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Technology Briefing: Signal Conditioning ICs
Technology Briefing: Signal Conditioning ICs
The increasing need to measure and connect the analog world using digital signal processing is driving the rapid development of sensor IC technology. The ability to integrate both analog and digital functions on a single chip has enabled the development of standard sensor signal conditioner (SSC) ICs especially for conditioning Wheatstone bridge sensors, most commonly used in pressure/force measurement applications. ZMDI addresses this application with its RBIC™ ‘Lite’ family of Signal Conditioning ICs.
In this Ismosys Technology Briefing we focus on a family of high performance signal conditioners for bridge type sensors that offers advanced features including; chopper stabilised analog front end, high resolution A-to-D conversion,
digital calibration math based with EEPROM stored coefficients and a variety of digital and analog outputs. Useful features such as power safe mode, internal regulator and diagnostic function are complemented by end-of-line calibration, a feature supported by ZMDI’s proprietary one-wire protocol.
The increasing need to measure and connect the analog world using digital signal processing is driving the rapid development of sensor IC technology. The ‘raw’ signal produced by analog electrical sensors is today transformed by advanced signal conditioning technology into a standardised digital/analog output signal capable of handling tasks such as signal amplification, error correction and the removal of unwanted environmental influences, i.e. temperature dependence. The ability to integrate both analog and digital functions on a single chip has enabled the development of standard sensor signal conditioner (SSC) ICs especially for conditioning Wheatstone bridge sensors, most commonly used in pressure/force measurement applications as well as magneto-resistive bridges used in linear and angular position measurement. The differential output signal and the ‘ratiometricity’, i.e. the strict proportionality of signal and supply voltage connected with this sensor principle are ideal for precise and very robust signal processing in relation to almost any kind of disturbance.
The method of calibrating the device is among the major cost-determining factors in sensor module production. The conventional calibration approach is to use potentiometers and laser-trimmed correction resistors in a complex, and usually iterative calibration process that produces insufficient accuracy (only gain and offset) and insufficient stability. The ideal solution is a universal SSC IC that interfaces with the widest variety of resistive bridge sensors and offers the broadest range of features and yet supports a wide variety of analog and digital standard outputs.
This is exactly the requirement that German manufacturer ZMDI set out to address when it developed the ZMD31050 universal SSC. The great variety of ZMD31050’s features includes selectable sensor bridge excitations (constant voltage or constant current), selectable output options (ratiometric voltage output, current loop output 4 to 20mA, I2C, SPI, PWM, ZACwire ), selectable internal/external temperature sensor types and digital compensation of sensor offset, sensitivity, temperature drift and non-linearity up to 3rd order as well. The proprietary end-of-line calibration implemented in this device eliminates multiple iterations.
The ZMD31050 is a very attractive solution to many custom transducer manufacturers but the company’s RBIC™ family of ‘Lite’ products provides additional solutions to applications that demand low power consumption, a high degree of miniaturisation and cost efficiency. Based upon the modular concept of the ZMD31050, the ZMDI Lite family comprises three devices:
ZMD31010 RBIC Lite
This device includes easy and precise end-of-line calibration. The calibration coefficients are stored in the on-chip EEPROM.
ZMD31015 RBICd Lite (‘d’ = diagnostics)
This device provides many diagnostic functions such as EEPROM signature, bridge connection check, bridge short detection and power loss detection.
ZMD31014 RBICi Lite (‘i’ = interface)
This device utilises the standard I²C™ interface and the SPI outputs are implemented with a higher number of programmable analog gain factors and calibration coefficients.
The core blocks and the associated features of the Lite IC family of SSC ICs are illustrated in figure 1
Analog Front End (AFE)
The analog front end (AFE) of ZMDI’s Lite product family is characterised by the chopper stabilise pre-amplifier block, the second-order charge balancing A/D converter block and the input multiplexer block, which features an auto-zero function.
To reach the required output resolution of 10 to 12 bits, the AFE must be configured, both in the GAIN (preamp) parameter and by an adjustable zero point shift in the ZERO parameter, to roughly match the actual output voltage range of the sensor (see figure2). The AFE is also responsible for sampling the temperature signal and the offset (Autozero).
The key differences between the AFE provided by ZMDI’s Lite products and the AFE provided by the company’s universal SSC are principally; the Lite product family is only applicable for bridges with voltage excitation, the devices only offer a two-stage pre-amplifier with 2 to 6 configurable GAIN values and with no analog pre-compensation of higher bridge offset voltages, and reduced adjustment capability of the14 bit A/D converter used to compensates for lost sensor signal range resolution.
Digital Core
A microcontroller can be used to provide functions such as clock control of the analog circuitry, correction calculations and to implement the serial communication interface, however many applications demand less complex calculations and would benefit from further chip size reduction. These applications are better served by the state machine concept realised in the ZMDI Lite product family. The16 bits processing, data wide EEPROM coefficients and 14 bit A/D conversion offered by Lite products guarantees high-level computational accuracy and overflow security in the fixed-point arithmetic. The stored calibration coefficients and the already auto-zero compensated ADC raw values of the bridge-signal and the temperature-signal ensures that the corrected output signals are calculated. For low power applications, which typically require only a few measurements per second, the devices adopt a ‘standby/update mode’ that deactivates the bridge between single measurements.
Focus on the ZMD31014 RBICi Lite
Back End
The I²C and SPI protocols are the most common digital interfaces for sensor signal readout. By only supporting these two protocols at the output the ZMD31014 RBICi Lite provides analog output current consumption as low as 10µA in its update mode. The digital interface also serves the so-called ‘end-of-line’ calibration, which prevents mechanical or thermal stress changes to sensor parameters during the module manufacturing process. If the IC doesn’t receive the required command after power ON it won’t enter the communication mode and will continue in the normal operation mode.
Diagnostic Functions
All devices in the ZMDI Lite product family detect and report on key error states including; sensor errors (loss or short-circuit of bridge connections), data errors in the on-chip EEPROM (check sum error) and power and ground loss. Today’s quality standards often demand additional diagnostic functions to aid traceability in failed units. The ZMD31014 RBICi Lite supports this requirement by reserving space in its EEPROM for customer data. A LOCK function prevents changes to this data after module manufacture.
One Shot “End of Line” Calibration
The calibration procedure is a particularly cost-sensitive part of sensor module mass production as numerous pressure and temperature values must quickly be applied to each module. Flexible and effective calibration can only be ensured if there are no iterative steps, and if raw data is collected in arbitrary order with correction coefficients and programming determined as separate processes. Further, the quality of the calibration result must be fully independent, regardless of whether the values are intended for pressure or temperature measurement. Data can be collected by accurate reproduction of the operating points or by accurate back measurement in the respective operating point. An example of the calibration procedure that determines the characteristics of a sensor is illustrated in figure 3.
In the example shown all seven coefficients possible with the ZMD31014 RBICi Lite are used for bridge correction. The coefficients are computed after acquisition of raw data at seven calibration points. In order to minimise the random measurement and rounding errors those points should lie close to the respective limits as well as in the centre of the pressure and temperature range:
RB = O * (1+ TCO1 * ZT + TCO2 * ZT2) + G * (1+ TCG1 * ZT + TCG2 * ZT2) * ZB + SOT * ZB2
• G is the Gain correction factor "digitally zoom " up to x64
• O is the Offset correction
• SOT is the second order term for the correction of the nonlinearity by a polynomial of second order
• Compensation ofTCO1 and TCO2 are terms for the the Offset TC (first and second order)
• TCG1 and TCG2 are terms for the compensation of the Gain TC (first and second order)
Note that the errors at the calibration points become zero. Although, the error in the ranges between them will be the smaller, the better equation (1) describes the real physically caused sensor behaviour. The solution of the polynomial equation set, particularly if the operating points are not strictly fixed (back-measured values), can be calculated by a nonlinear optimisation method (curve fitting). For that reason the Excel Solver development software that ZMDI offers to support the Lite family of signal conditioning ICs merges in steps into the DLL for coefficient calculation. Called ‘One-Shot’ this step process acquires raw digital data and calculates the coefficient by curve fitting and programming of all coefficients. In this way ZMDI’s Lite product family fulfils all conditions for an effective calibration procedure.
