I'm trying to design a simple bandpass filter with fast settling time after powerup. I have tried several methods and in every case am finding limitation between requirements of HPF frequency, gain, and powerup settling time. I am hoping to learn if my limitation is due to my requirements or if there might be a circuit topology I haven't considered.
At a high level the application of this circuit is to be a rudimentary sound level meter for specific frequency range. The plan is to take a signal from MEMs microphone, amplify/filter it, and then read the output via a fast ADC built into my MCU. The accuracy requirements are very low and the design is working well, but requires a 35ms delay before signal settles which is too long for this application. The MEMs mic I am using is Knowles SPU0410LR5H-QB, which biases the signal at ~0.7V. The output is stable after approximately 0.5ms.
The approximate specifications I'm designing towards are:
- Passband: 85Hz-500Hz
- Gain: 37 V/V
- Powerup Settling Time: <10ms
Note that here the powerup settling time is dependent upon both HPF and gain settings. I'm a little confused here because I believe that the delay is due to charging C1 through R1+R2, however that would be a time constant of 71ms (e.g. 213ms to being mostly settled), while I'm finding the signal settles within ~35ms. Irregardless, I find that reduction of C1-R1-R2 does reduce my settling time, however to get time under 10ms I need to cut gain in half and double HPF which is quite significant of a deviation from my desired spec.
My leading option right now is to use a non-inverting amp and let it cut out half of the signal. The issue with passing the full signal is adding a resistor bias increases the delay and this design does not have room for a dual-supply rail. Note that I've doubled the gain here to account for having only half the signal. The other consideration is that the LPF will distort the half-wave signal, but I believe this is fine as long as I only track the peak voltage. The powerup settling time is independent of gain and although it is dependent upon HPF frequency, at 85Hz it is just under 10ms.
I don't want to make my description too long, but here is a brief overview of some of the other options I've considered:
- With mentioned inverting amp circuit, I have tried adding a FET between IN- and IN+ controlled by the MCU at powerup. After optimizing the length of the control signal I find it still takes ~20ms before signal settles. Although the signal is never large enough to overcome the FET's body diode, I'm not a big fan of this approach of having a FET linked to the signal path.
- I have tried an IGMF filter with F0=250Hz, Q=0.707, and G=37. Although simulation showed a 6ms step response settling time, I find ~20ms settling after powerup when I breadboarded the circuit.
- I have tried a 2-stage approach with filtering in first stage and gain in second stage, however the second stage requires AC coupling capacitor and bias resistors which create an additional HPF stage and additional delay.
I'm happy to provide circuits on any of these if anyone would find it beneficial.
I think the non-inverting circuit will be okay, but thought it would be wise to reach out to this forum to see if I can gain a better understanding of the limitation and see if there might be an approach I haven't considered. Hopefully I'll learn something here and others will find the topic useful as well. I appreciate any insight I might receive.
EDIT: Here is the schematic and Scope captures of the IGMF Filter I tried: Here are scope captures of the response with CH1 (yellow): Power rail CH2 (blue): Input signal CH3 (pink): Output This is powerup with the mic: This is a step response from my function generator: