Digital panel meter design

M. Ganesh Raaja and Sachin Gupta, Cypress Semiconductor

July 30, 2012

M. Ganesh Raaja and Sachin Gupta, Cypress Semiconductor

Mixed Signal System-on-Chip approach
Mixed signal devices have both analog and digital peripherals on-chip. Apart from offering all the advantages of an MCU-based approach, these devices can integrate the analog front-end into the same device to further reduce the size and cost of the system. Figure 4 below shows the block diagram of a digital panel meter using a mixed signal SoC processor.

Click on image to enlarge.

Figure 4: PSoC 1-based digital panel meter solution
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 Analog 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 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.