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.
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