LTC1062 LPF values for 3KHz cut off?

[neazoi]

Hi,
In page 11 the circuit "Single 5V Supply 5th Order LP Filter" is what I need.
1. Can you please let me know What values should I put on R and the two C for a cut-off of 3KHz? I want to veryfy my calcs.
2. Should I leave the "DC accurate output" unconnected from the next stage (audio amplifier) and take the output from pin 8?

 

[d123]

Hi Neazoi,

It's like the blind leading the blind...

If you trust me (I wouldn't), 5k2 and 10nF compute to 3060Hz using the formula provided in the schematic, the 1/(2*pi*R*C) formula. Are you certain the right-hand C is related to the filter at all? I don't know about that IC.

I have no idea about the second question. Page 6, section: Internal Buffer may answer that.

 

[crutschow]

The clock frequency should be 100 times 3Hz or 300kHz.

That formula in the data sheet on pg. 11 appears to be in error.
It should be [1/(2*pi*R*C) * fc/100 * 1/1.63]
That gives an RC value of 86.47µs for a 3kHz corner frequency and a 300kHz clock.

Thus, for a capacitor value of 10nF for each capacitor, the resistor value is 8.66k.

I would use the DC accurate circuit as shown.

 

[neazoi]

The clock frequency should be 100 times 3Hz or 300kHz.

That formula in the data sheet on pg. 11 appears to be in error.
It should be [1/(2*pi*R*C) * fc/100 * 1/1.63]
That gives an RC value of 86.47µs for a 3kHz corner frequency and a 300kHz clock.

Thus, for a capacitor value of 10nF for each capacitor, the resistor value is 8.66k.

I would use the DC accurate circuit as shown.

Right, so 10nF and 8.6k for about 3KHz cut-off. Thanks a lot!
In the DC-accurate circuit, can I put an output coupling capacitor to DC isolate the next stage? Will this have any serious impact on the cut-off frequency?

 

[crutschow]

............
In the DC-accurate circuit, can I put an output coupling capacitor to DC isolate the next stage? Will this have any serious impact on the cut-off frequency?

It won't as long as the coupling capacitor is large enough to carry the lowest frequency of interest.

Note that the output load impedance must be much larger than 8.66k to avoid affecting the rolloff frequency.
Alternately make the parallel value of the input resistor and the output impedance equal to 8.66k ohm (or as close as you can).