OP270 Analog Devices, OP270 Datasheet - Page 16

• Manufacturer Part Number: OP270
• Description: Low Noise Precision Dual Op Amp
• Manufacturer: Analog Devices

Specifications of OP270

• -3db Bandwidth: 5MHz
• Slew Rate: 2.4V/µs
• Vos: 50µV
• Ib: 15nA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 3.2nV/rtHz
• Vcc-vee: 9V to 36V
• Isy Per Amplifier: 3.25mA
• Packages: DIP,SOIC

OP270
LOW PHASE ERROR AMPLIFIER
The simple amplifier depicted in Figure 41 utilizes a monolithic
dual operational amplifier and a few resistors to substantially
reduce phase error compared with conventional amplifier
designs. At a given gain, the frequency range for a specified
phase accuracy is more than a decade greater than that of a
standard single op amp amplifier.
The low phase error amplifier performs second-order fre-
quency compensation through the response of Op Amp A2 in
the feedback loop of A1. Both op amps must be extremely well
matched in frequency response. At low frequencies, the A1
feedback loop forces V
forces V
function of V
resistor divider at the output, V
the resistor divider around A2. Note that, like a conventional
single op amp amplifier, the dc gain is set by resistor ratios only.
Minimum gain for the low phase error amplifier is 10.
Figure 42 compares the phase error performance of the low
phase error amplifier with a conventional single op amp
amplifier and a cascaded two-stage amplifier. The low phase
error amplifier shows a much lower phase error, particularly for
frequencies where ω/βω
−0.1° occurs at 0.002 ω/βω
at 0.11 ω/βω
R2
K2
O
/(K1 + 1) = V
V
IN
ASSUME A1 AND A2 ARE MATCHED.
T
O
for the low phase error amplifier.
/V
Figure 41. Low Phase Error Amplifier
IN
= K1 + 1. The dc gain is determined by the
A
OP270E
O
OP270E
(s) =
2
2
1/2
/(K1 + 1) = V
1/2
A1
/(K1 + 1), yielding an overall transfer
A2
T
< 0.1. For example, a phase error of
ω
T
s
T
for the single op amp amplifier, but
O
, and is not directly affected by
R2
R1
IN
. The A2 feedback loop
R2 = R1
V
2
V
O
R1
K1
= (K
1
+ 1)V
V
O
IN
Rev. E | Page 16 of 20
FIVE-BAND, LOW NOISE, STEREO GRAPHIC
EQUALIZER
The graphic equalizer circuit shown in Figure 43 provides 15 dB
of boost or cut over a five-band range. Signal-to-noise ratio over
a 20 kHz bandwidth is better than 100 dB and referred to a 3 V
rms input. Larger inductors can be replaced by active inductors,
but consequently reduces the signal-to-noise ratio.
V
IN
0.47µF
47kΩ
C1
R1
–2
–3
–4
–5
–6
–7
–1
0.001
0
680Ω
680Ω
680Ω
680Ω
680Ω
R11
R3
R5
R7
R9
CONVENTIONAL DESIGN
Figure 43. Five-Band, Low Noise Graphic Equalizer
TANTALUM
TANTALUM
OP270E
0.047µF
0.022µF
0.22µF
6.8µF
+
+
+
+
+
1/2
1µF
SINGLE OP AMP.
C2
C3
C4
C5
C6
Figure 42. Phase Error Comparison
(TWO STAGES)
0.005
FREQUENCY RATIO (1/βω)(ω/ω
LOW PHASE ERROR
CASCADED
600mH
180mH
60mH
10mH
0.01
1H
L1
L2
L3
L4
L5
3.3kΩ
R2
AMPLIFIER
1kΩ
1kΩ
R10
1kΩ
R12
1kΩ
1kΩ
R6
R8
R4
200Hz
800Hz
10kHz
60Hz
3kHz
0.05
0.1
OP270E
1/2
3.3kΩ
R13
T
)
0.5
100Ω
R14
1
V
OUT