User`s manual

User’s Manual 81
Many other possible ranges are possible by physically changing the resistor values that
make up the attenuator circuit.
It is also possible to read a negative voltage on ADC_IN0–ADC_IN5 by moving the 0 
jumper (see Figure B-6) on header JP1, JP2, or JP9 associated with the A/D converter
input from analog ground to the reference voltage generated and buffered by the A/D con-
verter. Adjacent input channels are paired so that moving a particular jumper changes both
of the paired channels. At the present time Digi 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.
NOTE: THERM_IN7 was configured to illustrate the use of a thermistor with the sample
program, and so is not available for use as a differential input. There is also no resistor
attenuator for THERM_IN7, so its input voltage range is limited.
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.
The A/D converter inputs can also be used with 4–20 mA current sources by measuring the
resulting analog voltage drop across 249  1% precision resistors placed between the ana-
log input and analog ground for ADC_IN3 to ADC_IN6. Be sure to reconfigure the
jumper positions on header JP5 as shown in Section B.5 using the slip-on jumpers
included with the spare parts in the Development Kit.
Table B-4. Differential Voltage Ranges
Min. Differential
Voltage
(V)
Max. Differential
Voltage
(V)
Amplifier
Gain
mV per Tick
0 ±20.0 ×1 10
0 ±10.0 ×2 5
0±5.0×42.5
0±4.0×52.0
0 ±2.5 ×8 1.25
0 ±2.0 ×10 1.00
0 ±1.25 ×16 0.625
0 ±1.0 ×20 0.500