I am designing an interfacing circuit for PIN
photodiode which will be used in NIR
spectrophotometer project. The PIN diode is IG22x250S4i
and works in photovoltaic mode
. The specifications are:
- Capacitance (@0V): 40pF
- Forward Voltage: 0.56 V
- Reverse Voltage Max: 5V
- Forward Current: 1 mA Max
- Reverse Current: 0.5 mA (Max)
- Dark Current: 0.5 uA (at 5 V reverse voltage)
- Shunt Impedance: 400 - 500 k ohm
- Wavelength: 1150 - 2200 nm
The datasheet of the sensor is over here. The application notes and reference circuit is over here.
Initially, I tried connecting a 10 k ohm resistor in parallel to the diode and observe the response in a oscilloscope
. I was getting 500mV max when the light source is bought near to the diode. Rather, I realized that due to 50 ohm impedance of probe, I was getting much smaller signal.
Later, I picked a random op-amp
(input bias current in 100 pA) and designed the circuit similar to the one given below (with jumper wire):
I was getting rail-to-rail signal (3.3V) with that op-amp but it was very noisy.
I have now moved to designing a PCB
with an opamp. I have read this document and considered selecting an opamp with low input bias current
, low input offset voltage
.
But I am confused with bandwidth and slew rate of the opamp for my application. My setup has a single PIN
diode and a stepper motor. Light after passing through the sample and lens fall on grating
. The grating splits the light and falls on the diode. The motor moves to capture the whole spectrum. The time required to capture the spectrum is 1 seconds and 3000 points will be taken in that interval. So, one sample will be around 333.3 us
. If I hypothetical consider a peak in spectrum causes an instantaneous jump of 0
V to 3.3
V (of photodiode output) in single point (out of 3000 points), then maximum rate of change of input voltage (to opamp) will be 0.0099 V/ us
and that's how i concluded the slew rate of opamp should be more than 0.0099 V/ us
.
I am not confident about my calculation and are looking for similar calculations to calculate required bandwidth of the opamp.
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