A/D Converter - Rabbit RabbitCore RCM3400 User Manual

4.4 A/D Converter

The RCM3400 has an onboard ADS7870 A/D converter whose scaling and filtering are
done via the motherboard on which the RCM3400 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 3000.
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 8 for the analog inputs.
+ V
Figure 8. Resistor Divider Network for Analog Inputs
R0 and R1 are typically 20 kΩ to 100 kΩ, with a lower resistance leading to more accu-
racy, but at the expense of a higher current draw. R10 and R11 would then be 180 kΩ 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 chan-
nel of the ADS7870 A/D converter. Thus, you must scale the analog signal with an attenu-
ator 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 resistors R0 and R1 con-
nected to ground, your analog circuit is well-suited to perform positive A/D conversions.
When R0 and R1 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 ana-
log ground.
User's Manual
R10
R11
C C
R0
AGND
BVREF
LN0
ADC
LN1
R1
31