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OP291GP View Datasheet(PDF) - Analog Devices

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OP291GP Datasheet PDF : 20 Pages
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OP191/OP291/OP491
A +3 V, Cold Junction Compensated Thermocouple Amplifier
The OP291’s low supply operation makes it ideal for +3 V battery-
powered applications such as the thermocouple amplifier shown
in Figure 11. The K-type thermocouple terminates in an iso-
thermal block where the junctions’ ambient temperature is
continuously monitored using a simple 1N914 diode. The diode
corrects the thermal EMF generated in the junctions by feeding
a small voltage, scaled by the 1.5 Mand 475 resistors, to
the op amp.
To calibrate this circuit, immerse the thermocouple measuring
junction in a 0°C ice bath, and adjust the 500 pot to zero volts
out. Next, immerse the thermocouple in a 250°C temperature
bath or oven and adjust the Scale Adjust pot for an output
voltage of 2.50 V. Within this temperature range, the K-type
thermocouple is accurate to within ± 3°C without linearization.
1.235V
AD589
ISOTHERMAL
BLOCK 7.15k
1N914
1%
10k3.0V
24.3k
1%
SCALE
ADJUST
1.33M20k
ALUMEL
1.5M24.9k4.99k
1%
1%
1%
AL
COLD
JUNCTIONS
CR
CHROMEL
K-TYPE
THERMOCOUPLE
40.7V/؇C
11.2mV
475
1%
500
10-TURN
2.1k
1%
ZERO
ADJUST
8
2
OP291
3
4
VOUT
1
0V = 0؇C
3V = 300؇C
Figure 11. A 3 V, Cold Junction Compensated Thermo-
couple Amplifier
Single-Supply, Direct Access Arrangement for Modems
An important building block in modems is the telephone line
interface. In the circuit shown in Figure 12, a direct access
arrangement is utilized for transmitting and receiving data from
the telephone line. Amplifier A1 is the receiving amplifier, and
amplifiers A2 and A3 are the transmitters. The forth amplifier,
A4, generates a pseudo ground half-way between the supply
voltage and ground. This pseudo ground is needed for the ac
coupled bipolar input signals.
The transmit signal, TXA, is inverted by A2 and then reinverted
by A3 to provide a differential drive to the transformer, where
each amplifier supplies half the drive signal. This is needed
because of the smaller swings associated with a single supply as
opposed to a dual supply. Amplifier A1 provides some gain for
the received signal, and it also removes the transmit signal present
at the transformer from the receive signal. To do this, the drive
signal from A2 is also fed to the noninverting input of A1 to
cancel the transmit signal from the transformer.
390pF
0.1F
RXA
37.4k
A1
13
1/4
14 OP491
12
0.0047F
20k, 1%
3.3k
0.1F 20k, 1%
TXA
10
A2
1/4
OP491
8
9
37.4k, 1%
750pF
20k1%
20k, 1%
475, 1%
T1
0.033F
1:1
20k1%
6
A3
1/4
OP491
7
5
5.1V TO 6.2V
ZENER 5
+3V OR +5V
42
1
1/4
OP491
A4 11 3
100k
100k
10F
0.1F
Figure 12. Single-Supply Direct Access Arrangement for
Modems
The OP491’s bandwidth of 3 MHz and rail-to-rail output swings
ensures that it can provide the largest possible drive to the trans-
former at the frequency of transmission.
A +3 V, 50 Hz/60 Hz Active Notch Filter with False Ground
To process ac signals in a single-supply system, it is often best
to use a false-ground biasing scheme. A circuit that uses this
approach is illustrated in Figure 13. In this circuit, a false-ground
circuit biases an active notch filter used to reject 50 Hz/60 Hz
power line interference in portable patient monitoring equipment.
Notch filters are quite commonly used to reject power line
frequency interference which often obscures low frequency
physiological signals, such as heart rates, blood pressure readings,
EEGs, and EKGs. This notch filter effectively squelches 60 Hz
pickup at a filter Q of 0.75. Substituting 3.16 kresistors for
the 2.67 kresistors in the twin-T section (R1 through R5)
configures the active filter to reject 50 Hz interference.
–16–
REV. A
 

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