ADC - Analogue Digital Converter

The ADC will be used to sample the output from ADCS, sample the temperature reading from the CPU and sample the Payload output.

The ADC samples between two set reference voltages. It has a reference generator which can be used to generate 1.5v and 2.5v reference voltages for use with sampling.

To access the on-board temperature sensor, a pre defined input channel is sampled. This can be handled entirely by the board with no involvement of external subsystems, and should not affect their operation.

The onboard ADC can sample at a rate greater than 200 ksps, through eight independent input channels. The digital output is given by the formula

(1)
\begin{equation} N = 4095 * (Vin - Va)/(Vb - Va) \end{equation}

where Vin is the input voltage, and VA and VB are configurable internal voltages used to define the limits of the sampling such that

(2)
\begin{equation} Vb=-Va \end{equation}

and the magnitude of the voltages can be either 1.5V or 2.5V, giving a sampling range of 3V or 5V.

The ADC has 4 conversion modes set by the two CONSEQx bits in ADC12CTL1:

  • 00 – A single channel is converted once
  • 01 – A sequence of channels is converted once
  • 10 – A single channel is converted repeatedly
  • 11 – A sequence of channels is converted repeatedly

The first mode (00) will be used to sample the ADCS analogue output, as only a single channel is used and the ADCS control timing is such that converting repeatedly is not required.

Payload will likely use the third or fourth conversion modes.

When data is converted by an ADC channel, it is placed into a conversion buffer of 16 x 12 bits. Once 16 signal levels have been sent, the ADC generates an interrupt which clears the buffer. At 200 ksps this happens every 80 microseconds.

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