LTC6601-2
31
66012f
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).
Figures 14 to 17 show additional circuits highlighting the
use of R4 or C3 in the modifi ed second order cicuit to
set the f
3dB
frequency to 13MHz, 19MHz, 22.7MHz and
24.6MHz 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).
