RabbitCore eDisplay OP7200, User Manual

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User’s Manual 25
The A/D converter chip can make either single-ended or differential measurements
depending on the value of the
opmode
parameter in the software function call. Adjacent
A/D converter inputs are paired to make differential measurements. The default setup for
the OP7200 is to measure only positive voltages for the ranges listed in Table 2.
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 by moving the 0
Ω
jumper (see Figure 11) on
header JP4, JP5, JP6, or JP7 associated with the A/D converter input from analog ground
to the 2.048 V reference voltage generated and buffered by the A/D converter. Adjacent
input channels are paired so that moving a particular jumper changes both of the paired
channels. 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.
Table 2. Positive A/D Converter Input Voltage Ranges
Min. Voltage
(V)
Max. Voltage
(V)
Amplifier
Gain
mV per Tick
0.0 +20.0 1 10
0.0 +10.0 2 5
0.0 +5.0 4 2.5
0.0 +4.0 5 2.0
0.0 +2.5 8 1.25
0.0 +2.0 10 1.0
0.0 +1.25 16 0.625
0.0 +1.0 20 0.500