LMF100CIWM datasheet(24/28 Pages) NSC

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3.0 Applications Information

(Continued)

Mode 1b

Mode 2 and Mode 5

Mode 3

Mode 6a and 6c

Mode 6b

In many applications, the outputs are ac coupled and dc off-

sets are not bothersome unless large signals are applied to

the filter input. However, larger offset voltages will cause

clipping to occur at lower ac signal levels, and clipping at any

of the outputs will cause gain nonlinearities and will change

0 and Q. When operating in Mode 3, offsets can become ex-

cessively large if R

2 and R4 are used to make fCLK/f0 signifi-

cantly higher than the nominal value, especially if Q is also

high.

For example,

Figure 23 shows a second-order 60 Hz notch

filter. This circuit yields a notch with about 40 dB of attenua-

tion at 60 Hz. A notch is formed by subtracting the bandpass

output of a mode 3 configuration from the input using the un-

used side B opamp. The Q is 10 and the gain is 1 V/V in the

passband. However, f

CLK/f0 = 1000 to allow for a wide input

spectrum. This means that for pin 12 tied to ground (100:1

mode), R4/R2 = 100. The offset voltage at the lowpass out-

put (LP) will be about 3V. However, this is an extreme case

and the resistor ratio is usually much smaller. Where neces-

sary, the offset voltage can be adjusted by using the circuit of

Figure 24. This allows adjustment of V

OS1, which will have

varying effects on the different outputs as described in the

above equations. Some outputs cannot be adjusted this way

in some modes, however (V

OS(BP) in modes 1a and 3, for

example).

DS005645-12

FIGURE 22. Offset Voltage Sources

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