The third byte sent to a PCF8591 device is stored in the DAC data register and is converted to the corresponding analog voltage using the on-chip D/A converter. This D/A converter consists of a resistor divider chain connected to the external reference voltage with 256 taps and selection switches. The tap-decoder switches one of these taps to the DAC output line (see Fig.6).
The analog output voltage is buffered by an auto-zeroed unity gain amplifier. This buffer amplifier may be switched on or off by setting the analog output enable flag of the
control register. In the active state the output voltage is held until a further data byte is sent.
The on-chip D/A converter is also used for successive approximation A/D conversion. In order to release the DAC for an A/D conversion cycle the unity gain amplifier is equipped with a track and hold circuit. This circuit holds the output voltage while executing the A/D conversion.
The output voltage supplied to the analog output AOUT is given by the formula shown in Fig.7. The waveforms of a D/A conversion sequence are shown in Fig.8.