ADC0804LCWM datasheet(23/48 Pages) TI

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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 VDC applied to the VIN(−) pin to

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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