LMF100_15 Datasheet, PDF (24/40 Pages) Texas Instruments – LMF100 Dual High-Performance Switched Capacitor Filters

LMF100
SNOSBG9B – JULY 1999 – REVISED JUNE 2015
9 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
The LMF100 is a general purpose dual second-order state variable filter whose center frequency is proportional
to the frequency of the square wave applied to the clock input (f_CLK). The various clocking options are
summarized in Table 2.
POWER SUPPLY
−5 V and +5 V
−5 V and +5 V
0 V and 10 V
0 V and 10 V
−2.5 V and +2.5 V
0 V and 5 V
0 V and 5 V
Table 2. Clocking Options
CLOCK LEVELS
TTL (0 V to 5 V)
CMOS (−5 V to +5 V)
TTL (0 V to 5 V)
CMOS (0 V to 10 V)
CMOS
(−2.5 V to +2.5 V) TTL (0 V to 5 V)
CMOS (0 V to 5 V)
LSh
0V
0V
0V
+5 V
0V
0V
+2.5 V
V_D+
+5 V
+5 V
+10 V
+10 V
+2.5 V
0V
+5 V
By connecting pin 12 to the appropriate DC voltage, the filter center frequency, f₀, can be made equal to either
f_CLK/100 or f_CLK/50. f₀ can be very accurately set (within ±0.6%) by using a crystal clock oscillator, or can be
easily varied over a wide frequency range by adjusting the clock frequency. If desired, the fC_LK/f₀ ratio can be
altered by external resistors as in Figure 43 through Figure 49. This is useful when high-order filters (greater than
two) are to be realized by cascading the second-order sections. This allows each stage to be stagger tuned while
using only one clock. The filter Q and gain are set by external resistor ratios.
All of the five second-order filter types can be built using either section of the LMF100. These are illustrated in
Figure 25 through Figure 33 along with their transfer functions and some related equations. Figure 35 shows the
effect of Q on the shapes of these curves.
9.2 Typical Application
When designing a LP filter that has similar pass band characteristic as a Butterworth topology but requiring a
much steeper roll off then a fourth-order Chebyshev topology can implement the need with one LMF100.
Figure 53. Implement a Fourth-Order Chebyshev LP Filter Having a 1-kHz Cutoff Frequency and 1-dB PB
Ripple With an LMF100
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