[time-nuts] Voltage standards
Mike Monett
XDE-L2G3 at myamail.com
Mon Dec 1 09:58:14 UTC 2008
Warren wrote:
> If you want to reduce the measurement noise of a system you need
> to do one or more of the following:
> Lower the source impedance, by reducing the resistance of the
> thing you are measuring
> Lower the Bandwidth, by filtering over a longer time period
> Lower the temperature, by making it colder.
> Doing even any one of these things enough will in theory let the
> noise approach zero, When you do two (or all three) at once the
> noise will approach zero sooner.
> see "Johnson-Nyquist noise" for the details
>> From a practical standpoint, 1 nV of resolution is doable by
>> comparing the difference between two voltage sources if one uses
>> a lot of care and applies some form of extra filtering.
> A 1nV (1e-9) is way below the noise level of any voltage standard
> that puts out volts.
> This means reference measurements are not limited by the noise
> level when using a good but simple setup until the references gets
> to be in the 3e-10 precision range.
> The answer to "What can an amateur do to get a good low noise
> reference for less than, say, $500" IS shop at the US eBay site.
>WarrenS
The Keithley Low Level Measurements Handbook, Sixth Edition, has
some nice graphs that illustrate the limit due to noise. Figure 1-2,
on page 1-4, shows the theoretical limits of voltage measurements
due to noise. The next figure, 1-3 on page 1-5, shows the typical
limits of measurement with various source resistances:
https://www.keithley.com/servlet/Data?id=9538
The EM Electronics D.C. Picovoltmeter, Model P12, has a resolution
better than 5 picovolts, dependent on response time. From their web
page:
"The noise performance is such that, using a source impedance of 30
milliohms or higher, the detection level will be limited by the
Johnson noise of the source. The equivalent noise resistance of the
P12 is only 30 milliohms. To put this figure into perspective, this
is the noise generated by 30cm of 0.5mm diameter copper wire! To
make full use of the noise performance, it is desirable to use it
with very low source impedance. While the P12 can be operated with
source impedance of ten ohms or more, its characteristic will be
overwhelmed by the source noise if the source impedance is too
high."
http://www.emelectronics.co.uk/spec/P12.html
The EM DC Picovoltmeter Model P13 only goes down to 20 picovolts:
http://www.emelectronics.co.uk/spec/P13.html
For the creme de la creme, the Keithley Model 1801 Nanovolt Preamp
gives 1 picovolt resolution when used with the Model 2001 DMM.
This extends the range by a factor of 1,000, so the 20 millivolt
range becomes 20 microvolts. The Model 2001 DMM is 7 1/2 digits, so
the least significant digit becomes 1 picovolt (1e-12 volts.) It is
interesting to note the Model 1801 input connection uses copper
nuts.
From the brochure:
"The 1801's chopper-based design offers several advantages over
traditional preamps designed for high-frequency amplification. For
example, most DMMs and lock-in amplifiers are designed to filter out
random noise by averaging."
"However, low-level measurements are also subject to (1/f) noise,
which cannot be filtered out in this way. The chopper design cancels
out (1/f) noise completely, which lock-in preamps and most DMMs
can't. By continuously inverting the DC input and amplifying the
resulting AC waveform, the 1801's chopper measures the DC input
amplitude precisely, exclusive of voltage offsets and (1/f) noise in
the amplifier circuitry. Carefully designed feedback circuitry makes
the 1801 capable of rise times as fast as 500us 10 - 90% and DC
input resistance of up to 1GOhm. A one-eighth-inch-thick steel
shield and an internal mu-metal shield prevent magnetic coupling of
noise to the amplifier."
http://www.teknetelectronics.com/DataSheet/KEITHLEY/WEBKEITH1801.pdf
The 1801 web page is at:
http://www.keithley.com/products/lovoltloresist/?mn=1801
For those interested in making your own nanovoltmeter, the Analog
Devices AD8571 offers Rail-to-Rail Input and Output, 20 pA input
bias current, 1uV input offset, and 5 nanovolt/C input offset drift:
http://www.analog.com/static/imported-files/data_sheets/AD8571_8572_8574.pdf
Regards,
Mike Monett
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