Digi user manual A/D Converter RCM4000 only, Resistor Divider Network for Analog Inputs

4.4 A/D Converter (RCM4000 only)

The RCM4000 has an onboard ADS7870 A/D converter whose scaling and filtering are done via the motherboard on which the RCM4000 module is mounted. The A/D converter multiplexes converted signals from eight single-ended or four differential inputs to Serial Port B on the Rabbit 4000.

The eight analog input pins, LN0–LN7, each have an input impedance of 6–7 MΩ, depending on whether they are used as single-ended or differential inputs. The input signal can range from -2 V to +2 V (differential mode) or from 0 V to +2 V (single-ended mode).

Use a resistor divider such as the one shown in Figure 10 for the analog inputs.

R0				ADC
LN0
				(RCM4000)
LN1				ADC
R0
C	C	R1	R1
			3	BVREF
			1
AGND

Figure 10. Resistor Divider Network for Analog Inputs

The R1 resistors are typically 20 kΩ to 100 kΩ, with a lower resistance leading to more accuracy, but at the expense of a higher current draw. The R0 resistors would then be

180kΩ to 900 kΩ for a 10:1 attenuator. The capacitor filters noise pulses on the A/D converter input.

The actual voltage range for a signal going to the A/D converter input is also affected by the 1, 2, 4, 5. 8, 10, 16, and 20 V/V software-programmable gains available on each channel of the ADS7870 A/D converter. Thus, you must scale the analog signal with an attenuator circuit and a software-programmable gain so that the actual input presented to the A/D converter is within the range limits of the ADS7870 A/D converter chip (-2 V to + 2 V or 0 V to + 2 V).

The A/D converter chip 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 35