3-1/2 DIGIT A/D CONVERTERS
TC7106
TC7106A
TC7107
1
2
3
4
5
6
7
8
TC7107A
Integrating Capacitor – CINT
Oscillator Components
CINT should be selected to maximize the integrator
output voltage swing without causing output saturation. Due
to the TC7106A/7107A superior temperature coefficient
specification,analogcommonwillnormallysupplythediffer-
ential voltage reference. For this case a ±2V full-scale
integrator output swing is satisfactory. For 3 readings/
second (fOSC = 48kHz) a 0.22µF value is suggested. If a
different oscillator frequency is used, CINT must be changed
in inverse proportion to maintain the nominal ±2 V integrator
swing.
ROSC (Pin 40 to Pin 39) should be 100kΩ. COSC is
selected using the equation:
0.45
fOSC
=
RC
For fOSC of 48kHz, COSC is 100pF nominally.
NotethatfOSC isdividedbyfourtogeneratetheTC7106A
internal control 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,
48kHz, 40kHz, etc. should be selected. For 50 Hz rejection,
oscillatorfrequenciesof200kHz,100kHz,662/3kHz,50kHz,
40kHz, etc. would be suitable. Note that 40kHz (2.5 read-
ings/second) will reject both 50Hz and 60Hz.
An exact expression for CINT is:
VFS
1
(4000) (
) (
)
RINT
fOSC
CINT
=
VINT
Where:
Reference Voltage Selection
fOSC = Clock frequency at Pin 38
VFS = Full-scale input voltage
RINT = Integrating resistor
A full-scale reading (2000 counts) requires the input
signal be twice the reference voltage.
VINT = Desired full-scale integrator output swing
Required Full-Scale Voltage*
VREF
CINT must have low dielectric absorption to minimize
rollover error. A polypropylene capacitor is recommended.
200.0mV
2.000V
100.0mV
1.000V
Integrating Resistor – RINT
* VFS = 2 VREF
The input buffer amplifier and integrator are designed
with class A output stages. The output stage idling current
is 100µA. The integrator and buffer can supply 20µA drive
currents with negligible linearity errors. RINT is chosen to
remainintheoutputstagelineardriveregionbutnotsolarge
thatprintedcircuitboardleakagecurrentsinduceerrors. For
a 200mV full-scale, RINT is 47kΩ. 2.0V full-scale requires
470kΩ.
In some applications a scale factor other than unity may
exist between a transducer output voltage and the required
digitalreading. Assume, forexample, apressuretransducer
output is 400mV for 2000 lb/in2. 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.
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 VI–N. The transducer output is con-
nected between VI+N and analog common.
The internal voltage reference potential available at
analog common will normally be used to supply the convert-
er's reference. This potential is stable whenever the supply
potential is greater than approximately 7V. In applications
whereanexternally-generatedreferencevoltageisdesired,
refer to Figure 7.
Component
Nominal Full-Scale Voltage
Value
CAZ
200.0mV
0.47µF
47kΩ
2.000V
0.047µF
470kΩ
RINT
CINT
0.22µF
0.22µF
Note:1. fOSC = 48kHz (3 readings/sec)
TELCOM SEMICONDUCTOR, INC.
3-193
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
