ADC0801, ADC0802
ADC0803, ADC0804, ADC0805
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SNOSBI1B –NOVEMBER 2009–REVISED FEBRUARY 2013
Reference Voltage
Span Adjust
For maximum applications flexibility, these A/Ds have been designed to accommodate a 5 VDC, 2.5 VDC or an
adjusted voltage reference. This has been achieved in the design of the IC as shown in Figure 50.
Figure 50. The VREFERENCE Design on the IC
Notice that the reference voltage for the IC is either 1/2 of the voltage applied to the VCC supply pin, or is equal to
the voltage that is externally forced at the VREF/2 pin. This allows for a ratiometric voltage reference using the
VCC supply, a 5 VDC reference voltage can be used for the VCC supply or a voltage less than 2.5 VDC can be
applied to the VREF/2 input for increased application flexibility. The internal gain to the VREF/2 input is 2, making
the full-scale differential input voltage twice the voltage at pin 9.
An example of the use of an adjusted reference voltage is to accommodate a reduced span — or dynamic
voltage range of the analog input voltage. If the analog input voltage were to range from 0.5 VDC to 3.5 VDC
,
instead of 0V to 5 VDC, the span would be 3V as shown in Figure 51. With 0.5 applied to the VIN(−) pin to
VDC
absorb the offset, the reference voltage can be made equal to 1/2 of the 3V span or 1.5 VDC. The A/D now will
encode the VIN(+) signal from 0.5V to 3.5 V with the 0.5V input corresponding to zero and the 3.5 VDC input
corresponding to full-scale. The full 8 bits of resolution are therefore applied over this reduced analog input
voltage range.
Reference Accuracy Requirements
The converter can be operated in a ratiometric mode or an absolute mode. In ratiometric converter applications,
the magnitude of the reference voltage is a factor in both the output of the source transducer and the output of
the A/D converter and therefore cancels out in the final digital output code. The ADC0805 is specified particularly
for use in ratiometric applications with no adjustments required. In absolute conversion applications, both the
initial value and the temperature stability of the reference voltage are important factors in the accuracy of the A/D
converter. For VREF/2 voltages of 2.4 VDC nominal value, initial errors of ±10 mVDC will cause conversion errors of
±1 LSB due to the gain of 2 of the VREF/2 input. In reduced span applications, the initial value and the stability of
the VREF/2 input voltage become even more important. For example, if the span is reduced to 2.5V, the analog
input LSB voltage value is correspondingly reduced from 20 mV (5V span) to 10 mV and 1 LSB at the VREF/2
input becomes 5 mV. As can be seen, this reduces the allowed initial tolerance of the reference voltage and
requires correspondingly less absolute change with temperature variations. Note that spans smaller than 2.5V
place even tighter requirements on the initial accuracy and stability of the reference source.
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