OP270GPZ Datasheet, PDF (16/20 Pages) Analog Devices – Dual Very Low Noise Precision Operational Amplifier

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 V2/(K1 + 1) = VIN. The A2 feedback loop
forces VO/(K1 + 1) = V2/(K1 + 1), yielding an overall transfer
function of VO/VIN = K1 + 1. The dc gain is determined by the
resistor divider at the output, VO, and is not directly affected by
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.
R2 R2 = R1
R2
K2
1/2
OP270E
V₂
A2
1/2
OP270E
R1
R1
K1
A1
V_IN
ASSUME A1 AND A2 ARE MATCHED.
A_O(s)
=
ω_T
s
V_O
V_O = (K₁ + 1)V_IN
Figure 41. Low Phase Error Amplifier
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/βωT < 0.1. For example, a phase error of
−0.1° occurs at 0.002 ω/βωT for the single op amp amplifier, but
at 0.11 ω/βωT for the low phase error amplifier.
0
–1
SINGLE OP AMP.
CONVENTIONAL DESIGN
–2
–3
CASCADED
(TWO STAGES)
–4
–5
LOW PHASE ERROR
AMPLIFIER
–6
–7
0.001
0.005 ^0.01
0.05 ^0.1
FREQUENCY RATIO (1/βω)(ω/ω_T)
Figure 42. Phase Error Comparison
0.5 ¹
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.
C1
0.47µF
V_IN
R1
47kΩ
1/2
OP270E
R2
3.3kΩ
R3
680Ω
C2
6.8µF
+
L1
TANTALUM ^1H
R5
680Ω
C3
1µF
+
TANTALUM
L2
600mH
R7
680Ω
C4
0.22µF
+
L3
180mH
R9
680Ω
C5
0.047µF
+
L4
60mH
R11
680Ω
C6
0.022µF
+
L5
10mH
R4
1kΩ
60Hz
R6
1kΩ
200Hz
R8
1kΩ
800Hz
R10
1kΩ
3kHz
R12
1kΩ
10kHz
1/2
OP270E
R13
3.3kΩ
R14
100Ω
V_OUT
Figure 43. Five-Band, Low Noise Graphic Equalizer
Rev. E | Page 16 of 20