Converter Chips Measure Capacitance And Impedance

Two new ICs are described as fully integrated devices that offer direct capacitance-to-digital and impedance-to-digital conversion. The single-chip AD7745 integrates all stages of capacitance-to-digital conversion with 24-bit resolution. The AD5933 is an impedance-to-digital converter using direct digital synthesis, a/d conversion and digital signal processing. The AD7745 is software configurable, eliminating the need for expensive hardware reconfiguration and offers a programmable output from 5 to 90 Hz. With the AD5933, an external complex impedance ranging from 100Ω to 10 MΩ can be excited with a known frequency of up to 100 kHz. The response signal from the impedance is sampled by the on-chip a/d converter and processed by a digital signal processor using a discrete Fourier transform (DFT). The DFT algorithm returns a real (R) and imaginary (I) data word at each frequency point (in the case of a sweep). Both types are supplied in either 16-pin TSSOP packages or 16-pin SSOP packages. Pricing ranges from $4.60 to $4.95 each/1,000 for the capacitance converter and $4.35 to $6.65 each/1,000 for the impedance converter. ANALOG DEVICES INC., Norwood, MA. (800) 262-5643.