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ZL4AJS > TECH 16.11.05 03:52l 40 Lines 1507 Bytes #999 (0) @ WW
BID : 7C0297ZL4AJS
Read: GUEST DL1LCA OE7FMI
Subj: Frustrating Opamp
Path: DB0FHN<DB0RGB<DB0MRW<DB0WUE<DK0WUE<7M3TJZ<ON0AR<ZL2BAU<ZL4GQ
Sent: 051115/1900Z @:ZL4GQ.#95.NZL.OC #:37638 [Invercargill] FBB7.00i
From: ZL4AJS@ZL4GQ.#95.NZL.OC
To : TECH@WW
Hi All,
I've been designing a simple circuit to compare two voltages, one from a
sensor, the other from a fixed voltage reference. The idea is that when
the voltage from the sensor gets to a certain value, it will switch the
opamp's output high and thus trigger an alarm.
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.
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.
Is this normal? If it is, any ideas on how to accurately compare this
sensor voltage to the reference?
Of course there are such devices as comparators but I'm sure an opamp
should work?
73.
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:: Andrew ZL4AJS@ZL4GQ.#95.NZL.OC ::
:: Ohai, Southland, NZ ::
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Timed 14:15 on 15-Nov-2004
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