A/D Converter - RabbitCore RCM4500W User Manual

4.4.2 A/D Converter

The RCM4510W modules' XBee RF module has four inputs on header J4 that may be set up in
software as analog inputs.
The four analog input pins, ADC0–ADC3, each have an input impedance of 6–7 M, depending
on whether they are used as single-ended or differential inputs. The input signal can range from -
1.2 V to +1.2 V (differential mode) or from 0 V to +1.2 V (single-ended mode).
Use a resistor divider such as the one shown in Figure 10 to measure voltages above 1.2 V on the
analog inputs.
ADC0
ADC1
AGND
The R1 resistors are typically 20 k to 100 k, with a lower resistance leading to more accuracy,
but at the expense of a higher current draw. The R0 resistors would then be 180 k to 900 k for
a 10:1 attenuator. The capacitor filters noise pulses on the A/D converter input.
The A/D converter can only accept positive voltages. With the R1 resistors connected to ground,
your analog circuit is well-suited to perform positive A/D conversions. When the R1 resistors are
tied to ground for differential measurements, both differential inputs must be referenced to analog
ground, and both inputs must be positive with respect to analog ground.
User's Manual
R0
R0
C
Figure 10. Resistor Divider Network for Analog Inputs
(RCM4510W
ZigBee
modem)
R1
R1
C
3
1
ADC
ADC
VREF
38