Circuits to detect low voltage level signals

[Nicholas Kinar]

Hello--


I recently found this particular advertisement for an instrument to
measure picovolt levels:

http://www.emelectronics.co.uk/spec/P12.html


I am wondering how it might be possible to measure a signal with a
picovolt resolution. Does anyone know of a book, monograph, or paper on
the measurement of small voltage levels? Where should I look in the
literature to be able to design a similar circuit?

If the signal range is from picovolt levels to 5V levels, then how would
I resolve picovolt levels while taking into consideration the higher
voltages? Perhaps some sort of log amp would work well?

Obviously, the circuit may require an op amp with a low offset voltage,
BUT how would I deal with picovolt levels?!?

Nicholas

 

[Nicholas Kinar]

Measuring picovolts at DC is extremely hard at room temperature. Just
about everything has thermoelectric potentials of nanovolts to about a
millivolt per degree C.

It can be done if you convert it to AC using some modulation scheme,
amplify there, and convert back to DC, but it does not happen by accident.

Phil,

Thank you so much for your response! I would wonder if there would be
some sort of monograph on these types of measurements. It seems tricky.

Oh, and BTW, I can't wait until your new book is released. I'm going to
pick up a copy as soon as it becomes available here in Canada.

Nicholas

 

[Nicholas Kinar]

Agilent and Keithley make nanovoltmeters, and they have manuals and
probably appnotes online. Just this morning I designed a cable to
connect one of our gadgets to an Agilent 34420A. The input connector,
a custom Agilent thing, costs $112.

I'm going to have to take a look at the Agilent website, and do some
exploring on the Keithley website. BTW, it seems to be a pricey connector.

Picovolt measurements aren't easy or even practical in most
situations. Dissimilar metals, or even the same metals with impurity
differences, can generate microvolts or nanovolts of signal from small
temperature gradients. Just moving wires in the earth's magnetic field
can pin a nanovoltmeter.

Why would you want to measure picovolts?

I'm interested in this type of thing from a research perspective, and
stumbling across the idea of measuring picovolts led to me to consider
the possibility of being able to better measure outputs from a
transducer. Right now I am thinking of using a 24-bit ADC, and I
thought about the possibility of measuring more than just microvolts.


Anyway, it makes some interesting discussion, and helps to feed my
addiction for electronics--what else a better way to enjoy my time on a
newsgroup?


Thanks, John.

 

[Nicholas Kinar]

It's next to impossible. Measuring picovolts near DC requires a SQUID.

I vaguely remember something about superconductivity in the application
of a SQUID. So this is the reason why working with an AC signal is more
desirable?

 

[Tim Williams]

Could also be microphonics (in any insulators, in the resistor, in the
geometry itself). For instance, if the inside wire has a DC potential, its
capacitance and therefore voltage varies as it's moved around inside the
shield.

You can try a mu-metal tube instead, see if that has any effect. In the
mean time, it might be worthwhile adding some acoustic dampening inside the
tube to at least reduce the resonance, if not necessarily its amplitude.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

John said,
"Just moving wires in the earth's magnetic field can pin a
nanovoltmeter."

Hmm, I'm looking at the noise voltage from a 100k ohm resistor down
the bottom of a 6" brass tube. (one end of the resistor is soldered
to the bottom of the tube and the other to the one side of a shielded
twisted pair.) There is then 18" of cable connecting the probe to a
metal preamp box. This works great unless you 'bang' on the tube.
Banging on the tube with hard objects, (such as ice cubes in an ice
bath.) causes spikes in the noise voltage from the resistor at
acoustic frequencies. (2.5kHz and higher.) The spikes can increase
the noise measured in a 100kHz bandwidth by a factor of 2 or more.

If this is caused by motion of the conductors in the earth’s B field
can I reduce it by orienting the wires along the field?

Perhaps I should mention that the other half of the twisted pair and
the shield around the twisted pair is being driven by the output of
the first opamp. A driven shield.

(Friday thoughts after a few beers)

George H.

 

[John Larkin]

I'm going to have to take a look at the Agilent website, and do some
exploring on the Keithley website. BTW, it seems to be a pricey connector.




I'm interested in this type of thing from a research perspective, and
stumbling across the idea of measuring picovolts led to me to consider
the possibility of being able to better measure outputs from a
transducer. Right now I am thinking of using a 24-bit ADC, and I
thought about the possibility of measuring more than just microvolts.


Anyway, it makes some interesting discussion, and helps to feed my
addiction for electronics--what else a better way to enjoy my time on a
newsgroup?

Some of the cheap chopper opamps have drifts in the nv/K range. It
wouldn't be hard to temperature-regulate one of them, or several
working in parallel. A cmos analog switch could zero-check them. Then
the problem becomes noise and getting the signals into the amps
without thermal and magnetics problems.

Some of the cmos chopamps that I'm using have rather large offsets -
microvolts - that seem to depend on the capacitance seen by the input
pins. Some charge injection effect. I had to add a really tiny (0805
size) trimpot to one plug-in 6-pole active filter board to normalize
dc offsets to below a uV.

Picovolt signals, like picosecond signals, are inherently low
impedance.

John

 

[John Larkin]

The wiring (if there's a local B field) can induce voltage from
motion,
but there are lots of other suspects; if that 100k ohm resistor is
a carbon composition item, piezoconductivity is likely as well.
If it were a wirewound resistor, it'd be piezoresistivity to
worry about (probably a smaller effect). In insulators, one
finds triboelectric effects from bending of the insulator,
and the list goes on.

To some, the word 'electronics' means a kind of engineering
with off-the-shelf components. Those people don't measure
picovolts. To others, 'electronics' means everything that
electrons, positrons, even ions and other charged particles
get up to, in solids, gases, etc. Those aren't engineers,
they're physicists. And they don't generally know much
about picovolts, either.

Yes. Techs and scientists are often both bad circuit designers; one
trusts too little in theory, and the other too much. The horrors I've
seen!

We need a web site that posts the schematics of world-class-bad
commercial products. badschematics.com and badschematics.org are both
available.

John

 

[Okkim Atnarivik]

: >
: > Nicholas

: It's next to impossible. Measuring picovolts near DC requires a SQUID.

: Cheers

: Phil Hobbs


SQUID based picovoltmeters indeed exist. These guys make cooled transformers
for pV measurements IIRC: www.cmr.uk.com

Regards,
Mikko

 

[Nicholas Kinar]

Some of the cheap chopper opamps have drifts in the nv/K range. It
wouldn't be hard to temperature-regulate one of them, or several
working in parallel. A cmos analog switch could zero-check them. Then
the problem becomes noise and getting the signals into the amps
without thermal and magnetics problems.

Some of the cmos chopamps that I'm using have rather large offsets -
microvolts - that seem to depend on the capacitance seen by the input
pins. Some charge injection effect. I had to add a really tiny (0805
size) trimpot to one plug-in 6-pole active filter board to normalize
dc offsets to below a uV.

Picovolt signals, like picosecond signals, are inherently low
impedance.

I often wondered how to get rid of the large offsets. I suppose that
calibration is essential in this type of precision stuff.

Thanks, John.

 

[Nicholas Kinar]

The wiring (if there's a local B field) can induce voltage from
motion,
but there are lots of other suspects; if that 100k ohm resistor is
a carbon composition item, piezoconductivity is likely as well.
If it were a wirewound resistor, it'd be piezoresistivity to
worry about (probably a smaller effect). In insulators, one
finds triboelectric effects from bending of the insulator,
and the list goes on.

To some, the word 'electronics' means a kind of engineering
with off-the-shelf components. Those people don't measure
picovolts. To others, 'electronics' means everything that
electrons, positrons, even ions and other charged particles
get up to, in solids, gases, etc. Those aren't engineers,
they're physicists. And they don't generally know much
about picovolts, either.

I guess that measuring picovolts is something in-between engineering or
physics. Perhaps it could be considered something of an "art"?

 

[Nicholas Kinar]

Yes. Techs and scientists are often both bad circuit designers; one
trusts too little in theory, and the other too much. The horrors I've
seen!

We need a web site that posts the schematics of world-class-bad
commercial products. badschematics.com and badschematics.org are both
available.

As an environmental physicist and an engineer, I second this recommendation.

 

[Nicholas Kinar]

SQUID based picovoltmeters indeed exist. These guys make cooled transformers
for pV measurements IIRC: www.cmr.uk.com

Regards,
Mikko

Stuff like that couldn't easily be placed in a small plastic hand-held
electronics enclosure case. These types of instruments need to be
bolted to the lab floor.

 

[Jim Thompson]

If anyone has the time and web skills to set up a decent site, I'll
pay for it.

John

And populate it ?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

California is to Arizona as masturbation is to getting laid

 

[Fred Bartoli]

Jim Thompson a écrit :

And populate it ?

...Jim Thompson

Hem. Sure enough you have some good story to in there too
(Who has not?)

 

[Rich Grise]

Jim Thompson a écrit :

Hem. Sure enough you have some good story to in there too
(Who has not?)

I once worked as a Z-80 programmer on the control board of a box where
everything that could possibly be done wrong was. For example, they had
an X-Y carriage and a servo loop with 16-bit DACS, using AC Synchronous
Motors!!!!! AC synchronous motors in an alleged servo loop is insane! They
also had components that were moved with stepper motors; the guy before me
had no ides how to drive a stepper motor - I spent a few days stdying
them, and when I fixed the control loop and demo'd it, people went, "Ooh!
Aaaah!". But the guy had no provision for a zero stop, like an optical
interruptor - they just banged it into the mechanical backstop for long
enough to assume they were in zero position.

Unfortunately, before I (or anyone else) had a chance to do all the
design revisions, my contract ended and I found myself out twisting
in the wind. )-;

Cheers!
Rich