Datasheet

Rev 03 Jan 2013 19:42 | Page 6
subsequent low pass filter. The component values selected on the evaluation board yield good results
without the need of a buffer. Keep this in mind when changing component values.
Low-Pass Filter
The internal uncommitted op amp is used to build a two-pole low pass filter with gain, using a
Sallen-Key configuration. The following design equations set the low-pass cut-off frequency, gain and
Q.
f
c
= 1 / [2π √(R18 C14 R17 C7)]
Gain = 1 + R12/R13
Q = √{ (R17 C14 R18 C7) / [R17 C14 + R18 C14 + R18 C7 (1 - Gain) ] }
Notice that changing the gain or the cutoff frequency will have an effect on Q and vice versa.
The evaluation board components place the cutoff frequency for the low-pass filter at approximately
25Hz and the gain to 11. Keep the sum of R12 and R13 above 50kΩ to save power and to avoid
excessive loading of the output.
Figure 5 shows the transfer function of the signal from the differential input of the instrumentation
amplifier to VOUT with the default filter configuration.
Figure 5. Gain vs. Frequency