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Designing digital panel meter

Posted: 16 Aug 2012     Print Version  Bookmark and Share

Keywords:Panel meters  Analogue  digital 

These devices have on-chip programmable gain amplifiers (PGA) that can be used for signal conditioning. Multiple input ranges can be supported by changing the gain of the amplifier in firmware.

These devices also have internal low pass and band pass filters that can improve signal conditioning, an internal accurate band-gap reference, and the ability to provide buffered Analogue Ground (AGND) to bias bipolar and AC input signals. Different types of ADCs like incremental and Delta Sigma are available with configurable resolution.

Depending on the accuracy and speed requirements, the resolution may be selected. True RMS calculations may be performed using the on-chip MAC (Multiply Accumulate) registers which perform efficient hardware multi-multiplication and accumulation.

Since SoC devices contain the signal conditioning circuit on chip, dynamic offset compensation called correlated double sampling may be performed. In this method, the input of the signal conditioning circuit is shorted to AGND and the output of the ADC is measured.

This ADC result represents the total offset error of the system, including the signal conditioning circuit and the ADC. This value is saved temporarily. The input signal is now connected to the signal conditioning circuit, and the ADC result is measured. The saved offset is subtracted to get the offset compensated value.

The advantage of this offset compensation over the offset compensation done in two-point calibration is that this method takes care of the offset drift due to temperature changes as well.

Also, as the offset calibration can be performed using correlated double sampling, the digital calibration can be performed using a single point compared to the two points required for an MC-based approach. This further reduces the calibration time.

SoC devices also have on-chip comparators that can implement Hysteresis comparators, window comparators, or Zero Crossing detectors. These comparators may be used in implementing frequency meters where the input signal is fed to a comparator and the digital output of the comparator is fed to the capture input of a timer to measure the period of the input signal.

Window comparators may be used to generate hardware alarm signals when the input signal is outside a defined range. Integrated on-chip UARTs can implement communication protocols like MODBUS, and on-chip PWMs and DACs can be used to implement 4 to 20mA outputs.

Some SoCs also integrate capacitive buttons and sliders. These can be used to implement touch-based keypads which do not have the disadvantage of wear and tear associated with mechanical buttons. Apart from increasing reliability, this also reduces the cost of the user interface. Summarising, the advantages of a mixed signal SoC controller-based approach are:

 • Lowest possible external component count due to integration of analogue front end;
 • Programmable Gain Amplifiers and filters on chip;
 • True RMS calculation using hardware MAC;
 • Frequency measurement using on-chip comparators and timers;
 • Dynamic offset compensation using correlated double sampling; Single point digital calibration;
 • 4 to 20mA output; support for MODBUS communication protocol;
 • Capacitive touch interface capabilites; and
 • Reduced inventory since the same hardware can support multi-multiple input ranges.

Similar to the MCU-based approach, the disadvantage of an SoC-based approach is the need for knowledge in microcontroller system design and programming.

In price-sensitive markets where end-customers demand more features, batter accuracy, and flexible configurability, a mixed signal SoC device based approach is the best fit. SoCs enable multi-multiple functions to be integrated onto a single chip and reduce system footprint size. Also, they help in reducing inventory as only a single device is needed to support different configurations and the fact that SoCs do not need external signal conditioning and peripherals.

About the authors
Ganesh Raajareceived his degree in Electronics and Communication Engineering from Motilal Nehru Government Polytechnic, Pondicherry. His expertise lies in developing with analogue circuits, embedded systems, designing PCBs, and working with Assembly and C.

Sachin Guptais working as Product Marketing Engineer with Cypress Semiconductor. He holds B.Tech in Electronics and Communication from Guru Gobind Singh Indraprastha University, Delhi. He has several years of experience in mixed signal applications development.

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