OP191_15 Datasheet, PDF (21/24 Pages) Analog Devices – Micropower Single-Supply Rail-to-Rail Input/Output Op Amps

A 3 V, COLD JUNCTION COMPENSATED
THERMOCOUPLE AMPLIFIER
The OP291 low supply operation makes it ideal for 3 V battery-
powered applications such as the thermocouple amplifier
shown in Figure 70. The K-type thermocouple terminates in an
isothermal block where the junction 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 MΩ and 475 Ω resistors, to the
op amp.
To calibrate this circuit, immerse the thermocouple measuring
junction in a 0°C ice bath and adjust the 500 Ω potentiometer
to 0 V out. Next, immerse the thermocouple in a 250°C
temperature bath or oven and adjust the scale adjust
potentiometer 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
7.15kΩ
1N914
1%
10kΩ _3.0V
24.3kΩ
1%
SCALE
ADJUST
1.33MΩ 20kΩ
ALUMEL
1.5MΩ 24.9kΩ 4.99kΩ
1%
1%
1%
AL
COLD
JUNCTIONS
500Ω
10 TURN
CR
CHROMEL
K-TYPE
THERMOCOUPLE
40.7μV/°C
11.2mV
475Ω
1%
2.1kΩ
1%
ZERO
ADJUST
8
2
1/2
OP291
3
4
V_OUT
1
0V = 0°C
3V = 300°C
Figure 70. A 3 V, Cold Junction Compensated Thermocouple 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 71, a direct access
arrangement is used to transmit and receive data from the
telephone line. Amplifier A1 is the receiving amplifier;
Amplifier A2 and Amplifier A3 are the transmitters. The fourth
amplifier, A4, generates a pseudo ground halfway between the
supply voltage and ground. This pseudo ground is needed for
the ac-coupled bipolar input signals.
OP191/OP291/OP491
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 received 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
0.1μF
RXA
37.4kΩ
A1
13
1/4
14 _OP491
12
0.0047μF
20kΩ,1%
3.3kΩ
20kΩ,1%
0.1μF _20kΩ,1%
TXA
10
A2
1/4
OP491 ⁸
9
37.4kΩ,1%
750pF
20kΩ,1%
475Ω,1%
T1
0.033μF
1:1
20kΩ,1%
6
A3
1/4
OP491
7
5
5.1V TO 6.2V
ZENER 5
3V OR 5V
4
2
1/4
1 _OP491
A4 ^{11 3}
100kΩ
100kΩ
10μF
0.1μF
Figure 71. Single-Supply, Direct Access Arrangement for Modems
The OP491 bandwidth of 3 MHz and rail-to-rail output swings
ensure that it can provide the largest possible drive to the
transformer at the frequency of transmission.
Rev. E | Page 21 of 24