ZWorld Exp-A/D12, User Manual

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XP8500 I/O Configurations s 25
Unconditioned Inputs (AIN4–AIN10)
The seven unconditioned input channels, AIN4–AIN10, use 10 k
Ω
pulldown resistors at R9–R15 as shown in Figure 3-7 to keep the inputs
from floating when they are not being used.
Figure 3-7. Schematic of XP8500 Unconditioned Inputs
These channels are accessed with software by inserting the desired channel
number in the library functions that control the XP8500. These channels
are located on header H2. For optimum results, drive these channels with
low-impedance (< 50
Ω
) voltage sources such as LM660 op-amps. High-
impedance signal sources are susceptible to coupled noise and will become
distorted when loaded by the 10 k
Ω
pulldown resistors. In addition, only a
low-impedance source can charge the sampling capacitors accurately
within the A/D converter. When designing the signal sources to drive the
extra channels, be sure to consider whether the op-amps can handle the
capacitance of the cable used to connect them to header H2.
Internal Test Voltages
In addition to the 11 external input channels of the A/D converter chip,
three additional internal channels exist to measure reference points within
the chip. The A/D converter compares its internal nodes to REF+ and
REF- so the conversions yield either all 1s or all 0s. These channels are
accessed using ordinary library routines by specifying the appropriate
channel address when calling the functions.
See Chapter 4, “Software Reference,” for further details on
Channels 11, 12, 13, and 14.
Power-Down Mode
If Channel 14 on the A/D converter chip is called by the software, the chip
enters a power-down mode in which all circuits in the chip go into a low-
current, standby mode. The chip also goes into the power-down mode
when it is first powered on and before the first A/D conversion. The chip
remains in the power-down mode until a channel other than Channel 14 is
U3
AIN [4 –10]
VREF+
+5ANA
10 k
Ω
A/D
Converter
$