LTC6601-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC6601-1
Linear Technology
'LTC6601-1' PDF : 40 Pages View PDF
LTC6601-1
APPLICATIONS INFORMATION
COMPLEX FILTER CONFIGURATIONS
A Modified 2nd Order Lowpass Filter Topology
The basic filter topology of Figure 3 can be modified 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 flexibility, and enhance
the filter design options (the fO and Q equations shown
in Figure 13 reduce to equations of Figure 3 if C3 is zero
and R4 is infinite).
The modified second order filter topology provides for
setting the Q value (with R4) without changing the fO
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 (f3dB). If the Q value varies
and the fO value is constant then the f3dB frequency var-
ies in a second order lowpass function (refer to the f3dB
equation of Figure 13).
Figure 14 shows three configurations 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 fO and Q values and differ only in
the passband gain. The 150Ω R4 resistor sets a Q value
equal to 0.54 for an f3dB = 5MHz for fO = 6.954MHz.
Figures 15 to 17 show additional circuits highlighting the
use of R4 in the modified second order cicuit to set the f3dB
frequency to 7.5MHz, 10MHz and 15MHz respectively.
The design procedure for a specified f3dB frequency is
as follows:
1 Using the chosen C1, C2 and C3 values calculate the
fO value.
2. Using fO of step 1 and the specified f3dB calculate the
Q value.
3. Calculate the R4 value using the Q value of step 3.
4. Calculate the required external resistor REXT value for
the R4 value in step 3. Example, in Figure 14 the Q
value for f3dB = 5MHz is 0.54, the required R4 resistor
is 350Ω, the R4A and R4B resistors are the internal
100Ω and the REXT resistor is 150Ω [REXT = R4 – (R4A
+ R4B)].
Note: The modified second order filter topology requires
the use of at least two of the three input resistor pairs (two
of the three 400Ω, 200Ω and 100Ω pairs).
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