Design Ideas: March 14, 1996
The circuit in Figure 1 resembles a capacitive-ladder (C-2C) D/A converter. However, it does not provide conversion from a digital code to a voltage or current, but rather converts the digital code to a capacitor value. The C-2C topology is well-suited for IC or hybrid implementation, as it calls for only two nominal values. You can prove mathematically that the equivalent capacitance between terminals 1 and 2 is CEQ=2C·(Di·2-i), where Di represents the binary coefficients of the digital-input code.The follower IC1 keeps both buses I1 and I2 at equal potential. Thus, the total current from the network to ground is constant (code-independent) at any given voltage and frequency, but the distribution of this current between buses I1 and I2 depends on the digital code. The more ones in the input code (the right switch position corresponds to an input one), the larger the input current and, consequently, the larger the equivalent capacitance CEQ. The circuit uses a Linear Technology Corp LT1354 C-Load op amp, which is tolerant of high capacitive loads (to 10 nF). Possible applications of the D/C converter include programmable filters, measurement bridges, programmable-impedance networks, and biomedical instrumentation (for example, a galvanic skin-effect simulator). (DI #1840)