Electronic – Number between square brackets on FPGA schematic

No fundamental reason why not. Synchronous SRAM is truly random access, fairly inexpensive, and easy to interface to.

Its downside in that it occupies a fairly narrow niche between the on-chip BlockRam (not much smaller, free until it forces you to select a larger chip, massively parallel and more flexible) and external DRAM (massive storage capacity at a price SSRAM can't match).

So up to 0.5 or 1MB, external SSRAM is unnecessary, and above 8MB or 16MB (numbers may vary according to your budget and current prices!), SSRAM becomes expensive enough that DRAM takes over despite its limits. Then - if you need random access - you have to massively reorganise the computation to read chunks (bursts or pages) from DRAM into BlockRam where you can process it fast before writing back bursts etc....

But if you have a role for SSRAM within that window, go for it. I have added simple home-made SSRAM boards to augment commercial FPGA platforms where necessary.

1. Symbol indicates V bias (really fuzzy even on your link). This would be a fixed voltage to bias the Op-Amp. If you need more input on "bias", ask more question.

2. Different symbols are used for different ground (common) connections. Any ground symbol or common symbol isn't necessarily connected to earth. A connection to actual earth ground might be shown as a comment. In this case, the schematic is showing a voltage divider between two "common" or "ground" signals. This indicates that the two "ground" signals are likely at different potential (voltage), thus feeding the average of the two potentials to the minus (-) input of the comparator (right most Op-Amp). However, in this case, I think the designer likely meant that this is the "threshold" or set-point for the comparator.

3. The output on the right is the signal of interest and would be connected to Arduino. NOTE that none of the Op-Amps is shown with +/- power connected. It appears that this schematic is merely for showing the features, and not for use as an actual wiring diagram.

4. Each of the ground (common) points will need to be connected together in this circuit. There is not enough information in the schematic to say otherwise.

5. The left two Op-Amps positive input can both be connected to the same DC bias level. Additionally if a multitude of sensors are built, each of the sensors can use the same bias.

6. "Share the same threshold" : When making several of these sensor's, the same set-point (threshold) can be used among the several sensors.