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參數(shù)資料
| 型號(hào)： | ICL7149CM44 |
| 廠商： | INTERSIL CORP |
| 元件分類： | 模擬信號(hào)調(diào)理 |
| 英文描述： | ER 3C 3#12 SKT RECP |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, PQFP44 |
| 封裝： | PLASTIC, MS-022AB, MQFP-44 |
| 文件頁(yè)數(shù)： | 11/13頁(yè) |
| 文件大?。?/td> | 111K |
| 代理商： | ICL7149CM44 |
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3-43
Integrator Capacitor, C
lNT
As with all dual-slope integrating convertors, the integration
capacitor must have low dielectric absorption to reduce
linearity errors. Polypropylene capacitors add undetectable
errors at a reasonable cost, while polystyrene and
polycarbonate may be used in less critical applications. The
ICL7139 and ICL7149 are designed to use a 3.3nF
(0.0033
μ
F) C
lNT
with an oscillator frequency of 120kHz and
an R
lNTV
of 10M
. With a 100kHz oscillator frequency (for
50Hz line frequency rejection), C
lNT
and R
INTV
affects the
voltage swing of the integrator. Voltage swing should be as
high as possible without saturating the integrator. Saturation
occurs when the integrator output is within 1V of either V+ or
V-. Integrator voltage swing should be about
±
2V when using
standard component values. For different R
lNTV
and
oscillator frequencies the value of C
lNT
can be calculated
from:
Integrator Resistors
The normal values of the R
lNT V
and R
lNT l
resistors are
10M
and 1M
respectively. Though their absolute values
are not critical, unless the value of the current sensing resis-
tors are trimmed, their ratio should be 10:1, within 0.05%.
Some carbon composition resistors have a large voltage
coefficient which will cause linearity errors on the 400V scale.
Also, some carbon composition resistors are very noisy. The
class “A” output of the integrator begins to have nonlinearities
if required to sink more than 70
μ
A (the sourcing limit is much
higher). Because R
lNT V
drives a virtual ground point, the
input impedance of the meter is equal to R
lNT V
.
Deintegration Resistor, R
DElNT
Unlike most dual-slope A/D converters, the ICL7139 and
ICL7149 use different resistors for integration and deintegra-
tion. R
DElNT
should normally be the same value as R
lNT V
,
and have the same temperature coefficient. Slight errors in
matching may be corrected by trimming the reference voltage.
Autozero Capacitor, C
AZ
The C
AZ
is charged to the integrator’s offset voltage during
the autozero phases, and subtracts that voltage from the
input signal during the integrate phases. The integrator thus
appears to have zero offset voltage. Minimum C
AZ
value is
determined by: 1) Circuit leakages; 2) C
AZ
self-discharge;
3) Charge injection from the internal autozero switches.
To avoid errors, the C
AZ
voltage change should be less than
1/10 of a count during the 10,000 count clock cycle integra-
tion period for the 400mV range. These requirements set a
lower limit of 0.047
μ
F for C
AZ
but 0.1
μ
F is the preferred
value. The upper limit on the value of C
AZ
is set by the time
constant of the autozero loop, and the 1 line cycle time
period allotted to autozero. C
AZ
may be several 10s of
μ
F
before approaching this limit.
The ideal C
AZ
is a low leakage polypropylene or Teflon
capacitor. Other film capacitors such as polyester, polysty-
rene, and polycarbonate introduce negligible errors. If a few
seconds of settling time upon power-up is acceptable, the
C
AZ
may be a ceramic capacitor, provided it does not have
excessive leakage.
Ohm Measurement Resistors
Because the ICL7139 and ICL7149 use a ratiometric ohm
measurement technique, the accuracy of ohm reading is pri-
marily determined by the absolute accuracy of the
R
KNOWN1
and R
KNOWN2
. These should normally be 10k
and 1M
, with an absolute accuracy of at least 0.5%.
Current Sensing Resistors
The 0.1
and 9.9
current sensing resistors convert the
measured current to a voltage, which is then measured
using R
lNT l
. The two resistors must be closely matched, and
the ratio between R
lNT l
and these two resistors must be
accurate - normally 0.5%. The 0.1
resistor must be capa-
ble of handling the full scale current of 4A, which requires it
to dissipate 1.6W.
Continuity Beeper
The Continuity Beeper output is designed to drive a piezo-
electric transducer at 2kHz (using a 120kHz crystal), with a
voltage output swing of V+ to V-. The beeper output off state
is at the V+ rail. When crystals with different frequencies are
used, the frequency needed to drive the transducer can be
calculated by dividing the crystal frequency by 60.
Display
The ICL7139 and ICL7149 use a custom, duplexed drive dis-
play with range, polarity, and low battery annunciators. With
a 3V peak display voltage, the RMS ON voltage will be
2.37V minimum; RMS OFF voltage will be 1.06V maximum.
Because the display voltage is not adjustable, the display
should have a 10% ON threshold of about 1.4V. Most display
manufacturers supply a graph that shows contrast versus
RMS drive voltage. This graph can be used to determine
what the contrast ratio will be when driven by the ICL7139
and ICL7149. Most display thresholds decrease with
increasing temperature. The threshold at the maximum
operating temperature should be checked to ensure that the
“off” segments will not be turned “on” at high temperatures.
Crystal
The ICL7139 and ICL7149 are designed to use a parallel
resonant 120kHz or 100kHz crystal with no additional exter-
nal components. The R
S
parameter should be less than
25k
to ensure oscillation. Initial frequency tolerance of the
crystal can be a relatively loose 0.05%.
Switches
Because the logic input draws only about 5
μ
A, switches
driving these inputs should be rated for low current, or “dry”
operations. The switches on the external inputs must be able
to reliably switch low currents, and be able to handle
voltages in excess of 400V
AC
.
C
INT
---------------------------------------------------------------------------------------------------
)
=
10,000 x 2 x Oscillator Period
-------------------------------------------------------------------------------------------------------------------------
(
)
0.4V/R
×
=
ICL7139, ICL7149
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