LTC1061
12
1061fe
+
Σ
∫ ∫
AGND
R1
N
BP
LP
V
IN
1061 F20
+
–
S
R2
R3
R4
f
O
= ; f
n
= ; Q =
H
OLP
= ; H
OBP
= –
H
ON1
(f → 0) = ; H
ON2
= –
f
CLK
100(50)
R2
R1
R3
R2
–R2/R1
1 + (R2/R4)
R3
R1
f
CLK
2
f →
()
√
1 +
R2
R4
f
CLK
100(50)
√
1 +
R2
R4
–R2/R1
1 + (R2/R4)
–
f
IN
(kHz)
1
0
V
OUT
/V
IN
(dB)
–90
–50
–30
–10
10 100
1061 F19
–80
–70
–60
–40
–20
4
STANDARD 1%
RESISTOR VALUES
R11 = 30.9k
R31 = 16.2k
R
h
1 = 45.3k
R22 = 10.5k
R42 = 10k
R
l
2 = 15.8k
R33 = 28.7k
R
h
3 = 95.3k
R
g
= 28k
R21 = 10k
R41 = 26.7k
R
l
1 = 19.6k
R32 = 100k
R
h
2 = 52.3k
R23 = 10k
R43 = 12.7k
R
l
3 = 10k
NOTE: ADD A CAPACITOR C ACROSS R
g
TO CREATE A 7TH ORDER
LOWPASS SUCH AS (1/2πR
g
C) = (CUTOFF FREQUENCY) × 0.38
f
CLK
200kHz
f
CLK
500kHz
f
CLK
1MHz
f
IN
(kHz)
0
V
OUT
/V
IN
(dB)
–10
1.6
1061 F18
–30
–60
–70
0.2
0.8
1.2
2.0
–50
–40
–20
0
f
CLK
= 250kHz
0.4 0.6
1.0 1.4 1.8
STANDARD 1%
RESISTOR VALUES
R11 = 84.5k
R31 = 31.6k
R
h
1 = 48.7k
R22 = 10k
R42 = 97.6k
R
l
2 = 66.5k
R33 = 300k
R
h
3 = 10.2k
R
g
= 210k
R21 = 10.2k
R41 = 63.4k
R
l
1 = 287k
R32 = 232k
R
h
2 = 10.2k
R23 = 20k
R43 = 80.6k
R
l
3 = 63.4k
NOTE: CONNECT 39pF AND
100pF ACROSS R21 AND R22
RESPECTIVELY.
ODES OF OPERATIO
W
U
Mode 2 – This is a combination of Mode 1 and Mode 3,
Figure 20. With Mode 2, the clock-to-center frequency
ratio, f
CLK
/f
O
, is always less than 50:1 or 100:1. When
compared to Mode 3 and for applications requiring 2nd
order section with f
CLK
/f
O
slightly less than 100 or 50:1,
Mode 2 provides less sensitivity to resistor tolerances. As
in Mode 1, Mode 2 has a notch output which directly
depends on the clock frequency and therefore the notch
frequency is always less than the center frequency, f
O
, of
the 2nd order section.
Figure 18 shows the plotted amplitude responses of a 6th
order notch filter operating again with a clock-to-center
notch frequency ratio of 250:1. The theoretical notch
depth is 70dB and when the notch is centered at 1kHz its
width is 50Hz. Two small, noncritical capacitors were used
across the R21 and R22 resistors of Figure 16, to band-
limit the first two highpass outputs such that the practical
notch depth will approach the theoretical value. With these
two fixed capacitors, the notch frequency can be swept
within a 3:1 range.
When the circuit of Figure 16 is used to realize lowpass
elliptic filters, a capacitor across R
g
raises the order of the
filter and at the same time eliminates any small clock
feedthrough. This is shown in Figure 19 where the ampli-
tude response of the filter is plotted for 3 different cutoff
frequencies. When the clock frequency equals or exceeds
1MHz, the stopband notches lose their depth due to the
finite bandwidth of the internal op amps and to the small
crosstalk between the different sides of the LTC1061. The
lowpass filter, however, does not lose its passband accu-
racy and it maintains nearly all of its attenuation slope. The
theoretical performance of the 7th order lowpass filter of
Figure 19 is 0.2dB passband ripple, 1.5:1 stopband-to-
cutoff frequency ratio, and 73dB stopband attenuation.
Without any tuning, the obtained results closely approxi-
mate the textbook response.
Figure 19. Frequency Responses of a 7th Order Lowpass Elliptic
Filter Realized with Figure 16 Topology
Figure 18. 6th Order Band Reject Filter Operating with a Clock-
to-Center Notch Frequency Ratio of 250:1. The Ratio of 0dB to
the –65dB Notch Width is 8:1.
Figure 20. Mode 2: 2nd Order Filter Providing
Notch, Bandpass, Lowpass
