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M1BYT > TECH 16.11.05 21:14l 45 Lines 1937 Bytes #999 (0) @ WW
BID : 063822M1BYT
Read: GUEST DL7NDF DL1LCA OE7FMI
Subj: Re: Frustrating Opamp
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<GB7FCR
Sent: 051116/2011Z @:GB7FCR.#16.GBR.EU #:12475 [Blackpool] FBB-7.03a $:063822M1
From: M1BYT@GB7FCR.#16.GBR.EU
To : TECH@WW
ZL4AJS Opined with considerable skill:-
> I have the sensor voltage going to the + input, and the reference to the -
> input. No other components. The voltage from the sensor is for various
> reasons very low, around 0.4 V. The change in voltage between which I want
> the alarm to sound is also very small, just 20mV at most.
That is a relatively tiny voltage and possibly subject to interference from
local mains hum pickup.
>
> In theory, I believe, that while the - input is higher than the + input,
> the output of the opamp will be low. And as soon as the + input reaches a
> higher voltage than the - input, the output swings high almost
> immediately.
>
> This is not happening in my circuit. It appears the voltage on the + input
> must exceed the voltage on the - input by at least 0.8 V before the output
> changes! In my application this is not going to work! I've tried both
> LM741 and TL071 opamps with similar results.
I have never tried to compare such tiny voltages, so my experience on this is
limited, but I wonder what you have done about the off-set connections of the
741?
These might be the cause of your problem.
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º 73 from Harry, M1BYT @ GB7FCR.#16.GBR.EU º
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º International Museums Weekend 18th+19th June 2005 º
º www.ukradioamateur.co.uk/imw/index.html º
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º QTH: Garforth, W. Yorks.. º Timed 18:04 º
º harry.m1byt@tiscali.co.uk º 16 Nov 05 º
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