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參數(shù)資料
| 型號： | TC7136RCLW |
| 元件分類： | ADC |
| 英文描述： | 1-CH DUAL-SLOPE ADC, PQCC44 |
| 封裝： | PLASTIC, LCC-44 |
| 文件頁數(shù)： | 15/15頁 |
| 文件大?。?/td> | 134K |
| 代理商： | TC7136RCLW |
9
Low Power 3-1/2 Digit Analog-To-
Digital Converters
TC7136
TC7136A
TC7136-6
10/18/96
2001 Microchip Technology Inc.
DS21461A
Reference Voltage Capacitor (CREF)
The reference voltage, used to ramp the integrator
output voltage back to zero during the reference integrate
phase, is stored on CREF. A 0.1F capacitor is acceptable
when V
–
REF is tied to analog common. If a large common-
mode voltage exists (V
–
REF ≠ analog common) and the
application requires a 200mV full scale, increase CREF to
1
F. Roll-over error will be held to less than 0.5 count. A
Mylar-type dielectric capacitor is adequate.
Integrating Capacitor (CINT)
CINT should be selected to maximize integrator output
voltage swing without causing output saturation. Analog
common will normally supply the differential voltage refer-
ence this case, a
±2V full-scale integrator output swing is
satisfactory. For 3 readings per second (fOSC = 48kHz) a
0.047
F value is suggested. For one reading per second,
0.15
F is recommended. If a different oscillator frequency is
used, CINT must be changed in inverse proportion to main-
tain the nominal
±2V integrator swing.
An exact expression for CINT is:
Component
Nominal Full-Scale Voltage
200mV
2V
CAZ
0.47
F
0.1
F
RINT
180k
1.8M
CINT
0.047
F
0.04
F
NOTE: fOSC = 48kHz (3 readings per sec). ROSC = 180k, COSC = 50pF.
Oscillator Components
COSC should be 50pF. ROSC is selected from the equa-
tion:
fOSC =
.
Note that fOSC is
4 to generate the TC7136A's internal
clock. The backplane drive signal is derived by dividing fOSC
by 800.
To achieve maximum rejection of 60Hz noise pickup,
the signal integrate period should be a multiple of 60Hz.
Oscillator frequencies of 240kHz, 120kHz, 80kHz, 60kHz,
40kHz, etc. should be selected. For 50 Hz rejection, oscil-
lator frequencies of 200kHz, 100kHz, 66-2/3kHz, 50kHz,
40kHz, etc. would be suitable. Note that 40kHz (2.5 read-
ings per second) will reject both 50Hz and 60Hz.
Reference Voltage Selection
A full-scale reading (2000 counts) requires the input
signal be twice the reference voltage.
0.45
RC
In some applications, a scale factor other than unity may
exist between a transducer output voltage and the required
digital reading. Assume, for example, a pressure transducer
output for 2000 lb/in.2 is 400mV. Rather than dividing the
input voltage by two, the reference voltage should be set to
200mV. This permits the transducer input to be used di-
rectly.
The differential reference can also be used when a
digital zero reading is required when VIN is not equal to zero.
This is common in temperature measuring instrumentation.
A compensating offset voltage can be applied between
analog common and V
–
IN. The transducer output is con-
nected between V
+
IN and analog common.
DEVICE PIN FUNCTIONAL DESCRIPTION
Differential Signal Inputs
V
+
IN (Pin 31), V
–
IN (Pin 30)
The TC7136/A is designed with true differential inputs
and accepts input signals within the input stage common-
mode voltage range (VCM). The typical range is V+ – 1V to
V– +1V. Common-mode voltages are removed from the
system when the TC7136A operates from a battery or
floating power source (isolated from measured system), and
VIN
– is connected to analog common (V
COM). (See Figure 7.)
Required Full-Scale Voltage*
VREF
200mV
100mV
2V
1V
*VFS = 2 VREF.
where: fOSC = Clock frequency at pin 38
VFS = Full-scale input voltage
RINT = Integrating resistor
VINT = Desired full-scale integrator output swing.
CINT must have low dielectric absorption to minimize
roll-over error. A polypropylene capacitor is recommended.
Integrating Resistor (RINT)
The input buffer amplifier and integrator are designed
with Class A output stages. The output stage idling current
is 6
A. The integrator and buffer can supply 1A drive
currents with negligible linearity errors. RINT is chosen to
remain in the output stage linear drive region, but not so
large that PC board leakage currents induce errors. For a
200mV full scale, RINT is 180k. A 2V full scale requires
1.8M
.
CINT =,
1
fOSC
(
VFS
RINT
(
)
(4000)
VINT
Value
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