EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Gerharddc on August 26, 2022, 07:12:53 am
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I want to passively filter out short pulses from a current source (photodiode). For a voltage signal, a typical RC high-pass filter works fine, but for a current signal that circuit obviously just doesn't work. A simple solution of course is to add a resistor after the current source to obtain a voltage signal that can then be filtered. (Something like in the attached image)
This works, but I feel like a simple two component arrangement should also be possible for filtering current signals however I am struggling to think of one. Any ideas?
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The signal source has a very high output impedance- 10k
This will not drive an hp section with a low impedance 100R in the passband. The passband attenuation will be ~100/10000 !
For a passive hp response you want R2>>R1 typically 10x at least. You could reduce C1 and raise R1 proportionally. This would lead to silly values of course.
There's no passive network that's going to work well.
The simple solution is to buffer the input to the C1 R2 section from R1. Note that this buffer needs an input impedance >> R1 again typically 10x at least.
See https://www.analog.com/en/technical-articles/how-to-effectively-design-and-optimize-tia-interfaces-of-lidar-systems.html (https://www.analog.com/en/technical-articles/how-to-effectively-design-and-optimize-tia-interfaces-of-lidar-systems.html) on AC coupling moans
To get a decent hf response you need a photodiode "transimpedance" preamp to turn output current into a useful voltage.
Load of tools for that, eg https://tools.analog.com/en/photodiode/ (https://tools.analog.com/en/photodiode/) . Low frequency rejection can be added post preamp or by adding an integrator loop . The integrator is a preferable solution.
This might sound complicated but its not. Ambient light rejection circuits have a high pass effect. You just tune the integrator time constant to get the hp corner.