Electronic Design

Programmable Differential Amplifier Features Logarithmic-Type Gain

The circuit shown is a differential amplifier that not only offers programmability but also provides a logarithmic-type gain (Fig. 1). The traditional gain-setting resistors of the amplifier, R1 and R2, are used with a pair of digitally controlled potentiometers, DCP1 and DCP2. It’s the potentiometer portion of the DCPs that adds variability to the circuit. The circuit’s control and memory section provides programmability via the serial bus.

Two equivalent resistances, R1 and R2, are implemented with the potentiometer. They’re mathematically modeled as kR and (1-k)R, where k is a number that varies from zero to one and R is the end-to-end resistance. The value of k reflects the proportionate position of the wiper from one end of the pot (0) to the other end (1). In terms of k, the gain (G) of the circuit is equal to (1-k)/k. Due to its pseudo-logarithmic behavior, this circuit provides over three decades of gain through the 64 gain settings of the Xicor dual 64-tap potentiometer (Fig. 2).

The wipers of the two DCPs are software-ganged and the performance of the circuit takes advantage of the resistor matching inherent in the integrated circuit. Any error caused by the potentiometer’s wiper resistance is negligible. This is because the wiper resistance is in series with the high-input impedance of the operational amplifier. In Figure 2, the measured dc data shown fell within 2% of the calculated values. Using only two components, this circuit is appropriate for test and measurement, signal processing, and audio applications.

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