OP191 Datasheet, PDF (16/20 Pages) Analog Devices – Micropower Single-Supply Rail-to-Rail Input/Output Op Amps

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 64. The K-type thermocouple terminates in an
isothermal 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 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 pot to zero
volts out. Next, immerse the thermocouple in a 250°C tem-
perature 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%
10k _3.0V
24.3k
1%
SCALE
ADJUST
1.33MΩ 20k
ALUMEL
1.5M
1%
AL
COLD
JUNCTIONS
CR
CHROMEL
K-TYPE
THERMOCOUPLE
40.7µV/°C
11.2mV
475Ω
1%
24.9k
1%
2.1k
1%
4.99k
1%
500Ω
10-TURN
ZERO
ADJUST
8
2
OP291
3
4
V_OUT
1
0V = 0°C
3V = 300°C
Figure 64. 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 65, 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.
390pF
0.1µF
RXA
37.4kΩ
A1
13
14 1/4
OP491 ₁₂
0.0047µF
20kΩ, 1%
3.3kΩ
20kΩ, 1%
10
A2
1/4
8
9 ^OP491
37.4kΩ, 1%
0.1µF _{20kΩ, 1%}
TXA
750pF
20kΩ 1%
475Ω, 1%
T1
0.033µF
1:1
20kΩ 1%
6
A3
1/4
7
5 ^OP491
5.1V TO 6.2V
ZENER 5
+3V OR +5V
4
2
1 _1/4
OP491 ₃
A4 ₁₁
100k
100k
10µF
0.1µF
Figure 65. Single Supply Direct Access Arrangement for
Modems
The transmit signal, TXA, is inverted by A2 and then re-
inverted 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. The OP491’s bandwidth of 3 MHz and rail-to-rail
output swings ensures that it can provide the largest possible
drive to the transformer at the frequency of transmission.
–16–
REV. 0