Analog-to-digital (ADC) converters are key to the success of analog microcontrollers but there are tradeoffs depending upon what kind of ADC is being use (see The Real World Versus Your ADC). Often the tradeoff is between less precision with a higher sample rate or high precision, Sigma-Delta ADC with a lower sample rate. Designers often have to determine which type of ADC because a microcontroller is typically bundled with one type or another.
Microchip's PIC24FJ128GC010 family (Fig. 1) combines two different ADC types onto a single chip. This includes a high precision, 16-bit Sigma-Delta ADC plus a high speed, 10 Msample/s pipeline ADC. This 16-bit, 32 MHz micro is loaded for analog work. It also includes a pair of 10-bit, 1 Msample/s DACs, a pair of rail-to-rail op amps, three rail-to-rail comparators and a charge time measurement unit (CTMU). The CTMU can be used for a number of purposes including capacitive touch applications.
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The 16-bit Sigma-Delta ADC has two differential inputs with a configurable gain section. The ADC has a maximum clock source of 4 MHz but the actual sample rate will be slower and based on a number of configuration factors. The pipeline ADC supports up to 50-channels and incorporates an auto-accumulate feature along with channel scanning list support and threshold detection. It can also operate when the processor and other peripherals are in sleep mode.
The chip designers took great care with the ADC layout. The Sigma-Delta unit actually sits on its own silicon island to provide as much isolation as possible. This is one reason it has fewer inputs than the pipeline ADC.
One application where the PIC24FJ's dual ADCs come into play is glucose meters. There are a number of ways to measure glucose. Some employ many readings over a short time period where frequency is important than just precision. There are other techniques that require high precision to make accurate measurements. This chip can handle either. Likewise, these sensing chores may not be the only ones the micro handles. It could also have other sensors or even track there battery voltage that may also affect the results. The mixed approach can be utilized in many applications from motor and power control to monitoring other types of sensors.
The PIC24FJ uses Microchip's XLP (extreme low power) technology. Chips are available with up to 128 Kbytes of flash and 8 Kbytes of RAM. Multiple I2C, SPI and serial interfaces are provided. The digital pin select support allows digital I/O to be mapped to almost any pin combination. LCD displays up to 472 segments are supported. The also have a USB On-The-Go (OTG) interface.
Pricing starts at $2.59. The PIC2F Intelligent.Integrated.Analog starter kit (Fig. 2) is available for $89.99. The PIC24 family is supported by Microchip's free MPLAB X (see NetBeans Powers New PIC IDE).