Rabbit OP7100 User Manual download pdf (Page 31)

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 does not offer the software drivers to work with sin-

gle-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

1 2 ... 26 27 28 29 30 31 32 33 34 35 36 ... 171 172

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Rabbit OP7100 User Manual Page 31