Dear all
I am here looking for help on amplifier+bandpass filter design. We have built up a 6 order band pass filter with two stage amplifier. The performance, however, was not satisfactory.
The design goal is to measure a very weak 2kHz signal from photodiode. The current will first go through a Transimpedance Amplifer(1MOhm) then go to bandpass filter and then be amplified by 77dB.
The procedure is as following:
PreAmplifier (Made by an OpAmp)-> 4 order BPF(active filter, no gain) -> 34dB Amplifier(inverting OpAmp) -> 2 order BPF(active filter, no gain) -> 43dB Amplifier(inverting OpAmp).
We put one stage Amp between BPF to avoid saturation caused by BPF OpAmp bias.
Design requirements:
1. very low noise, especially at 1980Hz--2020Hz.
2. 77dB of gain. (under such gain, noise floor may lead to OpAmp saturation..)
3. stable, of course
4. Need less tune of center frequency, or preferable no tune.
What are not satisfactory for our current implementation:
1. Gain can only achieve 63dB. If gain is further increased, circuit will oscillate.
2. Too large noises. noise floor at 2.3k is -15dBu, which can be found in eclosed pircutre.
3. center frequency of BPF shifts by 300Hz, which is unacceptable since it leads to a -15dB gain.
My questions are:
1. Should we use passive filters to reduce noises? If so, I am afraid that 1kHz is too low for inductors..
2. If active filters are used:
a. Shall we use bpf with gain to reduce stages, therefore to reduce noises?
b. Shall we buy commercial active filter IC to avoid tuning? Such as this one: UA42:http://www.ti.com/lit/ds/sbfs002b/sbfs002b.pdf
c. Shall we use high pass+ low pass, instead of single frequency bp? Maybe they can be more stable? And no need to tune that much?
3. For the amplifier choices, can we use BJT/MOSFET? All the amplifiers I built are based on OpAmp, since most amplifiers on the internet are about OpAmp.. What are the tradeoffs of BJT/MOSFET amplifiers and OpAmp?
The enclosed pictures:
1. the noise floor at 2.3kHz(center frequency of BP) reaches -15dB, which is too large. No signal has been applied at this time.
2. When a 2kHz signal is applied, strange harmonics showed up. Where are they from? The OpAmp is far from saturation and there are no harmonics at the very beginning signal source.
Moreover, what are the "hills" besides the harmonics? We are using a single pass band BPF...
Thanks for reading up all this. Any comment will be appreciated.
Regards
Richie
I am here looking for help on amplifier+bandpass filter design. We have built up a 6 order band pass filter with two stage amplifier. The performance, however, was not satisfactory.
The design goal is to measure a very weak 2kHz signal from photodiode. The current will first go through a Transimpedance Amplifer(1MOhm) then go to bandpass filter and then be amplified by 77dB.
The procedure is as following:
PreAmplifier (Made by an OpAmp)-> 4 order BPF(active filter, no gain) -> 34dB Amplifier(inverting OpAmp) -> 2 order BPF(active filter, no gain) -> 43dB Amplifier(inverting OpAmp).
We put one stage Amp between BPF to avoid saturation caused by BPF OpAmp bias.
Design requirements:
1. very low noise, especially at 1980Hz--2020Hz.
2. 77dB of gain. (under such gain, noise floor may lead to OpAmp saturation..)
3. stable, of course
4. Need less tune of center frequency, or preferable no tune.
What are not satisfactory for our current implementation:
1. Gain can only achieve 63dB. If gain is further increased, circuit will oscillate.
2. Too large noises. noise floor at 2.3k is -15dBu, which can be found in eclosed pircutre.
3. center frequency of BPF shifts by 300Hz, which is unacceptable since it leads to a -15dB gain.
My questions are:
1. Should we use passive filters to reduce noises? If so, I am afraid that 1kHz is too low for inductors..
2. If active filters are used:
a. Shall we use bpf with gain to reduce stages, therefore to reduce noises?
b. Shall we buy commercial active filter IC to avoid tuning? Such as this one: UA42:http://www.ti.com/lit/ds/sbfs002b/sbfs002b.pdf
c. Shall we use high pass+ low pass, instead of single frequency bp? Maybe they can be more stable? And no need to tune that much?
3. For the amplifier choices, can we use BJT/MOSFET? All the amplifiers I built are based on OpAmp, since most amplifiers on the internet are about OpAmp.. What are the tradeoffs of BJT/MOSFET amplifiers and OpAmp?
The enclosed pictures:
1. the noise floor at 2.3kHz(center frequency of BP) reaches -15dB, which is too large. No signal has been applied at this time.
2. When a 2kHz signal is applied, strange harmonics showed up. Where are they from? The OpAmp is far from saturation and there are no harmonics at the very beginning signal source.
Moreover, what are the "hills" besides the harmonics? We are using a single pass band BPF...
Thanks for reading up all this. Any comment will be appreciated.
Regards
Richie
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