2.6.2.1 - Analog Inputs With DAC1 Enabled (Hardware Revisions 1.20 & 1.21 only) [U3 Datasheet]

This Section only applies to the older hardware revisions 1.20 and 1.21. Starting with hardware revision 1.30, DAC1 is always enabled and does not affect the analog inputs.

The previous information assumed that DAC1 is disabled. If DAC1 is enabled, then the internal reference (Vref = 2.44 volts) is not available for the ADC, and instead the internal regulator voltage (Vreg = 3.3 volts) is used as the reference for the ADC. Vreg is not as stable as Vref, but more stable than Vs (5 volt power supply). Following are the nominal input voltage ranges for the analog inputs, assuming that DAC1 is enabled.

Table 2.6.2.1-1. Nominal Analog Input Voltage Ranges (DAC1 Enabled)

Note that the minimum differential input voltage of -3.3 volts means that the positive channel can be as much as 3.3 volts less than the negative channel, not that a channel can measure 3.3 volts less than ground. The voltage of any analog input pin, compared to ground, must be in the range -0.3 to +3.6 volts, for specified performance. See Appendix A for voltage limits to avoid damage.

Negative channel numbers 30 and 32 are not valid with DAC1 enabled.

When DAC1 is enabled, the slope/offset calibration constants are not used to convert raw readings to voltages. Rather, the Vreg value is retrieved from the Mem area, and used with the approximate single-ended or differential conversion equations above, where Span is Vreg (single-ended) or 2Vreg (differential).