ADC0804 datasheet(10/17 Pages) INTERSIL

10 / 17 page

Such an adjusted reference voltage can accommodate a

reduced span or dynamic voltage range of the analog input

voltage. If the analog input voltage were to range from 0.5V to

3.5V, instead of 0V to 5V, the span would be 3V. With 0.5V

applied to the VlN(-) pin to absorb the offset, the reference

voltage can be made equal to 1/2 of the 3V span or 1.5V. The

A/D now will encode the VlN(+) signal from 0.5V to 3.5V with

the 0.5V input corresponding to zero and the 3.5V input

corresponding to full scale. The full 8 bits of resolution are

therefore applied over this reduced analog input voltage

range. The requisite connections are shown in Figure 13. For

expanded scale inputs, the circuits of Figures 14 and 15 can

be used.

Reference Accuracy Requirements

The converter can be operated in a pseudo-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. In absolute conversion applicatIons, both the initial

value and the temperature stability of the reference voltage

are important accuracy factors in the operation of the A/D

converter. For VREF/2 voltages of 2.5V nominal value, initial

errors of

±10mV 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 20mV (5V span) to

10mV and 1 LSB at the VREF/2 input becomes 5mV. 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.

In general, the reference voltage will require an initial

adjustment. Errors due to an improper value of reference

voltage appear as full scale errors in the A/D transfer

FIGURE 12. THE VREFERENCE DESIGN ON THE IC

FIGURE 13. OFFSETTING THE ZERO OF THE ADC080X AND

PERFORMING AN INPUT RANGE (SPAN)

ADJUSTMENT

V+

DGND

VREF/2

AGND

(VREF)

DIGITAL

CIRCUITS

ANALOG

CIRCUITS

810

DECODE

300

TO VREF/2

TO VIN(-)

ZERO SHIFT VOLTAGE

0.1

µF

VREF

(5V)

ADJ.

“SPAN”/2

ICL7611

FIGURE 14. HANDLING

±10V ANALOG INPUT RANGE

FIGURE 15. HANDLING

±5V ANALOG INPUT RANGE

VIN(-)

VIN ± 10V

VIN(+)

(VREF)

V+

µF

ADC0803-

ADC0804

VIN(-)

VIN ±5V

VIN(+)

(VREF)

V+

µF

ADC0803-

ADC0804

ADC0803, ADC0804

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