Capacitance and impedance converters

has taken the wraps off what it claims are the world’s first converters that address the signal processing challenges of direct capacitance-to-digital and impedance-to-digital conversion.

The first three devices in ADI’s capacitance-to-digital converter (CDC) family - the AD7745, AD7746 and AD7747 - combine 24-bit resolution, low noise of 5 aF (10-18) per root hertz, and low power (1 mA max) with a complete range of on-chip analog functions. These functions include a 24-bit, sigma-delta analog-to-digital converter (ADC), a precision low-noise, low-drift voltage reference, temperature sensor, clock, multiplexer, calibration registers and 2-wire (I2C compatible) serial interface.

For their part, the AD5933 and AD5934 impedance-to-digital converters (IDC) allow an external complex impedance (range 100 ohms to 10M ohms) to be excited with a known frequency of up to approximately 100 kHz. The response signal from the impedance is sampled by the on-board ADC and discrete Fourier transform (DFT) processed by an on-board DSP engine. The DFT algorithm returns a Real (R) and Imaginary (I) data word at each frequency point (in the case of a sweep) allowing impedance to be conveniently calculated based on an initial calibration.