I have designed a photodiode circuit using the OSD35-5T photodiode from Centronic. I am using the LTC 2054HV as a transimpedance amplifier (TIA) and the LTC2051HV (dual opamp) as the next stage followed by a LTC2052HV (quad opamp) for the final stages.
Previously, for measuring the light signal levels, I put all the gain onto the TIA stage and the next stage was an unity gain inverting amplifier whose output was shared by 5 other amplifiers. Each of these 5 amplifier had the gains of 4, 16, 64, 256 & 1024. The outputs of these amplifiers were connected via a Low pass filter and fed into a microcontroller.
The system runs on +/-5V rails.
The required bandwidth is 100Hz. Looking at the bandwidth graph of the TIA, I can see that for a gain of approx. 102dB, the maximum allowable bandwidth is way lower than required. Hence I decided to split the gain between the TIA and the inverting amplifier. The TIA would have approx. 60dB gain and the inverting amplifier would have approx. 42dB gain.
What I have found is that the output (as read by the microcontroller) is quite noisy compared to my initial design.
Please note that the bandwidth of the inverting amplifier and the other 5 amplifier stages and the LPF at the end is set to 400Hz so that there is reduced attenuation of 100Hz signal.
Any idea on how I could get rid of the noise?
Furthermore, can you please let me know if there is any problem if I set the bandwidth of the first stage more than what I can get for a certain gain? Will it cause oscillatory or instability problems? If so, how?
Please see the circuit below.
I reverted the circuit back to its original form where the first stage gain is concentrated fully at the TIA stage and the following amplifier is an unity gain inverter. This seemed to have fixed this noise issue as the output is now quiet. This begs the question as to why the cascaded amplifier design didn't work. I cannot get y head around it.
Any ideas or explanations?