The LOG200 achieves fast transient response using a new amplifier topology. The logarithmic amplifier stage dynamically changes the amplifier open-loop gain as a function of the input current. This circuit topology allows for a stable transient response across several current ranges without requiring a very high bandwidth amplifier. The LOG200 achieves transient response from low-to-high and high-to-low current measurements significantly faster than the previous-generation logarithmic amplifiers.

The following figures show oscilloscope captures of the LOG200 output as the device responds to one-decade shifts in the input current. The optical laser driver produces a sharp square waveform to measure the rising and falling steps between 100µA and 1mA, and between 10nA and 100nA. A 30MHz, 16-Bit voltage resolution function generator drives the laser driver for laser modulation.

Figure 4-3 and Figure 4-4 show the oscilloscope captures of the 20nA to 200nA current-step. The LOG200 measured a 154ns rise time and a fall time of approximately 472ns.

Figure 4-3 Oscilloscope Capture of a 20nA-200nA Current Step

Figure 4-4 Oscilloscope Capture of a 200nA-20nA Current Step

Figure 4-5 and Figure 4-6 show the oscilloscope captures of the 200µA to 2mA current-step. The LOG200 measured a 26ns rise time and a fall time of approximately 31ns.

Figure 4-5 Oscilloscope Capture of a 200µA-2mA Current Step

Figure 4-6 Oscilloscope Capture of a 2mA-200µA Current Step