> You are exceeding the common-mode range of the opamp by having
> 10v/2 going into each input

I did wonder about a 10V signal into a +/- 5V (actually +/- 5.42V)
amp. At the + and - inputs I measured 5.01V and 5.00V and amp
output of 80mV

I've breadboarded a voltage follower (12V supply) for each lead and
now find 5.01V on both the + and - inputs, with 0V out. The 380rpm
has gone from the meter. The linearity appears to have been preserved,
with 0V at 0rpm, 619mV at 3000rpm and 1238mV at 6000rpm
(subject to patient confirmation)

Thanks to Dwayne for the buffer suggestion . It was something I'd
considered but for no reason other than 'try it and see'

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