Digi RCM4000 user manual It is not referenced to

If a device such as a battery is
connected across two channels
				R5
for a differential measurement,
		AIN0					LN0 ADC
and it is not referenced to					2.2 nF	R13
analog ground, then the current	Device	+
			I
from the device will flow		-				+
through both sets of attenuator					2.2 nF	R14
resistors as shown in Figure 11.		AIN1		R6		–
							LN1
This will generate a negative
voltage at one of the inputs,
	Figure 11. Current Flow from Ungrounded
LN1, which will almost cer-
tainly lead to inaccurate A/D			or Floating Source

conversions. To make such dif-

ferential 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 J3 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 48 of header J3 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 12 shows the timing of a conversion start. The double falling arrow on CCLK indicates the actual start of the conversion cycle.

Conversion starts

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.

36 RabbitCore RCM4000