9.9 The SD16_A Sigma-Delta ADC

Fortunately it is easier to use a sigma-delta ADC than to understand its operation. The MSP430 currently offers three varieties of 16-bit sigma-delta ADC:

• The original module was the SD16, which contains three independent channels. In effect there are three complete ADCs, each with a single input. They are linked with logic so that they can sample their inputs simultaneously or with a specified delay. The F(E)42x provides this module.

• This was followed by the SD16_A, which has only a single core but a multiplexer on the input. It can therefore convert the different inputs sequentially but not simultaneously. Other enhancements include finer control over the frequency of the modulator, a high-impedance analog buffer, increased oversampling range, and an internal potential divider for measuring V _CC. This is now sometimes called the SD16_A1 for clarity.

• The latest module is a combination of these two. It has several SD16_A cores, each with a single input, linked by control logic. It is available only in the F47xx at the time of writing.

Most of this description applies to all versions but I pick out the SD16_A1 in detail. It is currently included in the F20x3 and F(G)42x0 although there are differences between the two implementations. In particular, the F20x3 lacks the analog input buffer.