Electronic Design

ADCs Digitize Capacitance And Impedance Directly

These monolithic devices simplify measurement-system design in applications ranging from automtove to medical.

Changes in capacitance, or in a complex impedance, are the input for many control applications. For example, capacitive sensing measures gas pressure and liquid volume. It's also used in proximity and humidity detectors. Impedance sensing is used in electro-impedance spectrometry, corrosion analysis, and liquid-condition analysis in industrial applications as well as for analyzing biological samples in medical equipment. Digitizing these changes has typically involved designing some kind of analog front end and applying its output to a data converter.

Two new families of monolithic analog-to-digital converters (ADCs) crafted by Analog Devices simplify the design of systems that use capacitive or impedance sensing.

The AD774x family of capacitance-to-digital converters (CDCs) will work with a total variation of ±4 pF in input capacitances up to 17 pF and resolve variations down to ±2 fF with ±0.01% linearity (see the figure). The ADC portion of the chip is a switched-capacitor-input, 24-bit sigma-delta that provides 19 bits of effective resolution at a 16.6-Hz data rate. Output is via an I2C serial interface. Compensation comes by way of an on-chip digital-to-capacitance converter (CAP DAC).

Three members make up the family. The single-input AD7745 and the multiplexed dual-input AD7746 both can be configured for single-ended or differential input from floating capacitive sensors. The AD7747 targets capacitive sensors that have one plate connected to ground.

The AD593x family of impedance-to-digital converters (IDCs) measures complex impedances from 0.1 kΩ to 10 MΩ with 12-bit resolution. It combines an on-board direct-digital-synthesis (DDS) frequency generator with a range from dc to 100 kHz and a 12-bit, 1-Msample/s ADC.

The frequency generator excites the external complex impedance. The chip samples the response with an on-board ADC, whose output is processed into I and Q components by an on-board DSP engine. These components then can be resolved into the actual impedance value at each frequency in the sweep.

The AD5933 runs at 1 Msample/s, and the AD5934 runs at 250 ksamples/s. System designers can program either chip's DDS excitation amplitude and frequency sweep to a 27-bit resolution (<0.1 Hz). Maximum frequency is 100 kHz.

At each point on the sweep, the ADC takes 1024 samples. The DSP engine then performs a discrete Fourier transform (DFT) on the data to resolve it into real and imaginary components. An on-chip temperature sensor allows for temperature compensation down to 628C accuracy.

All devices are currently sampling. Production quantities of the AD7745, AD7746, AD5933, and AD5934 should be available in May. Currently, the AD7747 is also sampling, with production quantities slated for August. The devices come in 16-lead TSSOP and SSOP packaging. In 1000-piece quantities, unit prices range from $4.60 to $4.95 for the CDCs and from $4.35 to $6.65 for the IDCs.

Analog Devices Inc.

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