Rabbit RabbitCore RCM3400 User Manual page 38

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 9.
This will generate a negative
voltage at one of the inputs,
LN1, which will almost cer-
tainly lead to inaccurate A/D
conversions. To make such dif-
ferential measurements, connect R0 and R1 to the A/D converter's internal reference volt-
age, which is software-configurable for 1.15 V, 2.048 V, or 2.5 V. This internal reference
voltage is available on pin 9 of header J1 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 20 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 10 of header J1 and can be used as
a hardware means of forcing the A/D converter to start a conversion cycle. 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 10 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 10. Timing Diagram for Conversion Start Using CONVERT Pin
Appendix B explains the implementation examples of these features on the Prototyping
Board.
32
R10
AIN0
+
Device
I
-
R11
AIN1
Figure 9. Current Flow from Ungrounded
or Floating Source
Conversion starts
LN0
1 nF R0
+
1 nF R1
–
LN1
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