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AD8307 4-pole low pass filter 32 / 5,000 Translation results AD8307 4-pole low pass filter design

Category: Hardware
Product Number: AD8307

AD8307 Datasheet page 20 shows a 4-pole low pass filter design. As far as I know it seems to be a sallen-key topology.

The datasheet states that changing the capacitors of C5~C7 will change the cut-off frequency.

But in reality, it seems that the cut-off changes only when C6, C7, and C8 are changed.

And the C5 doesn't seem to affect the cut-off.

What do you think?

  • Hi there,

    Figure 40 shows two active filters in cascade, C5, C6 in the first filter, and C7, C8 in the 2nd filter. All 4 capacitors should affect the aggregate corner frequency of the filter design. The 7.32k series resistor is also an important element of the 2nd stage filter. The first stage filter has an effective output resistance of 12.5k internally (ref. Figure 1), which gets external resistors VR2+32.4k in parallel to approximate some similar input series resistor as part of the first stage filter. 

    If any further questions, consider simulating the active filters with LTspice to see the filter frequency response. 

    It's important to not rely completely on the external filters for ripple reduction, as this will increase error down at low frequencies. Instead, design the detector external capacitors such that output ripple at AD8307 pin 4 OUT pin to be relatively low to begin with, at the lowest desired operating frequency. Use the active filters to reduce ripple even further, if necessary. The usage of the active filter helps keep detector response time as fast as possible, while also reducing output ripple to acceptable value and keeping accuracy within the required spec.