Electronic Design

Window Comparator Enhances Integrator Circuit's Flexibility

A simple to use window comparator can make short work of integrator, antiwindup, and reset circuits. Control systems that incorporate an integrator stage need some form of integrator reset or antiwindup function. An integrator drifts with time and is subject to saturation when driven by large error signals. This saturation effect causes the device's output information to become invalid and out of phase with the commanded system response. In this circuit, an LT1042 window comparator is used to detect and initiate integrator reset as required (see the figure).

Integrator design starts with determining the accuracy necessary for the selected integration time interval. Once the required accuracy is known, the components needed to meet it can be selected. The op amp and integrating capacitor are critical. For example, the µA741's 80-nA input bias current is enough to reach 1% error in 0.25 seconds. But an LTC1152, with its 100-pA input bias current, will reach the 1% error level in 200 seconds.

In order of preference, the integrating capacitor should be polystyrene, Teflon, or polypropylene. Even the best op amp and integrating capacitor won't do much good if proper attention isn't paid to pc-board layout and cleanliness. For longer integration times and/or lower error rates, temperature-compensated input-bias-current compensation should be used.

Once the integrator core is created, the need for integrator reset or integrator antiwindup is evaluated. Then the circuit is designed and tested.

As shown in the figure, window comparator U1 monitors the low-pass-filtered (via R5 and C3) integrator output signal from U3, pin 6. The low-pass filter reduces comparator trips due to noise. R3 and C2 determine U1's sample rate. (R3 must be between 100 kΩ and 10 MΩ.) The voltage at pin 5 of U1 sets the window width symmetrically around the center voltage set at pin 2. U1 compares the integrator's output to the center and window-width parameters. Its output ("within window"), at pin 1, goes low when the input value exceeds these parameters.

When U1's pin 1 goes low, it drives U4's pin 2 low. As a result, the integrator is reset for as long as its output is outside of the set window limits. Pin 5 of U2A also is driven low, triggering one-shot U2A. The one-shot output (pin 7) drives U4's pin 1 low for a set period of time \[equal to 0.7 × (R3 × C2)\]. Doing so ensures a minimum reset time. To reset the integrator, the output of U4's pin 4 controls sections S1A, S1C, and S1B of the LTC202A. While section S1C is used as an inverter, section S1B disconnects the input as the integrator is being reset.

This easy to use window-comparator building block enhances performance and adds intelligence to analog integrator circuits by way of its decision-based action. A flexible, decision-based analog-computer element, the comparator is used to achieve integrator reset. It does this by executing one important but simple task—comparing its input to a reference value and outputting a decision based on the input information.

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