This is the characteristic of voltage fluctuation when current fluctuates, which is often seen in the data sheets of power supply ICs and module products (Figure 1).

``Fast response'' and ``fast transient response'' mean that the power supply responds quickly when the FPGA changes from one steady state to another.

For example, when the FPGA transitions from sleep state (1. small current) to burst state (2. large current), the current and voltage change as shown in Figure 1. The change in voltage when the current value changes determines the responsiveness of the power supply.

It is not enough to simply look at the current change.

Figure 2 shows the power supply voltage drop over time for the FPGA current change. (result of simulation with Spice)
When the current changes at a rate of 10 μs, the voltage drops by 101 mV (yellow-green).
However, if the current change time is 300μs, the voltage drop becomes as small as 26mV (light blue).

In other words, the faster the current value is changed, the larger the voltage drop.

AC-like voltage fluctuations, such as voltage drops, affect the jitter characteristics of FPGA transceivers.

If you want to improve the jitter characteristics of your transceiver, it is desirable to select a power supply that has a small voltage drop (high-speed response).

In other words, FPGA logic design circuits that require faster current changes require a faster response power supply.

Ground inductance in power circuits adversely affects responsiveness.
Therefore, it is important to design a power supply using a product with the shortest ground wiring pattern.

To optimize the ground pattern, it is recommended to select leadless package module products.