icl8068a Intersil Corporation, icl8068a Datasheet - Page 10

• Manufacturer Part Number: icl8068a
• Description: Precision 4 1/2 Digit, A/d Converter
• Manufacturer: Intersil Corporation

Component Value Selection
For optimum performance of the analog section, care must
be taken in the selection of values for the integrator capacitor
and resistor, auto-zero capacitor, reference voltage, and
conversion rate. These values must be chosen to suit the
particular application.
Integrating Resistor
The integrating resistor is determined by the full scale input
voltage and the output current of the buffer used to charge
the integrator capacitor. This current should be small
compared to the output short circuit current such that
thermal effects are kept to a minimum and linearity is not
affected. Values of 5µA to 40µA give good results with a
nominal of 20µA. The exact value may be chosen by:
NOTE: If gain is used in the buffer amplifier, then:
Integrating Capacitor
The product of integrating resistor and capacitor is selected
to give 9V swing for full scale inputs. This is a compromise
between possibly saturating the integrator (at +14V) due to
tolerance buildup between the resistor, capacitor and clock
and the errors a lower voltage swing could induce due to
offsets referred to the output of the comparator. In general,
the value of C
R
I NT
=
R
Full Scale Voltage (See Note)
------------------------------------------------------------------------------ -
IN T
=
INT
(
------------------------------------------------------------------------------------------- -
Bu fferGa in
is given by:
20µA
WHEN APPLICABLE
WHEN APPLICABLE
FOR OVER-RANGE
FOR OVER-RANGE
) (Full Scale Voltage)
20µA
10
UNDER-RANGE
OVER-RANGE
INTEGRATOR
DIGIT SCAN
DIGIT SCAN
OUTPUT
STROBE
ICL8052A/ICL71C03, ICL8068A/ICL71C03
BUSY
FIGURE 5. TIMING DIAGRAM FOR OUTPUTS
x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
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x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x x x x x
COUNTS
10,001
/ 1,001
EXPANDED SCALE BELOW
ZERO
AUTO
D
FULL MEASUREMENT CYCLE
5
D
4
AUTO ZERO
D
40,002/4,002 COUNTS
3
D
2
D
COUNTS
SIGNAL
1
INTEG.
10,000
/ 1,000
1000
†
/100 COUNTS
COUNTS MAX
20,001 / 2,001
REFERENCE
INTEGRATE
INTEGRATE
A very important characteristic of the integrating capacitor is
that it has low dielectric absorption to prevent roll-over or
ratiometric errors. A good test for dielectric absorption is to
use the capacitor with the input tied to the reference.
This ratiometric condition should be read half scale 1.0000,
and any deviation is probably due to dielectric absorption.
Polypropylene capacitors give undetectable errors at
reasonable cost. Polystyrene and polycarbonate capacitors
may be used in less critical applications.
Auto-Zero and Reference Capacitor
The size of the auto-zero capacitor has some influence on
the noise of the system, with a larger value capacitor giving
less noise. The reference capacitor should be large enough
such that stray capacitance to ground from its nodes is
negligible.
When gain is used in the buffer amplifier the reference
capacitor should be substantially larger than the auto-zero
capacitor. As a rule of thumb, the reference capacitor should
be approximately the gain times the value of the auto-zero
capacitor. The dielectric absorption of the reference cap and
auto-zero cap are only important at power-on or when the
circuit is recovering from an overload. Thus, smaller or
cheaper caps can be used here if accurate readings are not
required for the first few seconds of recovery.
C
I NT
D
D
D
D
D
SIGNAL
5
4
3
2
1
=
------------------------------------------------------------------------------------------------------------------------ -
†
ONE COUNT LONGER
10,000(4-1/2 Digit)
FIRST D
1000(3-1/2 Digit)
Integrator Output Voltage Swing
5
REFERENCE
INTEGRATE
OF AZ AND REF INT
×
Clock Period
×
(
20µA
)