Capacitance And Impedance Converters—A High-Precision “First”

Paul Whytock reports on integrated converters that tackle the signal-processing challenges of direct capacitance-to-digital and impedance-to-digital conversion.

The world's first high-precision, integrated converters that address the signal-processing challenges of direct capacitance-to-digital and impedance-to-digital conversion have been developed by Analog Devices (ADI).

The company believes its capacitance-to-digital converter (CDC) and impedance-to-digital converter (IDC) devices attain a level of precision previously achieved with conventional analogue voltage-to-digital converters aided by a significant number of discrete components.

The devices aim at a range of high-performance instrumentation and sensing applications, from blood pressure monitors and glucose analysers to position sensors for automobiles and industrial corrosion analysis systems.

The AD7745 CDC integrates all stages of capacitance-to-digital conversion on one chip, reducing costs associated with traditional multichip solutions by 65%, says the company. It combines direct digital synthesis (DDS), analogue-to-digital conversion, and digital-signal-processing techniques.

"Designers of instrumentation equipment have long recognised the benefits of using capacitive and impedance techniques for applications requiring highly accurate measurements, but up until now they were constrained by complexities in implementation and by the significant design cost involved," explains Mike Britchfield, product line director, Precision Converters, Analog Devices.

Capacitive sensors are robust, accurate, and sensitive, but can be difficult to implement. Traditional capacitance sensor signal-processing solutions are either limited to power accuracy applications or require a multichip capacitance-to-voltage front end. ADI's CDCs eliminate these constraints by providing 24bit resolution and delivering on-chip analogue functionality. Because changes in the physical environment can change sensor parameters, the AD7745 is software configurable, eliminating the need for multiple customer product models and hardware reconfiguration. The software programmable output data rate can be varied from 5 to 90Hz. At 16.6 Hz, simultaneous rejection of 50 and 60Hz signals is achieved.

The first three devices in ADI's CDC family—the AD7745, AD7746, and AD7747—exploit ADI's experience in high-precision sigma-delta technology and system integration by combining 24bit resolution, low noise of 5aF (10-18) per root hertz, and low power (1mA max) with a range of on-chip analogue functions. These functions include a high-performance, 24bit, sigma-delta, analog-to-digital converter (ADC); precision low-noise, low-drift voltage reference; temperature sensor; clock; multiplexer; calibration registers; and a 2-wire (I2C-compatible) serial interface

Analog Devices also introduced a new XFP (10Gbit small form factor pluggable) optical transceiver chipset and reference design. XFP is rapidly becoming the leading standard for optical transceiver modules that connect to 10Gbps (gigabit per second) ports, such as those used in Ethernet, Fibre Channel, and SONET/SDH protocol applications. XFP is protocol agnostic, uses less than one-third the power, and is one-third the size of an MSA (Multisource Agreement) transceiver with parallel interface. It is also hot-pluggable.

For optical network engineers who must implement difficult 10 Gbps transceiver designs, this chip set achieves optical eye margins of greater than 20% over temperature to SONET specifications, and has receive sensitivities of better than —19dBm.

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