How good is the microcontroller's on-chip ADC?
An aid to selecting a microcontroller with high-resolution analogue/digital converters for measurement systems and metering designs by Ralf Hickl
The quality of analogue peripherals on microcontrollers has risen constantly. It allows for better integration and more compact switching solutions.
Performing measurements with utility meters, electrical bridges and the analysis of small, unamplified signals also requires a high nominal resolution. This is achieved by microcontrollers with analogue/digital converters (ADCs) that operate according to the delta-sigma principle.
Multipurpose microcontrollers (MCUs) are usually fitted with ADCs that perform conversion using a successive approximation register (SAR). This allows them to achieve nominal resolutions of between 10 and 12 bits.
For higher resolutions, microcontrollers with ADCs based on the delta-sigma principle are also offered. The most recent examples are the new 32-bit microcontrollers from Renesas Electronics, STMicroelectronics, Infineon and Microchip.
The RX21A with 32-bit CPU core is part of the low-power RX200 family from Renesas. Fitted with up to seven delta-sigma ADCs, each with 24-bit resolution, it achieves conversion rates of up to 12ksample/s for each ADC.
The ADC has at its disposal an internal bandgap voltage reference with a low temperature drift of maximum 30 ppm/°C as a reference voltage.
The inputs of the ADCs can be connected internally with the outputs of the two on-chip digital/analogue converters with a resolution of 10 bits. This is useful for calibration purposes or for measuring the supply voltage.
Each ADC is connected upstream of a PGA (programmable gain amplifier) with an amplification range between 1 and 4 or 1 and 64. For four of the seven PGAs, the inputs are differentially external. The other three inverted inputs are switched together and occupy a single common pin.
The RX21A has a decryption and encryption unit that operates on the basis of the Advanced Encryption Standard (AES) to provide secure remote readings. The capture function of the real time clock is used for timestamping in the case of attempted tampering (tamper detection). UART, IrDA and SPI interfaces are available.
Two comparators and two digital/analogue converters are further additions to the analogue features. An ADC that has a resolution of 10 bits and converts using the SAR principle is also one of the features. With these characteristics, the RX21A is suitable to be part of one-phase and three-phase electricity measurement device designs.
The RX21A is available with 64, 80 and 100-pin LQFPs. The integrated flash memory is implemented using Renesas’ own MONOS technology. A design feature of this flash memory is that it enables the microcontroller to execute code over its entire clock range of up to 50MHz without wait states. Memory can range from 256kbit to 512kbit of flash storage.
As a replacement for an EEPROM, 8kbit of flash memory with at least 100,000 specified erase/write cycles is reserved.
The STM32F373 is part of the 32-bit microcontroller family with an ARM Cortex-M4F CPU core and floating point unit (FPU). The modules offer up to three separate delta-sigma ADCs with resolutions of 16 bits. Conversion rates of 16.6ksample/s to 50ksample/s can therefore be achieved depending on the operating mode.
Added to this is an ADC with 12-bit resolution using SAR conversion, up to three digital/analogue converters, also with resolutions of 12 bits, and two rail-to-rail comparators.
The STM32F3 offers a range of interfaces and can be connected via USB or CAN. The USARTs have control signals for modems but also support IrDA and ISO7816-compliant transmission for smart cards.
Up to 24 contacts can be connected to the integrated touch sensing controller. The real time clock and a number of RAM cells are in a dedicated circuit with an exposed pin, which can be supplied with power via a back-up battery.
With this feature set, the STM32F3s are typically used in applications such as measurement and electrical drive systems.
Also, the small package WLCSP66, measuring just 3.8mm x 4.3mm, fits well in miniaturised sensors.
STMicroelectronics offers the STM32F3s in packages with between 48 and 100-pins. Storage capacity ranges from 64kbit to 256kbit of flash ROM. The RAM has parity bits which enables bit errors to be recognised automatically.
In the 100-pin package, the pins for the Embedded Trace Macrocell are external, thus providing additional debugging options.
With the XMC4500 range, Infineon is also backing the ARM Cortex-M4 with FPU and DSP add-on as a CPU core. A special feature of the XMC4500 is that no complete DS converter is implemented, just the demodulator part (DSD).
There are four of these fitted as peripherals on the microcontroller. A DS demodulator consists of low-pass filters connected in sequence combined with undersampling to reduce the data rate.
With the XMC4500, software settings can be used to cascade up to three of these SINC filters. The undersampling rate too can be configured using a special function register. At the end, each DSD provides a 16-bit result.
The DS modulator parts must be located in external components. This allows them to offer more for less, because this clever arrangement provides a number of advantages.
The first advantage is the simple galvanic isolation between the modulator and demodulator/microcontroller, as only the two digital signal lines clock and bitstream need to be isolated for each channel.
Secondly, the size of the surface where faults can arise on the transmission route from the sensor to the microcontroller is kept small because the digitalisation is performed in the external modulator element, thus near to the measurement point.
This approach is superb for determining mains currents as voltage drops across shunts. The principle is applied in particular in electrical drive technology. An d with consumption measurement devices, this provides the third advantage – protection against tampering – because the use of shunts to measure current is not sensitive to tampering using strong magnetic fields from outside.
Examples of power amplifiers with integrated DS modulators are Infineon’s MIPAQ-sense 3-phase IGBT bridges.
The applications that the XMC4500 is designed for are in inverters, switched mode power supplies, sensors and industrial control units.
This system-in-a-package stands out with its high level of integration. An 8-bit microcontroller from Microchip’s PIC18 range and an analogue front end (AFE) similar to the MCP3901 are located in the package.
The AFE is two-channel, comprising PGAs, two delta-sigma converters with synchronised reading at 24-bit resolution and adjustable phase correction. The internal reference voltage source offers a low temperature drift of typically just 12 ppm/°C. The microcontroller and AFE communicate via an SPI interface.
The use of the charge time measurement unit (CTMU) as a capacitive touchscreen controller and the LCD driver for up to 33×4 segments on the microcontroller also provides operating and display functions on this versatile low-cost IC. A clock with calendar, an ADC with 12-bit resolution operating using a SAR and serial interfaces are also available.
The PIC18F87J72 is delivered in a TQFP80 package with either 64kbit or 128kbit of flash memory. Parameter data can be stored in the program memory using the self write function.
Microchip has designed the PIC18F87J72 for applications such as single-phase energy meters, power meters, portable gauges and medical diagnostic equipment.
Ralf Hickl is product sales manager for microcontrollers at Rutronik