Hello All,
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
Thanks,