On Thu, 01 Apr 2010 10:19:20 +1300, "ivp" said:
> > You are exceeding the common-mode range of the opamp by having
> > 10v/2 going into each input.
> 
> Bob, do I have an issue with this circuit, bearing in mind that the meter
> now zeroes and the output (from subsequent stages) is a linear 4V / 
> 1000rpm, with no baseline offset ?

Looking at the specs of the TLC271, I'd say there is still a problem.
The common-mode range on that part extends down to the negative rail but
only to 1.5 volts below the positive rail(throughout temperature range).
You might try a heat gun alternating with cool spray on the opamp and
see if it still works at those temperature extremes.

If you have 12 volts available, why not power the opamp from +12 and -5
volts? You couldn't use the TLC271 since it's limited to 16 volts max,
but you could use an LM358 or do the whole circuit in a quad LM324.

Another source of offset could be whatever's driving the meter movement
might not have equal impedance on both terminals, so the new circuit
with buffering fixes that problem where the single amplifier circuit
loaded the meter driver.

Cheerful regards,

Bob

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