Electronic Design

Extend The Input Range Of Instrumentation Amplifiers

The XTR105, XTR112, and XTR114 from Burr-Brown are a family of two-wire, 4-20 mA current-loop transmitters with instrumentation-amplifier input stages. The front end is designed to be used with a bridge sensor as the signal source. Because of this optimization, the allowed input signal voltage is constrained to be within the limits 1.25 V to 3.5 V.

If the maximum input signal to the instrumentation-amplifier front end is greater than the allowed 3.5 V, a simple voltage divider could be used to bring it within the maximum limit. However, this won’t help the low end of the signal range. It’s possible to use the on-chip current sources and three resistors to shift the input signal to be within the bounds of the XTR105, XTR112, or XTR114.

Conventional instrumentation amplifiers that operate from single-supply rails exhibit similar input signal restrictions. This same technique, with an external dual-current reference, can be used to shift the input signal into the allowed signal-voltage range (see the figure).

The design values are calculated from the following relationships:

where

VZ = IC1’s input voltage for zero output,

VFS = IC1’s input signal for full scale output,

VMAX= the driving circuit’s maximum output,

VMIN = the driving circuit’s minimum output, and

I = the reference current source.

Note: For all relationships, the units are V, mA, and kΩ.

The gain of the amplifier is then given by the relationship:

Selecting the resistors to provide maximum signal swing at the transmitter’s input has the further advantage of IC1’s gain being kept to a minimum. This reduces the error contributions from the 4-20 mA transmitter to a minimum.

Substituting the values for the XTR105 yields the following results:

The only difference between the three transmitter products is the current from the on-chip current references (see the table). This technique can expand the usefulness of instrumentation amplifiers with the use of an external current reference, such as the REF200.

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