RabbitCore eDisplay OP7200, Руководство пользователя

Страница: 32 / 174

26 eDisplay (OP7200)
The A/D converter chip can only accept positive voltages. When the 0
Ω
resistor shown in
Figure 11 ties the A/D attenuator circuit to analog ground, both differential inputs must be ref-
erenced to analog ground, and
both inputs must be positive with respect to analog ground .
To make such differential measurements, move the 0
Ω
resistor jumper (see Figure 11)
associated with the A/D converter inputs (JP4, JP5, JP6, or JP7) from analog ground to the
2.048 V reference voltage. This allows input voltages that are negative with respect to
analog ground. Table 3 provides the differential voltage ranges for this setup.
If a device such as a battery is con-
nected across two channels for a
differential measurement, and it is
not referenced to analog ground,
then the current from the device
will flow through both sets of
attenuator resistors as shown in
Figure 12. This will generate a
negative voltage at one of the
inputs, AIN1, which will almost
certainly lead to inaccurate A/D
conversions.
Figure 12. Current Flow from Ungrounded
or Floating Source
Table 3. 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 ×4 2.5
0 ±4.0 ×5 2.0
0 ±2.5 ×8 1.25
0 ±2.0 ×10 1.00
0 ±1.25 ×16 0.625
0 ±1.0 ×20 0.500
180 k W
180 k W
1 nF
20 k W
20 k W
1 nF
ADC
AIN0
AIN1
+
–
I
0
1
+
-
Device