Figure 8-3 shows the test circuit used to measure the LMH32401-Q1 bandwidth and transient pulse response. The voltage source is dc biased close to the input bias voltage of the LMH32401-Q1 (approximately 2.47 V). The internal design of the LMH32401-Q1 is optimized to only source current out of the input pin (pin 3), and all the data shown previously are with the current flowing out of the pin. When the voltage input from the source exceeds 2.47 V, the LMH32401-Q1 input sinks the current. Set V_VOD = 0 V when the input must sink the current from the photodiode, or in this case, the voltage source. Set the dc bias so that sum of the input ac and dc component is always greater than the input voltage (2.47 V) when testing the LMH32401-Q1 with a network analyzer or sinusoidal source.

Figure 8-4 and Figure 8-5 shows the bandwidth of the LMH32401-Q1 when the device input is sinking the current. The input current range of the LMH32401-Q1 is reduced when the device input is sinking the current. This effect is seen by the decrease in bandwidth as the output swing increases and is more pronounced in a gain configuration of 20 kΩ. Compare Figure 8-4 with Figure 6-1 and Figure 6-3 to see the effect of current direction and input range in a 2-kΩ gain configuration. In a similar way, compare Figure 8-5 with Figure 6-2 and Figure 6-4 to see the effect of current direction and input range in a gain of 20 kΩ.

Figure 8-6 and Figure 8-7 show the pulsed-output response of the LMH32401-Q1 when the input current is increased past the amplifier linear input range. When the input is sinking current, a soft clamp aids in fast recovery; however, the pulse stretches slightly as the input current overrange increases. Compare Figure 8-6 with Figure 6-21 to see the pulse extension effect in a gain of 2 kΩ. Compare Figure 8-7 with Figure 6-22 to see the pulse extension effect in a gain of 20 kΩ. Knowledge of the pulse extension is used to determine the approximate input current, even under overrange situations that can occur because of the presence of retro-reflectors in the environment. As Figure 7-1 shows, each half of the differential output pulse swings greater than or less than the VOCM voltage, and the resulting maximum differential output swing is 0.75 V_PP because VOD is set to 0 V. Consequently, only half of the total ADC range is used in this photodiode configuration.