ICL71C03 Datasheet, PDF (10/19 Pages) Intersil Corporation – Precision 4 1/2 Digit, A/D Converter

ICL8052/ICL71C03, ICL8068/ICL71C03
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:
R_INT = ^F-----^u---^l-^l---^S----^c---^a----^l-^e-----^V----^o2---_0l--^t-^a-_µ---^g-_A--^e-----⁽--^S----^e----^e-----^N----^o---^t--^e---⁾
NOTE: If gain is used in the buffer amplifier, then:
R_INT = ⁽---^B----^u----^f--^f--^e----^r---^G-----^a----ⁱ--ⁿ---⁾---₂-⁽--^F-₀---^u_µ----^lA-^l---^S----^c---^a---^l--^e-----^V---^o----^l-^t--^a---^g----^e---⁾
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_INT is given by:
10,000(4-1/2 Digit)
× Clock Period × (20µA)
C_INT = -----¹----⁰---⁰---⁰---_I-⁽-_n-³--_t-^--_e-¹--_g--^/-²-_r--_a--^D-_t--_o--ⁱ-^gr---ⁱ-_O-^t-⁾---_u---_t--_p---_u---_t----_V---_o----_l-_t--_a---_g----_e-----_S----_w----_i-_n----_g-------------------
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 reason-
able cost. Polystyrene and polycarbonate capacitors may be
used in less critical applications.
10-50K
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.
Reference Voltage
The analog input required to generate a full scale output is:
V_IN = 2V_REF.
The stability of the reference voltage is a major factor in the
overall absolute accuracy of the converter. For this reason, it
is recommended that an external high quality reference be
used where ambient temperature is not controlled or where
high-accuracy absolute measurements are being made.
Buffer Gain
At the end of the auto-zero interval, the instantaneous noise
voltage on the auto-zero capacitor is stored and subtracted
from the input voltage while adding to the reference voltage
during the next cycle. The result of this is that the noise
voltage is effectively somewhat greater than the input noise
voltage of the buffer itself during integration. By introducing
some voltage gain into the buffer, the effect of the auto-zero
noise (referred to the input) can be reduced to the level of
the inherent buffer noise. This generally occurs with a buffer
gain of between 3 and 10. Further increase in buffer gain
merely increases the total offset to be handled by the auto-
zero loop, and reduces the available buffer and integrator
swings, without improving the noise performance of the
system. The circuit recommended for doing this with the
ICL8068/ICL71C03 is shown in Figure 6.
+15V -15V 100kΩ
-BUF IN BUF OUT -INT IN INT OUT
REF
OUT ₆
87 1
10
BUFFER
9
11
14
INTEG.
300pF
INT.
3 _REF.
-
A1
COMP.
-
A2
-
COMP
OUT
10kΩ
+
+
ICL8068
A3
+
2
1kΩ
5 +BUF IN 13
+INT IN 12
-1.2V
-15V
TO ICL7104
FIGURE 6. ADDING BUFFER GAIN TO ICL8068
3-43