Z-World eDisplay OP7200, User Manual

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22 eDisplay (OP7200)
The A/D converter chip can only accept positive voltages. When the 0
Ω
resistor shown in
Figure 10 ties the A/D attenuator circuit to analog ground, both differential inputs must be
referenced to analog ground, and both inputs must be positive with respect to analog
ground.
If a device such as a battery is
connected 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 11.
This will generate a negative
voltage at one of the inputs,
AIN1, which will almost cer-
tainly lead to inaccurate A/D
conversions. To make such dif-
ferential measurements, move the 0
Ω
resistor jumper (see Figure 10) 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.
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
Figure 11. Current Flow from Ungrounded
or Floating Source
180 k W
180 k W
1 nF
20 k W
20 k W
1 nF
ADC
AIN0
AIN1
+
–
I
0
1
+
-
Device