Instrumentation amp makes noninverting integrator

-September 19, 2002

Figure 1a shows the classic implementation of an integrator. The circuit has two properties that may be undesirable in same applications: It necessarily inverts, and it requires a split-supply or midsupply reference. Figure 1b shows an implementation of an integrator that uses an LT1789 instrumentation amplifier. This integrator does not invert, and it works with a single supply. In addition, because it has a positive-only output swing, the integrator capacitor can be a high-value, polarized electrolytic unit, as shown. Most of the circuit operates as a voltage-controlled current source. The LT1789 is a precision micropower instrumentation amplifier that can operate from 3 to 36V total-supply spans.

With a gain setting of 1, with pins 1 and 8 open, the voltage between the inputs also appears between the Output and Reference pins. The Output pin connects to one side of R1, and the voltage on the other side of R1 drives the Reference. The input voltage, VIN, appears across R1, causing the current-source action, with IOUT=VIN/R1. Dumping this current into a capacitor produces the integrator action, with the time constant R1C1. The LT1636 buffers the output voltage on C1, thereby eliminating the loading effects of approximately 200 kΩ of the LT1789's Reference pin and any downstream circuitry. The wide, single-supply range and micropower operation make the circuit suitable for battery-powered systems. As a positive-output-only integrator, this circuit is not generally applicable inside control loops. Suitable applications include accumulators, adjustable ramp generators, and voltage-to-frequency converters.

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