Part Number: TLV9051
I'm planning to make a light meter/exposure meter for photography, using TLV9051, a Hamamatsu photodiode, and possibly a Ti low power MCU.
I read the app note about the amp circuit, but have no idea how to reject 50/60/100/120Hz flicker from AC powered indoor light source.
Thanks for any advice!
Thanks for your question and welcome to the forum!
Could you please provide a link to the app note you read. We have several app notes that deal with this topic.
Your question is quite common, actually. You will want to filter out the frequencies that can be leaking in from the main (50 to 120Hz). It's also important to avoid filtering out your signal of interest. Do you know the frequency of your signal of interest? Knowing this will help to determine the type of filter you will need.
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In reply to Daniel Miller56:
The app notes I found are TIDU535 1 MHz, Single-Supply, Photodiode Amplifier Reference Design, and Analog Engineer's Circuit Cookbook: Amplifiers.
They describe so well about the basic and noise optimization, I still like to learn about the RC filter between Amp output and ADC.
The signal of interest might be in low frequency, It's sun light or any ambient light. I'm not so sure but I think measuring light for photography is similar to ALS sensing.
Light meter or exposure meter is quite rare application nowadays. It only require one-shoot measurement or sampling every 0.5s.
In reply to user4950967:
Would it be fair to say that you want to average (not reject) the light flicking frequencies?
If you only need the average total light from all sources, then a low bandwidth amplifier with low pass filter stage is a solution.
Regards,Ronald MichallickLinear Applications
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In reply to Ron Michallick:
I think you are right about this, averaging the light will be fine. Is there any app note I can learn about the low pass filter design? More importantly, how to determine the bandwidth.
Still I'm thinking an amp with diode feedback as log amp, this circuit is common in cameras, because the resolution is not that important when in high light.
If you'd like to learn more about the RC filter between the amplifier output and the ADC input, I'd recommend you watch section 5 of our Texas Instruments precision labs videos on ADCs. Here's a link to the first video in the section.
Could you please give a little more detail on what you are trying to do? I am not familiar with this application. So, I do not know if you would like to do the averaging in software or some other way.
If you would like to learn more about active filtering with op amps, you can see this app note. It is quite detailed. We also have a design tool called the "Filter Designer" which automates a lot of the design process based on your system's needs. Again, I still don't understand this application very well. So, perhaps a passive, low-pass filter will be enough.
Thank you so much, I will read all the materials you recommanded.
This application is as the ambient light sensor like OPT3001, photodiode + Amp + ADC. High speed reading is not needed, high resolution in high light situation is not needed. High dynamic range is needed, maybe Op Amp with two different value feedback resistors and analog switch, or maybe Op Amp with diode feedback which is commanly found in film cameras design from the old days.
Ok. I think I now have a better understanding of your application. Please correct me if I am wrong in any way.
The ambient light will activate the Hamamatsu photodiode and that information will be passed as the current from the photodiode. The current then passes through a log amplifier with a diode in the feedback loop. The output voltage will be related to the natural log of the input current.
So, do you then want to average artificial light at 50, 60, 100, and 120Hz? The low pass filter naturally averages in the time domain. I suppose you would like what Ron has stated, average the artificial light and keep the natural light at lower frequencies. This makes me think that the filter would need a cutoff below 60Hz. Is my understanding correct?
I think what Ron has suggested is sensible for this case. Perhaps we will need a photodiode + log amplifier + filter stage + ADC. I am not sure if the filter stage would be sufficient to drive the input of the ADC. I think it would depend on both the amplifier bandwidth and the ADC sampling rate. If not, then we might even need a photodiode + log amplifier + filter stage + buffer + ADC. But, I will need to look into this more to understand it properly.
Yes, your understanding is correct . I will start experiment on this.
Thanks all, Daniel and Ron!
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