I'm struggling on how the authors came up with the resistor and capacitor values. It seems that they focus the freq on 4.8 hz but I thought they needed 1 hz on the high pass filter. http://www.freescale.com/files/sensors/doc/app_note/AN1571.pdf

please see figure 1. thank you for any help!

 

A normal resting human hearbeat is about 1Hz (60 beats per minute).
4.8Hz would be from the heart of a running mouse at 288 beats per minute.

The filters are extremely simple using only one resistor and one capacitor each. Then their slopes are very gradual.
A 1Hz filter has an attenuation of almost nothing at 1Hz (-3dB). They use a 4.8Hz filter calculation so that 1Hz is attenuated 10dB and lower frequencies are attenuated more.

 

AudioGuru, I know almost nothing about double pole filters. Do you think you could explain just a little about how that works in this circuit? Thanks in advance for any wisdom.

 

To make sure I understand, some of the 1 hz signals will be filter out as well as the lower frequencies? Also may I ask why do they have two cutoff frequencies?

Thanks again!

 

Because of the op-amp input the two RC sections don't interact at all.
The input filter is just an RC high pass filter while the op-amp feedback gives you a high frequency gain.

 

To make sure I understand, some of the 1 hz signals will be filter out as well as the lower frequencies?

Look at the graph in the document. 1Hz is attenuated -10dB which is an amplitude of about 0.3 times.

Also may I ask why do they have two cutoff frequencies?

The graph shows the response is flat down to 30Hz, is down to -3db at 5Hz, is down to -10dB at 1Hz and is down -40dB (1/100th the amplitude) at 0.2Hz.

 

Just as a clarification that plot is normalized to the high frequency gain of 151 or 43.6 dB.
That is, the plot is gain minus 43.6 dB.
That's what they mean they say "the plot doesn't include the gain of the amplifier"

The op amp portion at low frequency is a voltage follower while at high frequency it's a gain of 43 dB.
The input RC portion is a high pass filter, which attenuates the low frequency signals.

They amplify the signal of interest using the op amp and attenuate the unwanted signal using the input RC. Because they normalize the plot it makes it look like extra attenuation.