# Four stage band pass filter and DC offset level detector

1. ## Four stage band pass filter and DC offset level detector

Hi,

I need to detect the "DC offset level" from the signal and "four stage band pass filter".

I am trying to measuring the Fluorescence decay time which is come from the Fiber optic thermometer. I am driving the LED by square pulse, then the square pulse is changed depending on the temperature and the signal behaves like exponential curve. But the problem is when I m bending the fiber cable, the dc offset level is changing. So, I need to "detect the dc offset level" as fast as possible and need to set the reference voltage depending on the dc offset level. Also, the signal is so noisy. So, I need to filter the signal by "four stage band pass filter".

Thank you so much!

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2. ## Re: Four stage band pass filter and DC offset level detector

Could you please share the spec, what is the range of DC level you are expecting, the signal level, frequency, etc. These are very much required to make a specific answer. Couldn't you eliminate the DC level by providing AC coupling?

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3. ## Re: Four stage band pass filter and DC offset level detector

Originally Posted by sijukrishnan
Could you please share the spec, what is the range of DC level you are expecting, the signal level, frequency, etc. These are very much required to make a specific answer. Couldn't you eliminate the DC level by providing AC coupling?
Hi,
The DC level with offset is 126-127 mv and the DC offset is 3.90-4.10mv (changing between specific interval). The frequency 300Hz.

So, I would like to detect the offset level to measure the falling time from Vmax to Vmin (Vmin should be 0v or detect the offset level and set the reference to the comparator). And the signal is so noisy, because the signal is come from photo-diode and I convert the photo-diode current to voltage with amplifier.

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4. ## Re: Four stage band pass filter and DC offset level detector

I believe, your signal is about 4mV amplitude which is sumperimposed over 126mV DC, right? But, the frequency, you mean the repetition rate of incident light pulse?
You are using a transresistance amplifier to convert photo diode current to output voltage, here, your gain is set by the feedback resistor. So, definitely you can expect huge amount of thermal noise. Also, even at normal lab environment itself, the noise floor would be as high as 40mV (switching of flurescent light etc.). Hence, you must amplify the signal at least as high as 80 to 100mV. So that you can set a marginal level of hysterisis to the final stage comparator. For the DC, at the final stage of amplification, before feeding to the comparator, provide a RC high pass filter, it will just kill the DC. I shall list..
1. Provide high gain stages, may be two stages to make the minimum amplitude at least 80mV
2. Kill the DC
3. Set reference to 35mV with hysterisis +/-20mV to the final comparator

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