HI7131CM44 INTERSIL [Intersil Corporation], HI7131CM44 Datasheet - Page 15

• Manufacturer Part Number: HI7131CM44
• Description: 3 1/2 Digit, Low Power, High CMRR, LCD/LED Display-Type A/D Converters
• Manufacturer: INTERSIL [Intersil Corporation]

Component Selection
Integrating resistors and capacitors (R
line to achieving the best performance from an integrating
A/D converter is to try to reduce the value of R
the value of C
swing at the output of the integrator. This will reduce the sen-
sitivity of the circuit to noise and leakage currents. In addition
to these guidelines the circuit limitations should also be con-
sidered.
To determine R
imum output drive current of the buffer amplifier (see Figure
5) while maintaining its linearity. This current for the buffer
amplifier is about 1 A. The R
from the expression:
R
The standard optimum values for R
full scale and 1.8M for 2V full scale. Type of resistor and its
absolute value is not critical to the accuracy of conversion,
as was discussed previously.
The integrating capacitor should be selected to yield the
maximum allowable voltage range to the integrator output
(INT pin). The maximum allowable range does not saturate
the integrator output. The integrator output can swing up or
down to 0.3V from either supply rail and still maintain its lin-
earity.
A nominal 2V maximum range is optimum. The maximum
range values are selected in order to leave enough room for
all the component and circuit tolerances and for a reason-
able common mode voltage range. The C
be calculated as:
C
Where T
before and I
I
For 48kHz nominal oscillator frequency (12kHz clock internal
frequency), R
mentioned swing, the optimum value for C
An additional requirement of the integrating capacitor is to
choose low dielectric absorption. This will minimize the con-
verter’s rollover, linearity and gain error. Furthermore, the
integrating capacitor should also have low leakage current.
Different types of capacitors are adequate for this applica-
tion; polypropylene capacitors provide undetectable errors at
reasonable cost and size. The absolute value of C
not have any effect on accuracy.
INT
INT
INT
=
=
=
V
---------------------------------------- - .
V
---------------------------------------- -
------------------------- - ,
V
T
IN
INT
IN
INTMAX
INT
Full Scale
R
INT
Full Scale
I
depends on clock frequency and was discussed
1 A
INT
INT
INT
INT
INT
is expressed as:
, while having the highest possible voltage
equals 180k
, the imposed circuit limitation is the max-
INT
resistor can be calculated
for 1.8M
INT
are 180k for 200mV
INT
INT
INT
, C
value can now
is 0.047 F.
INT
for the above
INT
): A guide-
, increase
HI7131, HI7133
INT
does
3-1840
Auto-Zero Capacitor (C
The value of the auto-zero capacitor has some influence on
the noise of the converter. A larger value C
tivity to noise. For 200mV full scale (resolution of 100 V),
where noise is important, a 0.47 F or greater is recom-
mended. On the 2V full scale, (resolution of 1mV), a 0.047 F
capacitor is adequate for low noise.
The auto-zero capacitor should be a low leakage type to
hold the voltage during conversion cycle. A mylar or
polypropylene capacitor is recommended for C
Reference Capacitor (C
As discussed earlier, the input to the integrator during the
deintegrate phase is the voltage at the reference capacitor.
The sources of error related to the reference capacitor are
stray capacitances at the C
currents. Where a large common mode voltage exists for
V
absorbing or pumping charge onto C
negative inputs are measured. Leakage of the capacitor
itself or leakages through circuit boards will drop the voltage
across C
boards should be designed to minimize stray capacitances
and should be well cleaned to reduce leakage currents.
A 0.1 F capacitor for C
applications. When common mode voltage exists or at
higher temperatures (where device leakage currents
increase) a 1.0 F reference capacitor is recommended to
reduce errors. The C
type, a mylar capacitor is adequate.
Those applications which have variable reference voltage
should also use a low dielectric absorption capacitor such as
polypropylene, for example, a ratiometric measurement of
resistance.
Oscillator Components
When an RC type of oscillator is desired, the oscillator
frequency is approximately expressed by:
(R in Ohms and C in Farads), where R > 50k and C > 50pF.
For 40kHz frequency which gives 2.5 readings per second, use
100K and 100pF or use 180k and 50pF for lower power loss.
There is a typical variation of about 5% between oscillator
frequencies of different parts. The oscillator frequency will
decrease 1% for each 25
normal mode rejection of 60Hz or 50Hz line frequency is critical,
a crystal or a precision external oscillator should be used.
Reference Voltage Selection
For a full scale reading the input signal is required to be
twice the reference voltage. To be more precise, the full
scale reading ( 1999) takes place when the input is 1.999
times the V
REF HI and REF LO inputs. Thus, for the nominal 200mV
and 2V ranges, V
f
OSC
REF
, the stray capacitances increase the rollover error by
=
0.45
---------- -
RC
REF
REF
,
and cause gain and rollover errors. The circuit
. V
REF
REF
REF
should be 100mV and 1V respectively.
o
is the potential difference between
REF
AZ
C rise. For those applications in which
REF
capacitor can be any low leakage
)
REF
)
should work properly for most
terminals, and the leakage
REF
AZ
when positive or
has less sensi-
AZ
.