TC7136 View Datasheet(PDF) - TelCom Semiconductor Inc => Microchip
Part Name
Description
Manufacturer
TC7136 Datasheet PDF : 12 Pages
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LOW POWER, 3-1/2 DIGIT
ANALOG-TO-DIGITAL CONVERTERS
1
TC7136
TC7136A
output voltage back to zero during the reference integrate
phase, is stored on CREF. A 0.1 µF capacitor is acceptable
when VR–EF is tied to analog common. If a large common-
mode voltage exists (VR–EF ≠ analog common) and the
application requires a 200 mV full scale, increase CREF to
1 µF. 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 = 48 kHz) a
0.047 µF value is suggested. For one reading per second,
0.15 µF is recommended. If a different oscillator frequency
is used, CINT must be changed in inverse proportion to
maintain the nominal ±2V integrator swing.
An exact expression for CINT is:
( ) ( ) (4000) 1
CINT =
fOSC
VFS
RINT ,
VINT
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 µA. The integrator and buffer can supply 1 µA 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
200 mV full scale, RINT is 180 kΩ. A 2V full scale requires
1.8 MΩ.
Component
Value
Nominal Full-Scale Voltage
200mV
2V
CAZ
0.47 µF
0.1 µF
RINT
180 kΩ
1.8 MΩ
CINT
0.047 µF
0.047 µF
NOTE:fOSC = 48 kHz (3 readings per sec). ROSC = 180kΩ, COSC = 50
TELCOM SEMICONDUCTOR, INC.
Oscillator Components
2 COSC should be 50 pF. ROSC is selected from the
equation:
fOSC =
0.45
RC
.
Note that fOSC is Ϭ4 to generate the TC7136A's internal
clock. The backplane drive signal is derived by dividing fOSC
3 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, 66-2/3 kHz, 50kHz,
40kHz, etc. would be suitable. Note that 40kHz (2.5 read-
ings per second) will reject both 50Hz and 60Hz.
Reference Voltage Selection
4
A full-scale reading (2000 counts) requires the input
signal be twice the reference voltage.
Required Full-Scale Voltage*
VREF
200 mV
100 mV
2V
*VFS = 2 VREF.
1V
5
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 400 mV. Rather than dividing the
input voltage by two, the reference voltage should be set to
200 mV. This permits the transducer input to be used
6 directly.
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
abneatwloegecnoVm+INmaonndanadnaVlIo–NgTchoemtrmanosnd.ucer output is connected
DEVICE PIN FUNCTIONAL DESCRIPTION
Differential Signal Inputs
7
VI+N (Pin 31), VI–N (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
8 when the TC7136A operates from a battery or floating power
source (isolated from measured system), and VI–N is con-
nected to analog common (VCOM). (See Figure 7.)
3-255
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