LMF100_15 Datasheet, PDF (29/40 Pages) Texas Instruments – LMF100 Dual High-Performance Switched Capacitor Filters

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Typical Application (continued)
Mode 3
V_OS(HP)
V_OS(BP)
V_OS(LP)
= V_OS2
= V_OS3
=
V_OS1
é
ê1+
êë
R4
R_p
ù
ú
úû
-
V_OS2
æ
çè
R4
R2
ö
÷ø
-
V_OS3
æ
çè
R4
R3
ö
÷ø
R_p = R1|| R2 || R3
Mode 6a and 6c
V_OS(HP)
= V_OS2
V_OS(LP)
=
V_OS1
æ
ç¹
+
è
R₃
R₂
+ ^R3
R₁
ö
÷
-
R₃
ø ^R2
V_OS2
Mode 6b
V_{OS(LP(N.INV ))}
V_{OS(LP(INV ))}
= V_OS2
=
æ
V_OS1
ç¹⁺
è
R₃
R₂
ö
÷
ø
-
R₃
R₂
V_OS2
LMF100
SNOSBG9B – JULY 1999 – REVISED JUNE 2015
Figure 59. Offset Voltage Sources
In many applications, the outputs are AC-coupled and DC offsets 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 f₀ and Q. When operating in
Mode 3, offsets can become excessively large if R₂ and R₄ are used to make f_CLK/f₀ significantly higher than the
nominal value, especially if Q is also high.
For example, Figure 60 shows a second-order 60-Hz notch filter. This circuit yields a notch with about 40 dB of
attenuation at 60 Hz. A notch is formed by subtracting the bandpass output of a mode 3 configuration from the
input using the unused side B operational amplifier. 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 output (LP) will be about 3 V. However, this is an extreme case
and the resistor ratio is usually much smaller. Where necessary, the offset voltage can be adjusted by using the
circuit of Figure 61. 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).
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