# Differential LPF cutoff frequency calculation

Discussion in 'General Electronics Chat' started by Jebasingh, Mar 7, 2011.

1. ### Jebasingh Thread Starter New Member

Mar 1, 2011
16
0
Hi all,

I have two Differentail RC Low pass filter cascaded, i found the cutoff frequency using simulation it comes around 12Hz, the maximum frequency of the input signal can be 50Hz.
kindly let me know the formula to calculate the cutoff frequency theoritically

Regards
Jebasingh

2. ### Vahe Member

Mar 3, 2011
75
9
If you have a RC lowpass filter, the cutoff (3dB) frequency would be
$
f_c = \frac{1}{2 \pi R C}
$

The single stage would provide a -20dB/dec roloff above the cutoff frequency. If you have two identical ones cascaded back to back and they are isolated, the cutoff frequency would still be the same but there would be a -40dB/decade roloff above the cutoff frequency. I am not sure about the structure of your circuit; however, if you post it I can provide more comments.

Cheers,
Vahe

3. ### Jebasingh Thread Starter New Member

Mar 1, 2011
16
0
Hi vahe,

Thanks for your reply
i have attached my schematics
The two differential filters are not identical ones
kindly let me know the method to calculate the cutoff frequency

Regards
Jebasingh

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4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
If you convert to the equivalent single-ended filter, analysis will be much simpler.

• ###### differential filter.PNG
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5. ### Jebasingh Thread Starter New Member

Mar 1, 2011
16
0
i have converted the same into single ended filter and found the cutoff frequency....thanks Ron
now i have a question how to find the number of poles available in this filter circuit it looks like a double pole filter but the simulation results is not showing the impact of the second pole
kindly advice me

Regards
Jebasingh

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
Sweep it from 1Hz to 100kHz. You'll see that the slope settles at -40dB/decade. That's indicative as a 2 pole filter.

7. ### Jebasingh Thread Starter New Member

Mar 1, 2011
16
0
i have done sweep already and there is no indication of -40dB/dec of slope settling. can you just tell me the method to do calculation.

8. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
Have you calculated the Laplace transfer function? It should have an s^2 term in the denominator. The order of s is the order of the filter.
I don't know why you didn't see the -40dB/decade slope. See the results below.

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