PSN-L Email List Message

Subject: Re: Op amp front end noise
From: George Bush ke6pxp@.......
Date: Sat, 12 Mar 2005 10:00:09 -0800

At 07:20 PM 3/10/05 -0500, you wrote:
>George Bush wrote:
>> I am building a mercury-cup tiltmeter and it uses the INA121 opamp. I would
>> be interested in seeing your analysis of it's noise also.
>I will include it in my survey, but it appears to be better with very
>high impedance signal sources.  

	Yes, the tiltmeter application is high impedance differential.

>I am interested in how the tiltmeter
>connects to the amplifier.  What sort of signal source impedance does
>this represent?  Do you need the instrumentation amplifier
>differential configuration?

	It uses RF connected to the mercury to excite antenna plates just above
two connected pools 24" apart. The antenna signals are detected (rectified)
and fed into the INA121 in a differential configuration. The DC impedance
is high (back impedance of the diodes), but since they are charging .1 uf
caps, the AC impedance is low. The mechanical design is old and originally
appeared in an early 1970's "Scientific American" Amatuer Scientist article
"A Sensitive mercury tiltmeter that serves as a seismometer". I still have
a copy of the pages (pp. 124-129), but SA didn't print the date on the
pages. Recently Ron Newton modernized the electronics and published an
article on the May 2003 "Nuts n Volts" magazine, pp. 47-51, "Measure Earth
Tides with a Tiltometer"

	I appreciate the OP-amp survey that you are doing, it will really help in
the construction of ametuer seismometers.
>John Popelish
>Public Seismic Network Mailing List (PSN-L)
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George Bush
Sea Ranch, CA, USA
38.73775N, 123.48882W

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