7
LTC1061
1061fe
20
19
18
17
16
15
14
13
12
11
1
2
3
4
5
6
7
8
9
10
R32
V
OUT
R13
V
IN
LTC1061 F05
LTC1061
R22
R12
R31
R21
R41
R33
R23
T
2
L CLOCK
< 2.5MHz
V
–
R11
V
+
HARMONIC DISTORTION WITH f
CLK
= 2MHz
f
IN
2ND HARMONIC
10kHz, 1V
RMS
20kHz, 1V
RMS
30kHz, 1V
RMS
40kHz, 1V
RMS
–74dB
–62dB
–62dB
–62dB
STANDARD 1%
RESISTOR VALUES
R11 = 20k
R31 = 11k
R12 = 20k
R32 = 14k
R13 = 10k
R21 = 20k
R41 = 20k
R22 = 20k
R23 = 10k
R33 = 17.8k
–
+
Σ
∫ ∫
AGND
R1
HP
BP
LP
V
IN
1061 F04
+
–
S
1/3 LTC1061
R2
R3
R4
C
C
NOTE: ADD C
C
FOR Q > 5 AND f
CLK
> 1MHz, SUCH AS C
C
≅
0.16
R4 × 1.2MHz
f
O
= ; Q =
H
OHP
= – ; H
OBP
= – ; H
OLP
= –
f
CLK
100(50)
R2
R1
R3
R1
R4
R1
R3
R2
√
R2
R4
√
R2
R4
–
+
Σ
∫ ∫
AGND
R1
N
BP
LP
V
IN
1061 F03
+
–
S
1/3 LTC1061
R2
R3
f
O
= ; f
n
= f
O
H
OLP
= – ; H
OBP
= – ; H
ON1
= –
Q =
f
CLK
100(50)
R2
R1
R3
R1
R2
R1
R3
R2
ODES OF OPERATIO
W
U
Description and Applications
1. Primary Modes: There are two basic modes of opera-
tion, Mode 1 and Mode 3. In Mode 1, the ratio of the
external clock frequency to the center frequency of each
2nd order section is internally fixed at 50:1 or 100:1. In
Mode 3, this ratio can be adjusted above or below 50:1 or
100:1. The side C of the LTC1061 can be connected only
in Mode 3. Figure 3 illustrates Mode 1 providing 2nd order
notch, lowpass, and bandpass outputs (for definition of
filter functions, refer to the LTC1060 data sheet). Mode 1
can be used to make high order Butterworth lowpass
filters; it can also be used to make low Q notches and for
cascading 2nd order bandpass functions tuned at the
same center frequency and with unity-gain. Mode 3,
Figure 4, is the classical state variable configuration pro-
viding highpass, bandpass and lowpass 2nd order filter
functions.
Since the input amplifier is within the resonant loop, its
phase shift affects the high frequency operation of the
filter and therefore, Mode 3 is slower than Mode 1. Mode
3 can be used to make high order all-pole bandpass,
lowpass, highpass and notch filters. Mode 3 as well as
Mode 1 is a straightforward mode to use and the filter’s
dynamics can easily be optimized. Figure 5 illustrates a 6th
order lowpass Butterworth filter operating with up to
40kHz cutoff frequency and with up to 200kHz input
frequency. Sides A, B are connected in Mode 1 while side
C is connected in Mode 3. The lower Q section was placed
in side C, Mode 3, to eliminate any early Q enhancement.
This could happen when the clock approaches 2MHz. The
measured frequency response is shown in Figure 6. The
attenuation floor is limited by the crosstalk between the
three different sections operating with a clock frequency
above 1MHz. The measured wideband noise was
150µV
RMS
. For limited temperature range the filter of
Figure 5 works up to 2.5MHz clock frequency thus yielding
a 50kHz cutoff.
Figure 4. Mode 3: 2nd Order Filter Providing Highpass,
Bandpass, Lowpass
Figure 3. Mode 1: 2nd Order Filter Providing Notch,
Bandpass, Lowpass
Figure 5. 6th Order Butterworth Lowpass Filter with
Cutoff Frequency up to 45kHz
