[Test-Equipment] Ultra-high input impedance amplifier

[Barry L. Ornitz]

Randy Zelick wrote:

> I will experiment with the 26A1 to see if I can increase the Zin
somewhat.
> This will take a couple of weeks, though, because I have to get some
> ancillary stuff going to do a proper test, and I want to get the
> physiology prep ready to see how much loading it can tolerate.
>
> Meanwhile, I will look into building a matched-pair FET probe as
some have
> suggested. If anyone has a ready-to-go design, do let me know. The
> requirements would be 2 Hz - 10 kHz bandwidth, unity gain, a gigohm
(or
> more) input impedance. I would try to put it in a small Pomona box
and run
> it off batteries. A shielded twisted pair cable would take the
outputs of
> each FET back to the Tek amp, which would provide differential
> amplification and filtering.

It will be far easier and cheaper to use an integrated circuit
instrumentation amplifier than trying to find the parts and
build a circuit with matched FET's.  The Analog Devices AD622
instrumentation amplifier is only a few dollars and it takes
only one resistor to set its gain.  You can place it close to
the source voltage to eliminate noise.  [Long coaxial cables
can generate considerable triboelectric noise. ]  Also if the
source impedance is exceptionally high, which is I suspect why
you need such a high input impedance to the amplifier, the
coaxial cable capacitance will completely kill your frequency
response.  FET source followers will take care of this, but
with less than unity gain, you will still see the
triboelectric noise.

The AD622 has a 10 Gohm, 2 pF input impedance.  With a gain of
1000, it has a DC to 12 kHz frequency response.  You might be
interested in the following application notes and data sheets:

A Designer's Guide to Instrumentation Amplifiers
http://www.analog.com/UploadedFiles/Associated_Docs/42329367appguide.p
df

A User's Guide to IC Instrumentation Amplifiers Application Note
(AN-244)
http://www.analog.com/UploadedFiles/Application_Notes/442106604AN244.p
df

AD622: Low Cost Instrumentation Amplifier Data Sheet (Rev. C, 4/99)
http://www.analog.com/UploadedFiles/Datasheets/218546992AD622_c.pdf

Tubes are fun to play with, and I enjoy doing so, but for
critical designs I'll take modern integrated circuits any
day.  I have no financial interest in Analog Devices, but I
have been happy with their products and especially happy with
their technical support since the early 1970's.

        73,  Barry L. Ornitz     WA4VZQ     ornitz@tricon.net