User`s manual

XP8500 and Exp-A/D12 Technical Circuit Details s 101
in a bridge circuit. In such cases, the ratiometric conversion will track any
drift or errors in the reference, permitting a more accurate conversion.
REF- is hard-wired to analog ground. REF+ connects to one of two
sources of 2.5 V. REF+ connects to a precision voltage referenceusually
an AD680 or alternatively an LM385for absolute conversion. The
voltage divider RP1RP2 derives a 2.5 V reference voltage for ratiometric
conversion using the +5 V analog supply.
Data Conversion
The U2 op-amps gain and bias resistors scale the four conditioned inputs
signal ranges to conform to the A/D converters input range of 0 V to
+2.5V. Because of the inverting configuration of the U2 op-amps, the
maximum conditioned input translates to a minimum input voltage at the
converter. Conversely, the minimum conditioned input translates to the
maximum input voltage at the converter.
In the unipolar conversion mode, a signal at the U2 op-amp
input that is lower than the minimum input voltage range will
cause the output of the op-amp to rise above 2.5 V. But the
converter will output only all ones. A signal at the U2 op-amp
input that exceeds the maximum input signal voltage will not
drive the op-amp output below 0 V because the op-amps do
not have a negative supply. Excessively low input signals are
converted to all zeros.
The A/D converter chip outputs a 12-bit digital value representing the
converted value of the input voltage. An input voltage at the A/D chip
equal to 0 V converts to all zeros, while an input at 2.5 V converts to all
ones.
The A/D converter can operate in either a unipolar or a bipolar mode.
Z-Worlds Dynamic C functions will return a reading in the appropriate
mode based on the arguments supplied to the function. Dynamic C
functions return 16-bit sign-extended values.
Limitations on Output Range
In actual practice, the U2 op-amp outputs can only approach ground (0 V)
but cannot actually reach it. The output low-voltage limit is about 10 mV
to 20 mV. The practical effect of this limitation is that approximately
0.4%0.8% of the upper end of the input-signal range is unusable. For
example, if the input signal range is selected as 0 V to 10 V, the useful
range is actually 0 V to about 9.92 V to 9.96 V.
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