I'm building an absorbance meter with photodiode. In the past I have been working with 4 op-amps to achieve good bandpass filtering performance. However now I'm trying to implement similar with only 2 op-amps and with single supply (3V).
I can achieve quite good performance with two active band pass filters, but in this configuration photodiode will be connected in series with a resistor, thus photocurrent creates the voltage across the resistor. The disadvantage of this circuit is that reverse voltage of photodiode is different at different photocurrent levels, which may cause non-linearity depending on e.g. ambient light level. In my application I consider linearity to be golden factor (Or is it really?).
I have read some posts and articles about DC-cancellation circuit with 2 op amps, but then I loose some of the band pass performance. Could anybody suggest any transimpedance amplifier topology that could cancel out DC and also filter out the 50Hz-120Hz+harmonics with 2 op amps? Or would this be almost impossible? I think the hardest part is to maintain the almost same voltage across the diode when also trying to block low frequency signals.
* Filter out the DC-signal of ambient light, thus protecting op-amp from saturating/clipping under the sunlight
* Supress 50Hz/60Hz and their harmonics
* Detect the modulated signal at 5kHz - 10kHz, which gives approx 1-2µA with selected photodiode (<25pF diode capacitance)
- Planning to use OPA2348 or OPA2314
The two-op-amp feedback circuits that you have seen on our forums create a single-pole high-pass filter function. This can provide some rejection of AC line/mains rejection, depending on how high the pole is positioned. I don't see an easy way to improve the filtering performance of this DC correction loop.
I assume that you are performing an a/d conversion in your system. You may want to consider using an a/d converter with ac line/mains rejection capability. Is this possible in your system? What a/d converter are you using?
In reply to Bruce Trump:
If I recall correctly the DC restoration circuit I once tested on TINA SPICE was one of your suggestions. Thank you for that. The A/Ds that we are using are Analog's AD7920 that don't have AC-rejection property. I don't know if the feedback on your circuit could also react on high frequencies as well, thus producing a bandpass. Though I expect that group delays may distort the waveform.
I'm thinking about if there is a robust way to use current mirror to remove DC-offset before TIA. Btw. does anybody have experience with die bonded amplifiers? If so, how did that turn out in your project?
In reply to Tianyu Huang:
Yes the DC restoration circuit was one I recommended in a few other postings. It was merely a recommendation--I never built it. I spent some time simulating and I believe it can be successfully implemented. It requires good understanding of the circuit to properly optimize it and I'm not sure whether others have succeeded. A number of forum readers appeared to have trouble understanding its operation.
Upon reflection, I'm not sure that my comments on an a/d with line rejection characteristics are applicable. You are probably modulating the light signal at some high frequency. Am I correct?
I'm not sure I understand your final comment. It seems to me that my DC restoration circuit is doing exactly what you describe.
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