TC7126 TelCom, TC7126 Datasheet - Page 9

• Manufacturer Part Number: TC7126
• Description: 3-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTERS
• Manufacturer: TelCom

Oscillator Components
equation:
For a 48 kHz clock (3 conversions per second), R = 180 k .
nal clock. The backplane drive signal is derived by dividing
f
the signal integrate period should be a multiple of 60 Hz.
Oscillator frequencies of 240 kHz, 120 kHz, 80 kHz, 60 kHz,
40 kHz, etc. should be selected. For 50 Hz rejection,
oscillator frequencies of 200 kHz, 100 kHz, 66-2/3 kHz, 50
kHz, 40 kHz, etc. would be suitable. Note that 40 kHz (2.5
readings per second) will reject both 50 Hz and 60 Hz.
Reference Voltage Selection
signal be twice the reference voltage.
3-1/2 DIGIT
ANALOG-TO-DIGITAL CONVERTERS
Integrating Resistor (R
with Class A output stages. The output stage idling current
is 6 A. The integrator and buffer can supply 1 A drive
current with negligible linearity errors. R
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, R
1.8 M .
Component
Value
C
R
C
NOTE:
Required Full-Scale Voltage*
200 mV
2V
*V
exist between a transducer output voltage and the required
digital reading. Assume, for example, a pressure transducer
output for 2000 lb/in.
OSC
AZ
INT
INT
FS
C
f
Note that f
To achieve maximum rejection of 60 Hz noise pickup,
A full-scale reading (2000 counts) requires the input
The input buffer amplifier and integrator are designed
In some applications, a scale factor other than unity may
OSC
= 2 V
by 800.
TELCOM SEMICONDUCTOR, INC.
OSC
f
OSC
REF
=
should be 50 pF; R
0.45
.
RC
= 48 kHz (3 readings per sec).
OSC
.
is
INT
2
is 400 mV. Rather than dividing the
4 to generate the TC7126A's inter-
is 180 k . A 2V full scale requires
200 mV
0.047 F
Nominal Full-Scale Voltage
0.33 F
180 k
INT
OSC
)
is selected from the
INT
is chosen to
100 mV
V
0.033 F
0.047 F
1V
REF
1.8 M
2V
input voltage by two, the reference voltage should be set to
200 mV. This permits the transducer input to be used
directly.
digital zero reading is required when V
This is common in temperature-measuring instrumentation.
A compensating offset voltage can be applied between
analog common and V
nected between V
DEVICE PIN FUNCTIONAL DESCRIPTION
(Pin Numbers Refer to 40-Pin DIP)
Differential Signal Inputs
V
and accepts input signals within the input stage common-
mode voltage range (V
+1V. Common-mode voltages are removed from the system
when the TC7126A operates from a battery or floating power
source (isolated from measured system), and V
nected to analog common (V
TC7126A's 86 dB common-mode rejection ratio minimizes
error. Common-mode voltages do, however, affect the inte-
grator output level. A worst-case condition exists if a large
positive V
differential signal. The negative signal drives the integrator
output positive along with V
applications, the integrator output swing can be reduced
below the recommended 2V full-scale swing. The integrator
output will swing within 0.3V of V
linearity error.
Differential Reference
V
within the V
common-mode voltages, C
stray node capacitance.
common temperature coefficient. This potential provides a
very stable voltage, suitable for use as a voltage reference.
The temperature coefficient of analog common is typically
35 ppm/ C for the TC7126A and 80 ppm/ C for the TC7126.
ANALOG COMMON (Pin 32)
approximately 3V below V
be between 2.7V and 3.35V below V
tied internally to an N-channel FET capable of sinking
IN
REF
+
The differential reference can also be used where a
The TC7126A is designed with true differential inputs
In systems where common-mode voltages exist, the
The reference voltage can be generated anywhere
To prevent roll-over type errors being induced by large
The TC7126A offers a significantly improved analog
The analog common pin is set at a voltage potential
(Pin 31), V
+
(Pin 36), V
CM
+
exists in conjunction with a full-scale negative
to V
IN
–
–
REF
IN
power supply range.
(Pin 30)
+
–
and analog common.
(Pin 35)
CM
IN
–
). Typical range is V
. The transducer output is con-
+
REF
. The potential is guaranteed to
CM
COM
should be large compared to
(see Figure 6.) For such
). (See Figure 5.)
+
or V
+
IN
. Analog common is
–
is not equal to zero.
without increased
TC7126A
TC7126
+
IN
–1V to V
–
is con-
3-225
–
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