Other Pin Features - RabbitCore RCM4500W User Manual

If a device such as a battery is con-
nected across two channels for a differ-
ential measurement, and it is not
referenced to analog ground, then the
current from the device will flow
through both sets of attenuator resistors
without flowing back to analog ground
as shown in Figure 11. This will gener-
ate a negative voltage at one of the
inputs, ADC1, which will almost cer-
tainly lead to inaccurate A/D conver-
sions. To make such differential
measurements, connect the R1 resistors
to the A/D converter's internal reference voltage, which is 1.2 V. This internal reference voltage
can be configured in software to be available on pin 6 of header J4 as VREF, and allows you to
convert analog input voltages that are negative with respect to analog ground.
NOTE: This software configuration option for VREF and differential measurements are not sup-
ported at this time.
NOTE: The amplifier inside the A/D converter's internal voltage reference circuit has a very lim-
ited output-current capability. The internal buffer can source up to 20 mA and sink only up to
20 µA. Use a separate buffer amplifier if you need to supply any load current.

4.4.3 Other Pin Features

There are two other features brought out on the auxiliary I/O header J4.
• There is a +3.3 V power supply point on pin 8, which can deliver up to 25 mA at 3.3 V DC.
This power supply point is essentially a filtered and isolated version of the regulated +3.3 V
DC power that is supplied to the RCM4510W RabbitCore module through pin 1 of header J1
from the motherboard.
• The SYS_PWR_ON signal on pin 9 is used by the XBee RF module to place the remaining cir-
cuitry on the RCM4510W RabbitCore module into a powered-down "sleep" mode or to bring
it back up to normal operation.
User's Manual
R0
AIN0
2.2 nF
+
Device
I
-
2.2 nF
R0
AIN1
Figure 11. Current Flow from Ungrounded
or Floating Source
ADC
ADC0
R1
+
R1
–
ADC1
39