**Part Number:**THS4531

**Other Parts Discussed in Thread:**TINA-TI, THP210

Hi,

Good day.

I want to use THS4531 to design a multiple feedback (MFB) low-pass filter (LPF) circuit, in order to amplify the differential signal.

The MFB LPF circuit is shown in the following figure:

Now, I have encountered some problems:

According to theoretical calculations, the transfer function of the filter is

and the cutoff frequency fc is

Q is

Here are the parameters in THS4531 Evaluation Module:https://www.ti.com/tool/THS4531DGKEVM

R1 = 2k ohms;

R2 = 2k ohms;

R3 = 255 ohms;

C2 = 200 pF;

C1 = 9100 pF;

Substitute the above parameters into the formula calculation to obtain the cutoff frequency fc:

fc = 58.986k Hz

However, I build a simulation model in Tina-Ti, as shown in the following figure:

Subsequently, the AC transmission characteristics of the circuit were analyzed, and the results are shown in the figure:

At -3db, the frequency is 6.97k which is not equal to 58.986k Hz.

My question is why is this happening? Why is the effective bandwidth smaller than what my calculations tell me they should be. I can't find out what I did wrong with my calculations, and I am not sure if there are any other equations to consider.

Thank you for any helpful answers and suggestions

Thank you and stay safe.

Regards,

Lewis