This is a circuit that useful for common filtering problems. This single supply circuit allows a signal to be rapidly acquired to final value but provides a long filtering constant. This characteristic is useful in multiplexed data acquisition systems and has been employed in electronic infant scales where fast, stable readings of infant weight are desired despite motion on the scale platform. This is the figure of the circuit.
How is the circuit work? When an input step is applied, C1's negative input will immediately rise to a voltage determined by the 1k pot-10 kX divider. C1's ``a'' input is biased through the 100 kX-0.01 mF time constant and phase lags the input. Under these conditions C1's output will go low, turning on Q1. This causes the capacitor to charge rapidly towards the input value. When the voltage across the capacitor equals the voltage at C1's positive input, C1's output will go high, turning off Q1. Now, the capacitor can only charge through the 100k value and the time constant will be long. Waveform B clearly shows this. The point at which the filter switches from short to long time constant is adjustable with the 1 kX potentiometer. Normally, this is adjusted so that switching occurs at 90%±98% of final value, but the photo was taken at a 70% trip point so circuit operation is easily discernible. A1 provides a buffered output. When the input returns to zero the 1N933 diode, a low forward drop type, provides rapid discharge for the capacitor. [Circuit’s source: National Semiconductor Notes].
How is the circuit work? When an input step is applied, C1's negative input will immediately rise to a voltage determined by the 1k pot-10 kX divider. C1's ``a'' input is biased through the 100 kX-0.01 mF time constant and phase lags the input. Under these conditions C1's output will go low, turning on Q1. This causes the capacitor to charge rapidly towards the input value. When the voltage across the capacitor equals the voltage at C1's positive input, C1's output will go high, turning off Q1. Now, the capacitor can only charge through the 100k value and the time constant will be long. Waveform B clearly shows this. The point at which the filter switches from short to long time constant is adjustable with the 1 kX potentiometer. Normally, this is adjusted so that switching occurs at 90%±98% of final value, but the photo was taken at a 70% trip point so circuit operation is easily discernible. A1 provides a buffered output. When the input returns to zero the 1N933 diode, a low forward drop type, provides rapid discharge for the capacitor. [Circuit’s source: National Semiconductor Notes].
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