Digi BL1800 user manual Schematic Diagram of A/D Converter

3.3 A/D Converter

The analog-to-digital (A/D) converter, shown in Figure 9, compares the DA0 voltage to AD0, the voltage presented to the converter. DA0 therefore cannot be used for the digital- to-analog (D/A) converter when the A/D converter is being used.

Vcc	Vcc
R34	R31
51.1 kW	10 kW
AD0			DA0 too low
	9 –
			8	R36
R35		LM324			PE7
200 W	10	+		0 W
DA0	13	–
R33				R30
200 W		LM324 14			PE6
	12			0 W
R32		+
			DA0 too high
51.1 kW

Figure 9. Schematic Diagram of A/D Converter

The A/D converter transforms the voltage at DA0 into a 20 mV window centered around DA0. For example, if DA0 is 2.0 V, the window in the A/D converter would be 1.990 V to 2.010 V. If AD0 > 2.010 V, PE7 would read high and PE6 would read low. If 1.990 V < AD0 < 2.010 V, PE7 would read low and PE6 would read low. This is the case when the A/D input is exactly the same as DA0. If AD0 < 1.990 V, PE7 would read low and PE6 would read high.

PE6 can be imagined to be a “DA0 voltage is too high” indicator. If DA0 is larger than the analog voltage presented at AD0, then PE6 will be true (high). If this happens, the pro- gram will need to reduce the DA0 voltage.

PE7 can be imagined to be a “DA0 voltage is too low” indicator. If DA0 is smaller than the analog voltage presented at AD0, then PE7 will be true (high). If this happens, the pro- gram will need to raise the DA0 voltage.

The A/D input, AD0, is the same as DA0 only when PE6 and PE7 are low. Because the A/D converter circuit uses a 20 mV window, the accuracy is ±10 mV. DA0 can range from

0.1V to 2.8 V, which represents 270 steps of ±10 mV. This is better than 8-bit accuracy. Since the D/A converter is able to change the DA0 output in 3.88 mV steps, there are 697 steps over the range from 0.1 V to 2.8 V. This represents a resolution of more than 9 bits.

User’s Manual 21