A/D Converter (Rcm4100 Only) - Rabbit RabbitCore RCM4100 User Manual

C-programmable core module

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4.4 A/D Converter (RCM4100 only)

The RCM4100 has an onboard ADS7870 A/D converter whose scaling and filtering are

done via the motherboard on which the RCM4100 module is mounted. The A/D converter

multiplexes converted signals from eight single-ended or four differential inputs to Serial

Port B on the Rabbit 4000.

The eight analog input pins, LN0–LN7, each have an input impedance of 6–7 MΩ,

depending on whether they are used as single-ended or differential inputs. The input signal

can range from -2 V to +2 V (differential mode) or from 0 V to +2 V (single-ended mode).

Use a resistor divider such as the one shown in Figure 9 to measure voltages above 2 V on

the analog inputs. See Appendix B.4.3 for information on how to measure negative voltages.

LN0

LN1

AGND

Figure 9. Resistor Divider Network for Analog Inputs

The R1 resistors are typically 20 kΩ to 100 kΩ, with a lower resistance leading to more

accuracy, but at the expense of a higher current draw. The R0 resistors would then be

180 kΩ to 900 kΩ for a 10:1 attenuator. The capacitor filters noise pulses on the A/D

converter input.

The actual voltage range for a signal going to the A/D converter input is also affected by

the 1, 2, 4, 5, 8, 10, 16, and 20 V/V software-programmable gains available on each channel

of the ADS7870 A/D converter. Thus, you must scale the analog signal with an attenuator

circuit and a software-programmable gain so that the actual input presented to the A/D

converter is within the range limits of the ADS7870 A/D converter chip (-2 V to + 2 V or

0 V to + 2 V).

The A/D converter chip can only accept positive voltages. With the R1 resistors connected

to ground, your analog circuit is well-suited to perform positive A/D conversions. When

the R1 resistors are tied to ground for differential measurements, both differential inputs

must be referenced to analog ground, and both inputs must be positive with respect to

analog ground.

ADC

(RCM4100)

ADC

BVREF

RabbitCore RCM4100

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