Rabbit RabbitCore RCM4200 User Manual page 48

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 without flowing back
to analog ground as shown in
Figure 11. This will generate a
negative voltage at one of the
inputs, LN1, which will almost
certainly lead to inaccurate A/D
conversions. To make such differential measurements, connect the R1 resistors to the A/D
converter's internal reference voltage, which is software-configurable for 1.15 V, 2.048 V,
or 2.5 V. This internal reference voltage is available on pin 49 of header J2 as VREF, and
allows you to convert analog input voltages that are negative with respect to analog ground.
NOTE: The amplifier inside the A/D converter's internal voltage reference circuit has a
very limited output-current capability. The internal buffer can source up to 20 mA and
sink only up to 200 µA. Use a separate buffer amplifier if you need to supply any load
current.
The A/D converter's CONVERT pin is available on pin 48 of header J2 and can be used as
a hardware means of forcing the A/D converter to start a conversion cycle at a specific time.
The CONVERT signal is an edge-triggered event and has a hold time of two CCLK periods
for debounce.
A conversion is started by an active (rising) edge on the CONVERT pin. The CONVERT
pin must stay low for at least two CCLK periods before going high for at least two CCLK
periods. Figure 12 shows the timing of a conversion start. The double falling arrow on
CCLK indicates the actual start of the conversion cycle.
CCLK
BUSY
CONV
Figure 12. Timing Diagram for Conversion Start Using CONVERT Pin
Appendix B explains the implementation examples of these features on the Prototyping
Board.
42
AIN0
+
Device
I
-
AIN1
Figure 11. Current Flow from Ungrounded
or Floating Source
Conversion starts
R0
LN0
2.2 nF R1
+
2.2 nF R1
R0
–
LN1
RabbitCore RCM4200
ADC