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参数资料
| 型号： | HI7133CM44 |
| 厂商： | INTERSIL CORP |
| 元件分类： | ADC |
| 英文描述： | 3 1/2 Digit, Low Power, High CMRR, LCD/LED Display-Type A/D Converters |
| 中文描述： | 1-CH 3-BIT DUAL-SLOPE ADC, PARALLEL ACCESS, PQFP44 |
| 封装： | PLASTIC, MQFP-44 |
| 文件页数： | 15/21页 |
| 文件大小： | 209K |
| 代理商： | HI7133CM44 |
3-1840
Component Selection
Integrating resistors and capacitors (R
INT
, C
INT
): A guide-
line to achieving the best performance from an integrating
A/D converter is to try to reduce the value of R
INT
, increase
the value of C
INT
, while having the highest possible voltage
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
INT
, the imposed circuit limitation is the max-
imum output drive current of the buffer amplifier (see Figure
5) while maintaining its linearity. This current for the buffer
amplifier is about 1
μ
A. The R
INT
resistor can be calculated
from the expression:
The standard optimum values for R
INT
are 180k
for 200mV
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
INT
value can now
be calculated as:
Where T
INT
depends on clock frequency and was discussed
before and I
INT
is expressed as:
For 48kHz nominal oscillator frequency (12kHz clock internal
frequency), R
INT
equals 180k
for 1.8M
for the above
mentioned swing, the optimum value for C
INT
is 0.047
μ
F.
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
INT
does
not have any effect on accuracy.
Auto-Zero Capacitor (C
AZ
)
The value of the auto-zero capacitor has some influence on
the noise of the converter. A larger value C
AZ
has less sensi-
tivity to noise. For 200mV full scale (resolution of 100
μ
V),
where noise is important, a 0.47
μ
F or greater is recom-
mended. On the 2V full scale, (resolution of 1mV), a 0.047
μ
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
AZ
.
Reference Capacitor (C
REF
)
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
REF
terminals, and the leakage
currents. Where a large common mode voltage exists for
V
REF
, the stray capacitances increase the rollover error by
absorbing or pumping charge onto C
REF
when positive or
negative inputs are measured. Leakage of the capacitor
itself or leakages through circuit boards will drop the voltage
across C
REF
and cause gain and rollover errors. The circuit
boards should be designed to minimize stray capacitances
and should be well cleaned to reduce leakage currents.
A 0.1
μ
F capacitor for C
REF
should work properly for most
applications. When common mode voltage exists or at
higher temperatures (where device leakage currents
increase) a 1.0
μ
F reference capacitor is recommended to
reduce errors. The C
REF
capacitor can be any low leakage
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
o
C rise. For those applications in which
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
. V
REF
is the potential difference between
REF HI and REF LO inputs. Thus, for the nominal 200mV
and 2V ranges, V
REF
should be 100mV and 1V respectively.
R
INT
V
Full Scale
(
)
----------------------------------------
=
C
INT
T
I
INTMAX
-------------------------
,
=
I
INT
V
Full Scale
INT
(
)
----------------------------------------
.
=
f
OSC
-----------
=
HI7131, HI7133
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