LTC6601-1
31
66011f
APPLICATIONS INFORMATION
COMPLEX FILTER CONFIGURATIONS
A Modifi ed 2nd Order Lowpass Filter Topology
The basic fi lter topology of Figure 3 can be modifi ed as
shown in Figure 13. The Figure 13 circuit includes an
impedance path between the two summing nodes (the
circuit nodes common to resistors R1, R2 and R3). A
resistor and/or a capacitor connection between the sum-
ming nodes provide even more fl exibility, and enhance
the fi lter design options (the f
O
and Q equations shown
in Figure 13 reduce to equations of Figure 3 if C3 is zero
and R4 is infi nite).
The modifi ed second order fi lter topology provides for
setting the Q value (with R4) without changing the f
O
value and increasing the passband gain to greater than
one without changing the Q value (in the Q equation of
Figure 13 the value of Q does not change if the value of
the [1 + GAIN + 2(R2/R4)] denominator factor does not
change). Using R4 to set the Q value allows the option
to design the –3dB frequency (f
3dB
). If the Q value varies
and the f
O
value is constant then the f
3dB
frequency var-
ies in a second order lowpass function (refer to the f
3dB
equation of Figure 13).
Figure 14 shows three confi gurations using a capacitor
(C3) and a resistor (R4) between the summing nodes.
The external 49.9Ω resistor isolates the LTC6601 outputs
from driving directly a capacitive load. The three circuits
of Figure 14 have equal f
O
and Q values and differ only in
the passband gain. The 150Ω R4 resistor sets a Q value
equal to 0.54 for an f
3dB
= 5MHz for f
O
= 6.954MHz.
Figures 15 to 17 show additional circuits highlighting the
use of R4 in the modifi ed second order cicuit to set the f
3dB
frequency to 7.5MHz, 10MHz and 15MHz respectively.
The design procedure for a specifi ed f
3dB
frequency is
as follows:
1 Using the chosen C1, C2 and C3 values calculate the
f
O
value.
2. Using f
O
of step 1 and the specifi ed f
3dB
calculate the
Q value.
3. Calculate the R4 value using the Q value of step 3.
4. Calculate the required external resistor R
EXT
value for
the R4 value in step 3. Example, in Figure 14 the Q
value for f
3dB
= 5MHz is 0.54, the required R4 resistor
is 350Ω, the R4A and R4B resistors are the internal
100Ω and the R
EXT
resistor is 150Ω [R
EXT
= R4 – (R4A
+ R4B)].
Note: The modifi ed second order fi lter topology requires
the use of at least two of the three input resistor pairs (two
of the three 400Ω, 200Ω and 100Ω pairs).
