Photodiode transimpedance amplifier with single supply
I have a photodiode and I want to connect it to an ADC to read the ambient light.
For the context, non linearity is not an issue as I'm interested in knowing the amount of light in 3-4 levels. The sensor itself pays a huge role on it anyway.
I don't have a part number of any details about the photodiode but I have a few questions about the circuits in those kind of applications.
The most common circuit I see on the internet is this one:
I have a few questions regarding its operation:
Let's assume the op amp operates with a single supply voltage VCC = 3.3V
In order for the circuit to work properly we will need an op amp where its inputs can operate close to its power rails (GND - VCC). Is this correct?
I assume this is because current (Ip) is flowing from cathode to anothe and inverted (V-) input of the op amp momentarily goes below non-inverted (V+) input, ground in that case, and the op amp tries to null out the difference by increasing the Vout voltage.
However that would make the voltage across the diode (as its shown in the circuit) positive with a reverse biased polarity (cathode positive). So, how does that work?
In addition to this I also saw the circuit below:
source: eeweb
In the second circuit I don't understand why the bias (R1, R2) at non-inverted input (V+) is actually needed.
The only case I could think of where this circuit would be a better fit is where the op-amp can not operate its inputs close to the power rails (GND - VCC) so we bring both of them at VCC/2. Is this correct?
Or is it just something that is used to zero out the leakage current (dark current) when no light is applied in the diode?
I have also seen some other circuits where they reverse bias the photodiode (negative supply on its anode).
Where to my understanding is that it has the same effect as the second circuit (bias on V+ using a divider). Is this correct?
1 answer
synopsis
Achievable by using a rail-to-rail op-amp. A small bias on the non-inverting input improves the transient response of the op-amp. That’s the second circuit in the O.P. I use such topology myself for sensing flashing light, as well as steady ambient light.
rationale
An op-amp may have a worse transient response when the output is at the rail. If you bias the non-inverting input to a modest voltage V_bias (0.1 to 0.3 mV) then the output of the TIA will never go below V_bias and never reach the lower rail.
Such op-amp bias also reverse-biases the photodiode with the same V_bias, as a side effect. What are the consequences of the photodiode reverse bias? The dark current increases, the noise increases, the response time decreases. Check experimentally.
If the ambient light is relatively stable [if the ambient light is DC] then the transient response may be less of a concern. My suggestion would be to design the circuit with the voltage divider for bias. If bias isn’t wanted, then leave out the upper resistor.
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