Designing Embedded Hardware, 2nd Edition

Interfacing an External ADC

There is a very wide range of ADCs available, for every considerable purpose. Choose from very low-cost, low-speed ADCs for simple voltage conversion to very high-speed, precise (and expensive) ADCs for sampling video streams. Many microcontrollers have inbuilt ADC subsystems, making analog interfacing simple. However, if the processor doesn't incorporate an ADC, or its ADC is not suited to your application, it becomes necessary to add an external device.

A good general-purpose ADC for sensor applications is the Maxim MAX1245. It has eight channels of analog input and can sample at 100,000 samples per second, with a resolution of 12 bits. (There are similar devices with resolutions ranging from 8 bits to 16 bits, and with interfaces such as SPI, I²C, and processor bus.) The MAX1245 has an internal track and hold, preventing a changing signal from corrupting the result during a conversion. The MAX1245 is interfaced to a host processor via an interface that is compatible with SPI, Microwire, and the serial interfaces found in Texas Instruments TMS320-series DSP processors (Figure 13-10). As you can see, the MAX1245 is very easy to use. In this schematic, the analog input comes in via an IDC header, the 16-pin connector on the left of the figure. Note that every second pin on the connector is tied to ground. This means that every second wire in the connected cable will be grounded, providing a degree of noise immunity to our analog signals.

Figure 13-10. MAX1245 ...