LTC1061_09 Datasheet, PDF (13/16 Pages) Linear Technology – High Performance Triple Universal Filter Building Block

W
U
ODES OF OPERATIO
Figure 21 shows the side A of the LTC1061 connected in
Mode 2 while sides B and C are in Mode 3a. This topology
can be used to synthesize elliptic bandpass, highpass and
notch filters. The elliptic highpass of Figure 17 is synthe-
sized again, Figure 22, but the clock is now locked onto the
R43
R33
R23
T²L, CMOS
CLOCK INPUT
V⁺
V_IN
R_h2
R_l2
1
20
2
19
3
18
4
17
5
16
LTC1061
6
15
7
14
8
13
9
12
10
11
V_OUT
R42
R32
R22
V ^{– Rl1}
R41
R31
R21
R_h1
R11
1061 F21
Figure 21. LTC1061 with Side A is Connected in Mode 2 While
Side B, C are in Mode 3a. Topology is Useful for Elliptic
Highpass, Notch and Bandpass Filters.
STANDARD 1%
0
RESISTOR VALUES
–10
R11 = 54.9k
R31 = 34.8k
R21 = 24.3k
R41 = 10k
–20
R_h1 = 28.7k
R_l1 = 280k
R22 = 68.1k R32 = 18.2k
–30
R42 = 10k
R_h2 = 10.2k
–40
R_l2 = 16.2k
R33 = 75k
R23 = 10k
R43 = 14k
–50
NOTE: FOR CLOCK FREQUEN-
–60
CIES ABOVE 300kHz, ADD
A CAPACITOR C ACROSS
–70
R21 AND R22 SUCH AS
–80
(1/2πR21C) = f_CLK
–90
0 1 2 3 4 5 6 7 8 9 10
f_IN (kHz)
1061 F22
Figure 22. 6th Order Elliptic Highpass Filter Operating with a
Clock-to-Cutoff Frequency Ratio of 75:1 and Using the Topology
of Figure 21
LTC1061
higher frequency notch provided by the side A of the
LTC1061. As shown in Figure 22, the highpass corner
frequency is 3.93kHz and the higher notch frequency is
3kHz while the filter operates with a 300kHz clock. The
center frequencies, Qs, and notches of Figure 22, when
normalized to the highpass cutoff frequency, are (f_O1 =
1.17, Q1 = 2.24, f_n1 = 0.242, f_O2 = 1.96, Q2 = 0.7, f_n2 = 0.6,
f_O3 = 0.987, f_n3 = 0.753, Q3 = 10). When compared with the
topology of Figure 16, this approach uses lower and more
restricted clock frequencies. The obtained notch in Mode
2 is shallower although the topology is more efficient.
Output Noise
The wideband RMS noise of the LTC1061 outputs is nearly
independent from the clock frequency. The LTC1061
noise when operating with ±2.5V supply is lower, as Table
3 indicates. The noise at the bandpass and lowpass
outputs increases rough as the √Q. Also the noise in-
creases when the clock-to-center frequency ratio is al-
tered with external resistors to exceed the internally set
100:1 or 50:1 ratios. Under this condition, the noise
increases square root-wise.
Output Offsets
The equivalent input offsets of the LTC1061 are shown in
Figure 23. The DC offset at the filter bandpass output is
always equal to V_OS3. The DC offsets at the remaining two
outputs (Notch and LP) depend on the mode of operation
and external resistor ratios. Table 4 illustrates this.
It is important to know the value of the DC output offsets,
especially when the filter handles input signals with large
dynamic range. As a rule of thumb, the output DC offsets
increase when:
1. The Qs decrease
2. The ratio (f_CLK/f_O) increases beyond 100:1. This is
done by decreasing either the (R2/R4) or the R6/(R5
+ R6) resistor ratios.
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