ADC-0803 Harris, ADC-0803 Datasheet - Page 10

• Manufacturer Part Number: ADC-0803
• Description: (ADC0802 - ADC0804) 8-Bit/ Microprocessor- Compatible/ A/D Converters
• Manufacturer: Harris

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Input Source Resistance
Large values of source resistance where an input bypass
capacitor is not used will not cause errors since the input
currents settle out prior to the comparison time. If a low-
pass filter is required in the system, use a low-value series
resistor ( 1k ) for a passive RC section or add an op amp
RC
applications ( 1k ), a 0.1 F bypass capacitor at the inputs
will minimize EMI due to the series lead inductance of a long
wire. A 100
capacitor (both the R and C are placed outside the feedback
loop) from the output of an op amp, if used.
Stray Pickup
The leads to the analog inputs (pins 6 and 7) should be kept
as short as possible to minimize stray signal pickup (EMI).
Both EMI and undesired digital-clock coupling to these inputs
can cause system errors. The source resistance for these
inputs should, in general, be kept below 5k . Larger values of
source resistance can cause undesired signal pickup. Input
bypass capacitors, placed from the analog inputs to ground,
will eliminate this pickup but can create analog scale errors as
these capacitors will average the transient input switching cur-
rents of the A/D (see Analog Input Current). This scale error
depends on both a large source resistance and the use of an
input bypass capacitor. This error can be compensated by a
full scale adjustment of the A/D (see Full Scale Adjustment)
with the source resistance and input bypass capacitor in
place, and the desired conversion rate.
Reference Voltage Span Adjust
For maximum application flexibility, these A/Ds have been
designed to accommodate a 5V, 2.5V or an adjusted voltage
reference. This has been achieved in the design of the IC as
shown in Figure 12.
Notice that the reference voltage for the IC is either
voltage which is applied to the V+ supply pin, or is equal to
the voltage which is externally forced at the V
allows for a pseudo-ratiometric voltage reference using, for
the V+ supply, a 5V reference voltage. Alternatively, a volt-
age less than 2.5V can be applied to the V
internal gain to the V
reduction in the reference voltage.
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 V
voltage can be made equal to
A/D now will encode the V
the 0.5V input corresponding to zero and the 3.5V input corre-
sponding to full scale. The full 8 bits of resolution are therefore
applied over this reduced analog input voltage range. The req-
uisite connections are shown in Figure 13. For expanded
scale inputs, the circuits of Figures 14 and 15 can be used.
active
low-pass
series resistor can be used to isolate this
lN(-)
REF
pin to absorb the offset, the reference
/2 input is 2 to allow this factor of 2
filter.
lN(+)
1
/
signal from 0.5V to 3.5V with
2
For
of the 3V span or 1.5V. The
low-source-resistance
ADC0802, ADC0803, ADC0804
REF
REF
/2 input. The
/2 pin. This
1
/
2
of the
6-14
FIGURE 13. OFFSETTING THE ZERO OF THE ADC0802 AND
V
V
V
REF
REF
IN
FIGURE 14. HANDLING 10V ANALOG INPUT RANGE
ADJ.
/2
(5V)
FS
FIGURE 12. THE V
10V
9
“SPAN”/2
PERFORMING AN INPUT RANGE (SPAN)
ADJUSTMENT
AGND
2R
ICL7611
R
R
ZERO SHIFT VOLTAGE
+
-
8
R
2R
DECODE
REFERENCE
5V
6
7
V
V
ADC0802-
ADC0804
IN(+)
IN(-)
300
0.1 F
DESIGN ON THE IC
V+
(V
V+
REF
CIRCUITS
ANALOG
20
)
DGND
20
+
CIRCUITS
DIGITAL
10 F
TO V
TO V
10
(V
REF
IN(-)
5V
REF
/2
)