DC accurate low-pass filter

Posted on Aug 24, 2012

The LTC1050 is confi gured as unity gain 2nd order low-pass fi lter which center frequency is (1.2πR´C´) = 1.72 • fCUT-OFF = 17.2Hz and Q = 0.5. Figure 2 shows the amplitude response of the fi lter, and Figure 3 shows a well behaved transient response for which Bessel fi lters are famous. The power supplies used were ±8V to provide a total DC input common-mode range of ±6V. The measured wideband noise was 52μVrms. The clock, and R, C values of Figure 1 can be easily modifi ed to provide a 7th order Butterworth 10Hz fi lter, such as: fCLK = 1kHz, R = 26.7k, C = 1μF, R´ = 165k, C1 = 0.2μF and C2 = 0.047μF. The diode at LTC1062 pin 3 should be used to protect the device from incoming signals above the power supplies.

DC accurate low-pass filter
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Monolithic switched-capacitor low-pass fi lters, although they offer precise frequency responses, cannot usually be used for DC accurate applications because of their prohibitive DC offsets and poor gain linearity. The LTC®1062, however, is quite different from currently available low-pass switched capacitor fi lters because it uses an external (R, C) to isolate the IC from the input-signal DC path and to provide antialiasing for incoming signals larger than half its clock frequency. The LTC1062 is ideal when used in conjunction with high performance chopper-stabilized op amps. The LTC1050 is an ultra low offset, low noise chopper with the sampling capacitors internal. It can remove residual clock noise without adding further DC error. Also, the internal capacitor minimizes board area.

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