Tool/software:
I have designed a single supply ( 5 Volt) audio circuit based on OPA1664. the circuit is as below:
Now I am getting noise at the output (VF2) . Also I noticed that Biasing voltage at all the non inverting terminals are different and more than 2.5 Volt . at some biasing pin , it is 2.6 v, 2.9 v and even 3.4 v. I am feeding biasing voltage of 2.5 V through 100k resistor. Kindly help me to resolve the issue.
Hi Pal
Now I am getting noise at the output (VF2) . Also I noticed that Biasing voltage at all the non inverting terminals are different and more than 2.5 Volt .
It is not clear what you are trying to do. The circuit has narrow BPF near 1kHz. Please specify the design requirements.
You packed multiple MFB filters and RC filters to achieve a design requirements. I need to know what it is. More op amp does not necessary to product a better filter. In addition, OPA1664 is a low noise op amp, and you use all high resistor values, which the thermal noises in resistors become dominated throughout the circuit.
In addition, OPA1664 has input Vcm and Vout swing requirements, you may not optimize the design with only 5Vdc rail (unless the signal is small). I do not know how 2.5Vdc reference voltage is generated. You should use bandgap reference or 2.5Vref with an op amp buffer to generate the stable reference. If the 2.5Vref is generated from voltage divider, then this is likely your issues.
Please let me know what you are trying to do.
Best,
Raymond
Thank you for your reply and valuable inputs . Please find below answers to your queries:
1) The circuit has narrow BPF near 1kHz. Please specify the design requirements: Ans: I want to design a audio preamplifier circuit with Frequency Response : ± 1dB for a frequency range from 300 Hz to 3400 Hz.
I used multiple MFB filters and RC filters to achieve a design requirements
2) The output of this circuit will be fed to a Audio codec board ( Raspberry PI Codec Zero Board).
3) 2.5Vdc reference voltage is generated using RT9193-25GB Regulator IC
4) It will be used as input of a voice logger ( for Radio and Railway control Telephone)
Awaiting your valuable reply on the above
Hi Pal,
I want to design a audio preamplifier circuit with Frequency Response : ± 1dB for a frequency range from 300 Hz to 3400 Hz.
You want to try the BPF filter similar to the one below. It is 3rd order MFB filter and the total noise is significantly lower than the one shown previously.
1kHz BPF 3rd MFB Filter 08272025.TSC
Regarding to the 2.5Vref, I do not recommend to use LDO. The reference does not draw much current at all, and LDO is not stable enough over temperature. Please try REF35 or similar bandgap reference with low ppm/C temperature coefficient, see the thread below.
https://www.ti.com/lit/wp/slpy003a/slpy003a.pdf?ts=1756278597004
If you have other questions, please let us know.
Best,
Raymond
