參數(shù)資料
型號(hào): ICL7109
廠商: Intersil Corporation
英文描述: 12-Bit,Microprocessor-Compatible A/D Converter(12位、微處理器兼容的A/D轉(zhuǎn)換器)
中文描述: 12位A/D轉(zhuǎn)換器,帶有三態(tài)二進(jìn)制輸出
文件頁數(shù): 9/23頁
文件大?。?/td> 227K
代理商: ICL7109
5-12
The ICL7109 has, however, been optimized for operation
with analog common near digital ground. With power sup-
plies of +5V and -5V, this allows a 4V full scale integrator
swing positive or negative thus maximizing the performance
of the analog section.
Differential Reference
The reference voltage can be generated anywhere within the
power supply voltage of the converter. The main source of
common mode error is a roll-over voltage caused by the
reference capacitor losing or gaining charge to stray capacity
on its nodes. If there is a large common mode voltage, the ref-
erence capacitor can gain charge (increase voltage) when
called up to deintegrate a positive signal but lose charge
(decrease voltage) when called up to deintegrate a negative
input signal. This difference in reference for positive or
negative input voltage will give a roll-over error. However, by
selecting the reference capacitor large enough in comparison
to the stray capacitance, this error can be held to less than 0.5
count worst case. (See Component Value Selection.)
The roll-over error from these sources is minimized by having
the reference common mode voltage near or at analog
COMMON.
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.
The most important consideration is that the integrator out-
put swing (for full-scale input) be as large as possible. For
example, with
±
5V supplies and COMMON connected to
GND, the normal integrator output swing at full scale is
±
4V.
Since the integrator output can go to 0.3V from either supply
without significantly affecting linearity, a 4V integrator output
swing allows 0.7V for variations in output swing due to com-
ponent value and oscillator tolerances. With
±
5V supplies
and a common mode range of
±
1V required, the component
values should be selected to provide
±
3V integrator output
swing. Noise and roll-over will be slightly worse than in the
±
4V case. For larger common mode voltage ranges, the inte-
grator output swing must be reduced further. This will
increase both noise and roll-over errors. To improve the per-
formance, supplies of
±
6V may be used.
Integrating Resistor
Both the buffer amplifier and the integrator have a class A output
stage with 100
μ
A of quiescent current. They supply 20
μ
A of
drive current with negligible nonlinearity. The integrating resistor
should be large enough to remain in this very linear region over
the input voltage range, but small enough that undue leakage
requirements are not placed on the PC board. For 409.6mV full-
scale, 200k
is near optimum and similarly a 20k
for a
409.6mV scale. For other values of full scale voltage, R
INT
should be chosen by the relation :
Integrating Capacitor
The integrating capacitor C
INT
should be selected to give the
maximum voltage swing that ensures tolerance build-up will
not saturate the integrator swing (approximately. 0.3V from
either supply). For the ICL7109 with
±
5V supplies and analog
common connected to GND, a
±
3.5V to
±
4V integrator output
swing is nominal. For 7
1
/
2
conversions per second (61.72kHz
clock frequency) as provided by the crystal oscillator, nominal
values for C
INT
and C
AZ
are 0.15
μ
F and 0.33
μ
F, respectively.
If different clock frequencies are used, these values should be
changed to maintain the integrator output swing. In general,
the value C
INT
is given by:
.
An additional requirement of the integrating capacitor is that
it have low dielectric absorption to prevent roll-over errors.
While other types of capacitors are adequate for this applica-
tion, polypropylene capacitors give undetectable errors at
The integrating capacitor should have a low dielectric
absorption to prevent roll-over errors. While other types may
be adequate for this application, polypropylene capacitors
give undetectable errors at reasonable cost up to 85
o
C.
Teflon capacitors are recommended for the military tem-
perature range. While their dielectric absorption characteris-
tics vary somewhat from unit to unit, selected devices should
give less than 0.5 count of error due to dielectric absorption.
Auto-Zero Capacitor
The size of the auto-zero capacitor has some influence on
the noise of the system: a smaller physical size and a larger
capacitance value lower the overall system noise. However,
C
AZ
cannot be increased without limits since it, in parallel
with the integrating capacitor forms an R-C time constant
that determines the speed of recovery from overloads and
the error that exists at the end of an auto-zero cycle. For
409.6mV full scale where noise is very important and the
integrating resistor small, a value of C
AZ
twice C
INT
is opti-
mum. Similarly for 4.096V full scale where recovery is more
important than noise, a value of C
AZ
equal to half of C
INT
is
recommended.
For optimal rejection of stray pickup, the outer foil of C
AZ
should be connected to the R-C summing junction and the
inner foil to pin 31. Similarly the outer foil of C
INT
should be
connected to pin 32 and the inner foil to the R-C summing
junction. Teflon, or equivalent, capacitors are recommended
above 85
o
C for their low leakage characteristics.
Reference Capacitor
A 1
μ
F capacitor gives good results in most applications.
However, where a large reference common mode voltage
exists (i.e., the reference low is not at analog common) and a
409.6mV scale is used, a large value is required to prevent
roll-over error. Generally 10
μ
F will hold the roll-over error to
0.5 count in this instance. Again, Teflon, or equivalent capac-
itors should be used for temperatures above 85
o
C for their
low leakage characteristics.
R
INT
-------------------------------------------
˙
.
=
C
INT
)
---------------------------------------------------------------------------------
=
ICL7109
Teflon
is a trademark of DuPont Corporation
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