Interpolating Scanner Circuit

Posted on Feb 6, 2014

This is an original design for an interpolating scanner, a circuit with a number of signal inputs, a control voltage input and a signal output. The output effectively selects between the inputs, fading from one to the next, as the control voltage rises. A two-channel panner circuit might be considered a degenerate form of a scanner. The data sheet

Interpolating Scanner Circuit
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for the Precision Monolithics, Inc. SSM-2024 current-controlled amp IC shows a stereo panner, they call it an "Exponential Cross-Fade Controller", where two CCAs are controlled from a PNP differentialamp. The circuit is pretty straightforward and I`m sure similiar circuits have been used before. The circuit presented here is inspired by and similiar to JH`s scanner except that drive circuit is an unusual extrapolation of the differential amplifier topology used in the PMI panner. This approach has some advantages: the total drive current (the sum of all the drive currents) is from a constant current source assuring that the sum of the individual gains is held constant Figure 1 below shows a block diagram of the Interpolating Scanner. There is a Current Controlled Amplifer (CCA) of traditional design for each signal input. Each CCA interfaces appropriately to the input signal, has a current source output, and a gain (transconductance) that is directly proportional to its control current input. The CCA outputs are summed by shorting them directly together. A buffer amplifier stage provides the appropriate output level and impedance. The CCAs are transconductance amplifiers; the input is a voltage, the output is a current, the gain is linearly proportional to the control current, and in this case the control current is expected to be from 0. 0 to 2. 0 mA. More on this later. In the schematic above, the opamp provides a...

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