TLV9002: Please fine-tune the Second-order Butterworth Bandpass Filter circuit

Kungyeh Chen

Part Number: TLV9002

I hope this message finds you well. I wanted to reach out regarding the Second-order Butterworth Bandpass Filter circuit (kung_tlv9002.TSC) that you assisted me in designing around 7 months ago. You can find the original discussion we had on the TI E2E Amplifiers Forum at this link: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1207108/tlv170-butterworth-filter

And https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1259319/tlv9002-distortion-in-the-second-order-butterworth-bandpass-filter-circuit/4769364#4769364

I tested the frequency response by inputting 150mVpp at 1kHz and found that the low-frequency (below 400Hz) in the output waveform were too low, as shown in the diagram below.

If bypassing Bandpass Filter circuit (kung_tlv9002.TSC) . It appears that there is no attenuation.

I tried changing C4 from 10nF to 33nF. The low frequencies passed, but the high frequencies failed.

Please assist in fine-tuning the circuit so that it maintains -3dB or higher at 300Hz.

Thank you!

Kungyeh

10 months ago

0 Robert Clifton56 10 months ago

TI__Mastermind 22790 points

Hi Kungyeh,

Alright so you want at minimum -3dB for the 300Hz. What is the high frequency bandwidth you want? I don't want to fix one part and break another.

Best Regards,

Robert Clifton

0 Kungyeh Chen 10 months ago in reply to Robert Clifton56

Prodigy 70 points

Hi Robert Clifton56 ,

Thank you for your response.

Here are the specifications:

Bandwidth 300 Hz to 3 kHz.
0dB ref = 1 kHz.
At 300 Hz og 3 kHz +1dB / -3dB.
Steep roll off <200 Hz and >4 KHz

Best Regards,

Kungyeh

0 Robert Clifton56 10 months ago in reply to Kungyeh Chen

TI__Mastermind 22790 points

Hi Kungyeh,

I'm sorry I forgot to also ask if you are still using this circuit?

If so there's a big discrepancy between the results shown and what I saw in the sims shown here:

Best Regards,

Robert Clifton

0 Kungyeh Chen 10 months ago in reply to Robert Clifton56

Prodigy 70 points

Hi Robert Clifton56 ,

Yes, I am using this circuit.

I've tested it, and it's already below -3 dB at 400 Hz.

How to modify the components to compensate for this difference?

Thanks,

Kungyeh

0 Robert Clifton56 10 months ago in reply to Kungyeh Chen

TI__Mastermind 22790 points

Hi Kungyeh,

That's interesting. How accurate are your passive components?

The fact that we are seeing such a difference in the frequency response cutoff could be a result of capacitors having a tolerance of 20% and resistors with 10%.

There's something strange going on. You could also double check the AP by just passing the frequency response from it's output to the input, though I'm leaning more to the passive component values being inaccurate.

Best Regards,

Robert Clifton

0 Kungyeh Chen 9 months ago in reply to Robert Clifton56

Prodigy 70 points

Hi Robert Clifton56 ,

Thank you for your response,

We've identified the root cause. The previous stage of the circuit is connected to a microphone, resulting in a 1.9V DC. To isolate this DC, a 0.1uF capacitor was placed between the microphone and the bandpass circuit. This led to the low-frequency portion of the frequency response failing. After adding the 0.1uF capacitor in the TINA simulation, the simulated frequency response matches the waveforms we measured in the lab.

If we change C10 to 1uF, it looks good.

Thanks,

Kungyeh

+1 Robert Clifton56 9 months ago in reply to Kungyeh Chen

TI__Mastermind 22790 points

Hi Kungyeh,

I'm glad to hear you were able to figure it out what was going on with this circuit.

Let us know if there's anything else you need help with.

Best Regards,

Robert Clifton

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TLV9002: Please fine-tune the Second-order Butterworth Bandpass Filter circuit