AN-242 Datasheet, PDF (7/16 Pages) National Semiconductor (TI) – Applying a New Precision Op Amp

The complete meter circuit in Figure 12 uses a different
scheme A floating supply is available so that the power
ground and the signal ground can be separated with R12 At
full-scale the meter current plus the measured current flow
through this resistor establishing the degeneration This
method has the advantage of allowing even-value range re-
sistors on the lower ranges but increases degeneration as
the measured current approaches the meter current
Bias-current compensation is used to increase the meter
sensitivity so there are two zeroing adjustments current bal-
ancing that is best done on the most sensitive range where
it is needed and voltage balancing that should be done with
the inputs shorted on a range below 100 mA where the
degeneration is minimal
With separate grounds error could be made dependent on
offset current This would eliminate bias-current compensa-
tion at the expense of more complicated range switching
The op amp input has internal back-to-back diodes across
it so R6 is added to limit current with overloads This type of
protection does not affect operation and is recommended
whenever more than 10 mA is available to the inputs The
output buffers are added so that input overloads cannot
drag down the op amp output on the least-sensitive range
giving a false meter indication These would not be required
if the maximum input current did not approach the output
current limit of the op amp
TL H 7479 – 14
Figure 12 Current meter ranges from 100 pA to 3 mA full-scale Voltage across input is 100 mV at lower ranges rising
to 3 mV at 3 mA Buffers on op amp are to remove ambiguity with high-current overload Output can also
drive DVM or DPM
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