OP250 Datasheet, PDF (13/16 Pages) Analog Devices – CMOS Single-Supply Rail-to-Rail Input/Output Operational Amplifiers

+V + 5V
50k
LEFT
INPUT
50k
10 F
10 F
100k
+V + 5V
1 F/0.1 F
1/2
OP250
20 270 F
50k
LEFT
HEADPHONE
+V
50k
RIGHT
INPUT
50k
10 F
10 F
100k
1/2
OP250
20 270 F
50k
RIGHT
HEADPHONE
Figure 37. A Single-Supply Stereo Headphone Driver
1
V_SY = 2.5V
A_V = +1
V_IN = 300mV rms
0.1
R_L = 500
R_L = 2k
R_L ≥ 10k
0.01
0.001
20
100
1k
FREQUENCY – Hz
Figure 38. THD vs. Frequency
10k 20k
Headphone Driver
The audio signal is coupled into each input through a 10 µF ca-
pacitor. This large value insures the resulting high pass filter
cutoff is below 20 Hz, preserving full audio fidelity. If the input
already has the proper dc bias, then the coupling capacitor and
biasing resistors are not required. A 270 µF capacitor is used at
the output to couple the amplifier to the headphone speaker.
This value is much larger than the input capacitor because of
the low impedance of the headphones, which can range from
32 Ω to 600 Ω or more. An additional 20 Ω resistor is used in
series with the output capacitor to protect the op amp’s output
in the event the output accidentally becomes shorted to ground.
OP250/OP450
Direct Access Arrangement for Modems
Figure 39 illustrates a +5 V transmit/receive telephone line inter-
face for 600 Ω systems. It allows full duplex transmission of sig-
nals on a transformer coupled 600 Ω line in a differential manner.
Amplifier A1 provides gain which can be adjusted to meet the
modem output drive requirements. Both A1 and A2 are config-
ured so as to apply the largest possible signal on a single supply to
the transformer. Because of the OP450’s high output current
drive and low dropout voltages, the largest signal available on a
single +5 V supply is approximately 4.5 V p-p into a 600 Ω trans-
mission system. Amplifier A3 is configured as a difference ampli-
fier for two reasons: (1) It prevents the transmit signal from
interfering with the receive signal and (2) it extracts the receive
signal from the transmission line for amplification by A4. Ampli-
fier A4’s gain can be adjusted in the same manner as A1’s to meet
the modem’s input signal requirements. Standard resistor values
permit the use of SIP (Single In-line Package) format resistor ar-
rays. Couple this with the OP450 14-lead TSSOP or SOIC foot-
print and this circuit offers a compact, cost-effective solution.
TO TELEPHONE
LINE
R3
360
1:1
Z_O
6.2V
600
6.2V
T1
MIDCOM
671-8005
R9
10k
R11
10k
R12
10k
A1, A2, A3, A4 = 1/4 OP450
P1
TX GAIN
ADJUST
R2
9.09k
2k
2
1
R5
A1 ₃
10k
R1 ^C1
10k ^{0.1 F}
TRANSMIT
TXA
R6
10k
7
R10
10k
2
1
3 ^A3
6
A2 ₅
+5V DC
R7
10k
R8
10 F _10k
R13
10k
R14
14.3k
P2
RX GAIN
ADJUST
RECEIVE
RXA
6
2k
7
5 ^A4
C2
0.1 F
Figure 39. A Single-Supply Direct Access Arrangement for
Modems
REV. 0
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