Member level 3
I studied the service manual 34420, I understand most probably, but I do not compensate for switching losses in the JFET.
Page 163, transistors Q201, 202, capacitive losses are settled through capacitors C201, C202.Everything is managed by a programmable amplifier with DAC.
I do not know how to Sets gain compensation amplifier?

aeneumann

Newbie level 1
Hi -

I just found this thread during a google search.

Re the nanovolt amplifier issue, I am working this problem now and I am looking at the ADA4528 and also the MAX44250 chopper op amps.

Beware resistor end-cap thermocouples as these can show voltage coefficients of 40uV/Deg c. It is best to try to control temperature as much as possible.

I appreciate the schematic you posted. It appears to show a chopper-type amplifier front-end. If you have a better image, or any idea where I can get a better image, I would appreciate.

Thanks

Alfred

Looking for design, radiometric DC amplifier.
I found see Appendix.Sorry for bad scann.
Amplifier with 2x JFET Q + MC34081 in the upper left corner is 1x, 10x, 100x with offset null with (DAC).
Second amplifier is LT1124 again with 100x 10x 1x.
This work with range 1mV x10000 , 10mV x1000, 100mV x100, 1V x10, 10V x1.
It is a very accurate solution from meter about \$ 4,000, but it is a solution for about 20 years old.
Do you know of any solutions to today's modern amplifier?
Primarily, I want the first amplifier with a differential pair of JFET.
Ideally, the sample solutions from AD, LT, Maxim, etc.
Thanks

View attachment 53255

FvM

Super Moderator
Staff member
The 34420A input amplifier, which is also shown in post #1, isn't exactly a chopper rather than a regular ampifier with autozero respective automatic offset compsensation feature. During active measurement period, it's avoiding switch activity.

Classiacl discrete as well as first generation IC chopper amplifiers like ICL7600 showed a number of unpleasant effects like demodulation of interferences near the switching frequency.

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