I'm trying to improve on a transimpedance amplifier design by changing out the op-amp that I'm currently using for another one. The issue I'm facing now is that I'm unsure as to how to pick a good op-amp based on my needs.
My transimpedance amplifier circuit is shown below. I'm using the SensL MicroFJ-60035 silicon photomultiplier (SiPM), and the amplifier I'm currently using is the TI OPA656.
SensL requires you to give your email for their datasheet, but I'll try and put some info on the device here. If you need more information, please let me know, and I'll add more info here. The spectral range of the sensor ranges from 200-900 nm, and its dark count rate is 80 kHz/mm squared. It has a typical gain of 5.3*10^6, a dark current parameter of 4.1 uA, and a rise time of 300 ps. Also, the sensor is rated for a max current of 15 mA.
The OPA656 is a voltage feedback amplifier with a slew rate of 295 V/us, and it has a gain bandwidth product of 230 MHz and a bandwidth of 500 MHz. It has an input bias current of 1 pA, and it has an input offset voltage of 100 uV. Also, the capacitance at the anode output is 4000 pF. The datasheet never said about the capacitance at the cathode output.
Right now, the issue I'm facing with my design is the speed. Currently, it's too slow, with the rise time at 14 ns. My team and I have managed to reduce the rise time of the circuit down to around 6.5-9 ns by reducing the feedback resistor from 470 ohms to 25 ohms, but the gain is so tiny, with the max amplitude being around 160 mV. I am trying to find a new op-amp that would allow for a faster rise time without the necessary drop in gain.
As I did some research, I don't necessarily have a full understanding of the nuances of op-amp design, so I focused primarily on the slew rate. However, I found that as the slew rate increases, the bandwidth decreases. Here's my first issue. My team needs a certain bandwidth requirement, but if the relationship between the slew rate and the op-amp bandwidth is inversely proportional, how do I determine the amount of bandwidth I need for my application? Is it based on my SiPM, or is it from the test environment on which the SiPM is tested?
My next question also relates to the bandwidth of the op-amp. A smaller BW means that the amplifier won't work on high frequency signals outside of its range, but how does that affect the output voltage? I found some SPICE models of my SiPM as well as the OPA656 and some possible replacement amplifiers (specifically the AD8014, ADA4895, ADA4860, and the AD8001). These replacements have a faster slew rate, but the bandwidth is smaller. Would a smaller bandwidth result in a smaller but faster output voltage? If this is the case, then this isn't too big a problem. We don't necessarily need a large voltage signal; having one at around 600 to 850 mV is enough for our needs.