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, V_IN, appears across R₁, causing the current-source action, with I_OUT=V_IN/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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