DAQ chips simplify sensor interfaces
By Graham Prophet, EDN Europe -- EDN, March 30, 2006
You can connect a wide variety of sensors to digital systems using austriamicrosystems' AS8500 and AS8501 chips to perform the analog-front-end and analog-to-digital-conversion function. The devices comprise a chopper input, a programmable-gain preamplifier, a 16-bit ADC, and a DSP filter block. They allow you to make direct connection to very-low-level sensor signals, such as low-value current-sense resistors or thermocouples, and operate from a single 5V supply. The chips offer resolution of less than 1 µV and linearity error below 0.1%, with sampling rates as high as 16 kHz. Applying correction in the digital section of the chip provides offset-free operation, and external averaging allows systems to achieve resolution of 21 bits. The chopper-input stage handles a maximum of four inputs, which you can use as single-ended, ground-referring connections or in pairs in a fully differential mode.
The company delivers the 8500 with a standard trim setting; for higher precision, the 8501 provides a calibration routine that characterizes each chip's functional analog parameters at final test and writes a set of trimming values into internal registers on the silicon. Internal-register settings also control the chips' configuration, and you can rewrite them at any time. The delivered default configuration is a read-only converter. Likewise, you set register values to select programmable gain of 6, 24, 50, or 100 to select full-scale measurement ranges of 7 to 120 mV. A three-wire serial interface outputs filtered digital data to a host system.
With austriamicrosystems' focus on mixed-signal design, the chip's designers gave particular attention to internal decoupling between analog and digital circuitry, part of a strategy to achieve low noise. Offset is less than 500 nV, and temperature-coefficient offset is negligible.
The chip also integrates a precision voltage reference, a temperature-measurement element, and a digital comparator. The parts operate over –40 to +125°C and suit use in automotive and industrial applications. To monitor current in a vehicle-battery lead, a 100-μΩ sense resistor provides measurement resolution of 10 mA in a full-scale range of 1500A. Self- and system-calibration routines run at power-up. The AS8500/1 chips sell for €5.50 ($6.60) and €6.60 ($7.90), respectively (1000).
