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參數(shù)資料
| 型號： | TC811CPL |
| 廠商： | Microchip Technology Inc. |
| 元件分類： | 顯示驅(qū)動器 |
| 英文描述： | 3-1/2 Digit Analog-To-Digital Converter with Hold and Differential Reference Inputs |
| 中文描述： | 3-1/2數(shù)字模擬到數(shù)字轉(zhuǎn)換器與保持和差分基準(zhǔn)輸入 |
| 文件頁數(shù)： | 8/12頁 |
| 文件大?。?/td> | 171K |
| 代理商： | TC811CPL |
3-144
TELCOM SEMICONDUCTOR, INC.
3-1/2 DIGIT A/D CONVERTER WITH HOLD AND
DIFFERENTIAL REFERENCE INPUTS
TC811
COMPONENT VALUE SELECTION
Auto Zero Capacitor - C
AZ
The value of the Auto Zero capacitor (C
AZ
) has some
influence on system noise. A 0.47
μ
F capacitor is recom-
mended for 200mV full-scale applications where 1LSB is
100
μ
V. A 0.10
μ
F capacitor should be used for 2.0V full-
scale applications. A capacitor with low dielectric absorption
(Mylar) is required.
Reference Voltage Capacitor -C
REF
The reference voltage used to ramp the integrator
output voltage back to zero during the reference integrate
cycle is stored on C
REF
. A 0.1
μ
F capacitor is typical. If the
application requires a sensitivity of 200mV full-scale, in-
crease C
REF
to 1.0
μ
F. Rollover error will be held to less than
1/2 count. A good quality, low leakage capacitor, such as
Mylar, should be used.
Integrating Capacitor - C
INT
CINT should be selected to maximize integrator output
voltage swing without causing output saturation. Analog
common will normally supply the differential voltage refer-
ence. For this case a
±
2V integrator output swing is optimum
when the analog input is near full-scale. For 2 or 2.5 reading/
second (f
OSC
= 32kHz or 40kHz) and V
FS
= 200mV, a .068
μ
F
value is suggested. If a different oscillator frequency is used,
C
INT
must be changed in inverse proportion to maintain the
nominal
±
2V integrator swing. An exact expression for C
INT
is :
4000 V
FS
V
INT
R
INT
f
OSC
C
INT
=
where:
f
OSC
= Clock frequency at Pin 39
V
FS
= Full-scale input voltage
R
INT
= Integrating resistor
V
INT
= Desired full-scale integrator output swing
C
INT
must have low dielectric absorption to minimize
roll-over error. A polypropylene capacitor is recommended.
Integrating Resistor -R
INT
The input buffer amplifier and integrator are designed
with class A output stages which have idling currents of 6
μ
A.
The integrator and buffer can supply 1
μ
A drive currents with
negligible linearity errors. R
INT
is chosen to remain in the
output stage linear drive region but not so large that printed
circuit board leakage currents induce errors. For a 200mV
full-scale, R
INT
should be about 180k
. A 2.0V full-scale
requires abut 1.8M
.
Oscillator Components
The internal oscillator has been designed to operate
with a quartz crystal, such as the Statek CX-1V series. Such
crystals are very small and are available in a variety of
standard frequencies. Note that f
OSC
is divided by four to
generate the TC811 internal control clock. The backplane
drive signal is derived by dividing f
OSC
by 800.
To achieve maximum rejection of ac-line noise pickup,
a 40kHz crystal should be used. This frequency will yield an
integration period of 100msec and will reject both 50Hz and
60Hz noise. For prototyping or cost-sensitive applications a
32.768kHz watch crystal can be used, and will produce
about 25dB of line-noise rejection. Other crystal frequen-
cies, from 16kHz to 48kHz, can also be used.
Pins 39 and 40 make up the oscillator section of the
TC811. Figures 6a and 6b show some typical conversion
rate component values.
The LCD backplane frequency is derived by dividing the
oscillator frequency by 800. Capacitive loading of the LCD
may compromise display performance if the oscillator is run
much over 48kHz.
Reference Voltage (V
REF
)
A full-scale reading (2000 counts) requires the input
signal be twice the reference voltage.
In some applications a scale factor other than unity may
exist, such as between a transducer output voltage and the
required digital reading. Assume, for example, a pressure
transducer output is 400mV for 2000lb/in
2
. Rather than
dividing the input voltage by two, the reference voltage
should be set to 200mV. This permits the transducer input to
be used directly.
9V
+
470k
22M
40.0 kHz
10pF
20pF
OSC2
39
V+
38
40
OSC1
TC811
Figure 6a. TC811 Oscillator
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