LTC1061 Просмотр технического описания (PDF) - Linear Technology
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LTC1061 Datasheet PDF : 16 Pages
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LTC1061
W
U
ODES OF OPERATIO
VS ≥ ±5V
0
TA = 25°C
VIN = 1VRMS
–10
fCLK = 2MHz
–20
fC = 40kHz
–30
–40
fCLK = 1MHz
–50 fC = 20kHz
–60
–70
10k 20k 40k 100k 200k
1M
fIN (Hz)
1061 F06
Figure 6. Measures Frequency Response of
the Lowpass Butterworth Filter of Figure 3
2. Secondary Modes: Mode 1b – It is derived from Mode
1. In Mode 1b, Figure 7, two additional resistors, R5 and
R6, are added to attenuate the amount of voltage fed back
from the lowpass output into the input of the SA (SB)
switched capacitor summer. This allows the filter clock-
to-center frequency ratio to be adjusted beyond 50:1 (or
100:1). Mode 1b still maintains the speed advantages of
Mode 1. Figure 8 shows the 3 lowpass sections of the
LTC1061 in cascade resulting in a Chebyshev lowpass
filter. The side A of the IC is connected in Mode 1b to
provide the first resonant frequency below the cutoff
frequency of the filter. The practical ripple, obtained by
using a non-A version of the LTC1061 and 1% standard
resistor values, was 0.15dB. For this 6th order lowpass,
the textbook Qs and center frequencies normalized to the
ripple bandwidth are: Q1 = 0.55, fO1 = 0.71, Q2 = 1.03, FO2
= 0.969, Q3 = 3.4, FO3 = 1.17. The design was done with
speed in mind. The higher (Q3, FO3) section was in Mode
1 and placed in the side B of the LTC1061. The remaining
two center frequencies were then normalized with respect
to the center frequency of side B; this changes the ratio of
clock-to-cutoff frequency from 50:1 to 50 × 1.17 = 58.5:1.
As shown in Figure 9, the maximum cutoff frequency is
about 33kHz. The total wideband output noise is 220µVRMS
and the measured output DC offset voltage is 60mV.
R51
R11
VIN
R61
fCLK < 2MHz
V+
1
R31 2
R21 3
4
5
6
7
8
9
10
LTC1061
20
19 R32
18 R22
17
16 R13
15
V–
14
13 R43
12 R33
11 R23
R12
VOUT
STANDARD 1% RESISTOR VALUES
R11 = 35.7k
R31 = 11.5k
R51 = 5.49k
R12 = 11k
R32 = 36.5k
R13 = 15.8k
R33 = 13k
R21 = 12.1k
R61 = 2.87k
R22 = 11k
R23 = 10.5
R43 = 15.8k
1061 F08
Figure 8. 6th Order Chebyshev, Lowpass Filter Using 3
Different Modes of Operation for Speed Optimization
R1
VIN
R6
R5
R3
R2
N
S
BP
LP
–
+–
Σ∫
∫
+
1061 F07
AGND
√ √ fO
=
fCLK
100(50)
R6
R5 + R6
;
fn
=
fO;
Q
=
R3
R2
R6
R5 + R6
( ) HON1 (f → 0) = HON2
f
→
fCLK
2
=
–
R2
R1
HOLP
=
–R2/R1
R6/(R5 + R6)
;
HOBP
=
–
R3
R1
;
(R5//R6)
<5k
Figure 7. Mode 1b: 2nd Order Filter Providing
Notch, Bandpass, Lowpass
8
VS > ±5V
0
TA = 25°C
VIN = 1VRMS
–10
fCLK = 1.9MHz
–20
–30
–40
–50
–60
–70
10k
30k
100k
1M
fIN (Hz)
1061 F09
Figure 9. Amplitude Response of the 6th Order
Chebyshev Lowpass Filter of Figure 8
1061fe
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