Precision Strain Gage Measurement

Compact chip offers analog alternative with lower current, faster updates

Designed for strain or piezo gage measurement, the purely digital Picostrain PS01 chip, made by ACAM-Messelectronic GMBH, Germany, and distributed by Transducers Direct LLC, Cincinnati, OH, capitalizes on time-to-digital (TDC) technology to offer product designers a more flexible alternative to analog for measuring metal strain gages or temperature. The PS01 determines resistor ratio from a highly precise time interval measurement, done with a resolution of 14 ps, which corresponds to a respectable 70 GHz for a counter. This digital measuring principle provides the advantages of faster updates and lower total current requirements. It eliminates complex analog circuitry to increase battery life and simplify integration. Measuring only 9 mm2, the compact chip is suited for consumer and industrial products, such as weight scales, postage scales, pressure transducers, load cells, solar-cell driven equipment and portable instrumentation, as well as temperature measurement. Operating from single or dual power supplies, the PS01 is suited for use in microcontroller- or DSP-based systems.

Strain gages used with the PS01 chip require no separate power supply or reference voltage and connect directly to the chip. The device can work with a full bridge or half bridges — bridge offset trimming is unnecessary — and the user determines how much current is supplied to the strain gage. Depending on the application, the total current of the system can be reduced down to only 5 µA. It is designed for resistor elements from 250O to 10 kO. Battery powered devices, such as those using 3V lithium coin cells, can experience extended battery life up to ten times, depending upon the application, because there is lower current draw than with analog circuitry.

The unit offers programmable update rates up to 65 KHz. Digital design provides excellent output stability over a wide operating temperature range of –40 to 120°C. The chip has a standard 8-bit parallel or alternative 3-wire serial SPI-compatible serial interface, housed within a 48-lead TQFP enclosure. It supports temperature measurement of up to four sensors.

The optional PSA1 setup and evaluation shortens development time for boards using the PS01 chip by allowing all of the chip’s setup parameters to be configured and checked quickly via a graphical PC interface. The PSA1 connects to a PC via an RS232 cable and operates in a Windows environment. It can also be used as a stand-alone unit with 0 to 10 VDC analog output. Several settings for core and I/O supply voltage can be selected with jumpers. Additional jumpers allow current measurement. Two optional clocks with 32,768 Hz (low power mode) or 10 MHz (high power mode) provide a reference frequency.

—SG