Digi RCM4000 - Manual (Page 107)

User’s Manual 101
Many other possible ranges are possible by physically changing the resistor values that
make up the attenuator circuit.
NOTE: Analog input LN7_IN does not have the 10 k
Ω resistor installed, and so no
resistor attenuator is available, limiting its maximum input voltage to 2 V. This input is
intended to be used for a thermistor that you may install at header location JP25.
It is also possible to read a negative voltage on LN0_IN–LN5_IN by moving the 0
Ω
jumper (see Figure B-6) on header JP23 or JP24 associated with the A/D converter input
from analog ground to the reference voltage generated and buffered by the A/D converter.
Adjacent input channels are paired; moving the jumper on JP 23 changes both of the
paired channels (LN4_IN–LN5_IN), and moving the jumper on JP24 changes LN0_IN–
LN3_IN. At the present time Rabbit Semiconductor does not offer the software drivers to
work with single-ended negative voltages, but the differential mode described below may
be used to measure negative voltages.
Differential measurements require two channels. As the name differential implies, the dif-
ference in voltage between the two adjacent channels is measured rather than the differ-
ence between the input and analog ground. Voltage measurements taken in differential
mode have a resolution of 12 bits, with the 12th bit indicating whether the difference is
positive or negative.
The A/D converter chip can only accept positive voltages, as explained in Section 4.4. Both
differential inputs must be referenced to analog ground, and both inputs must be positive
with respect to analog ground. Table B-4 provides the differential voltage ranges for this
setup.
Table B-4. Differential Voltage Ranges
Min. Differential
Voltage
(V)
Max. Differential
Voltage (with
prescaler)
(V)
A/D Converter
Gain
mV per Tick
0 ±22.528 1 11
0 ±11.264 2 5.5
0 ±5.632 4 2.75
0 ±4.506 5 2.20
0 ±2.816 8 1.375
0 ±2.253 10 1.100
0 ±1.408 16 0.688
0 ±1.126 20 0.550