Capacitance and impedance converters

Analog Devices has introduced new families of capacitance-to-digital converters and impedance-to-digital converters to simplify instrumentation and sensor design in industrial, automotive, and medical applications.

Analog Devices

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.

All the devices are currently sampling with production quantities of the CDCs (AD7745 and AD7746) and the IDCs (AD5933: 12-bit, 1 MSPS; and AD5934: 12-bit, 250 ksps) available in May 2005. The AD7747 CDC is currently sampling with production quantities available in August 2005.

The devices are available in small 16-lead TSSOP (thin shrink small outline package) and 16-lead SSOP (shrink outline package) with prices ranging from $4.60 to $4.95 (CDC) and $4.35 to $6.65 (IDC) per unit in 1,000-piece quantities.