101-1102 Rabbit Semiconductor, 101-1102 Datasheet - Page 42

• Manufacturer Part Number: 101-1102
• Description: KIT DEV RABBIT RCM4100 INTL
• Manufacturer: Rabbit Semiconductor
• Series: RabbitCore 4000r
• Type: MPU Moduler

Specifications of 101-1102

• Contents: RabbitCore Module, Dev. Board, Cable and Dynamic C® CD-Rom
• For Use With/related Products: RCM4100
• Lead Free Status / RoHS Status: Vendor undefined / Vendor undefined

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.
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.
38
RabbitCore RCM4100