LT1567
8
1567fa
Functional Description
The LT1567 contains two low noise rail-to-rail output,
wideband operational amplifiers, one of them connected
internally as a unity-gain inverter. These two amplifiers
can form a second order multiple feedback filter configu-
ration (Figure 1) for megahertz signal frequencies, with
exceptionally low total noise. The amplifier in the dedi-
cated inverter (INV) is optimized for better high fre-
quency linearity while the uncommitted operational
amplifier (OA) is optimized for lower input noise voltage,
addressing the different sensitivities to these effects
when used as a filter section. This combination produces
a low noise filter with better distortion performance than
would be possible with identical amplifiers.
LT1567 Free Design Software
A spreadsheet-based design tool is available at
www.linear.com for designing lowpass and bandpass
filters using the LT1567.
APPLICATIO S I FOR ATIO
WUU
U
FREQUENCY (Hz)
100k
–30
GAIN IS MEASURED TO EITHER OUTPUT ALONE.
IF OUTPUT USED DIFFERENTIALLY, V
OUT
+
– V
OUT
–
= 2× V
IN
GAIN (dB)
–21
–15
–9
0
1M 10M
1567 F01b
–27
–18
–12
–3
3
–6
–24
CHEBYSHEV
BUTTERWORTH
f
O
=
TRANSFER FUNCTION H(s) =
Q =
GAIN + 1
1
2π√R2R3 C2
R2
R3
(2πf
O
)
2
(2πf
O
)
Q
s
2
+ s + (2πf
O
)
2
Figure 1. 2nd Order Lowpass Filter and Gain Response for f
C
= 1MHz
(Butterworth: C1 = C2 = 390pF, R1 = R2 = 576Ω, R3 = 280Ω
Chebyshev: C1 = C2 = 390pF, R1 = R2 = 453Ω, R3 = 174Ω)
Gain vs Frequency
–
+
–
+
7
V
OUT
+
1567 F01a
V
OUT
–
DESIGN EQUATIONS:
R1 = R2, C1 = C2, C1 ≤
6
600Ω
600Ω
2
1
C1
3
5
8
V
IN
LT1567
0.1µF
7pF
C2
R3
150Ω
V
+
V
+
R1
R2
0.1µF
1
1000 • f
C
R2
R1
BUTTERWORTH R2 =
1
4.44 • C1 • f
C
R2
2
R3 =
CHEBYSHEV 0.25dB RIPPLE R2 =
1
5.65 • C1 • f
C
R2
2.62
R3 =
4
V
–
V
–
0.1µF
f
C
IS THE FILTER’S CUTOFF FREQUENCY
GAIN = 1 AND f
C
≤ 1MHz
()
GAIN =
