Figure 8-3 shows the LMH32401 device test circuit used to measure its bandwidth and transient pulse response. The voltage source is DC biased close to the input bias voltage of the LMH32401 device (approximately 2.47 V). The internal design of the LMH32401 device is optimized to only source current out of the input pin (pin 3), and all the data shown previously is with the current flowing out of the pin. When the voltage input from the source exceeds 2.47 V, the LMH32401 device input will sink the current. Set VOD = 0 V when the input has to 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 device with a network analyzer or sinusoidal source.

Figure 8-4 and Figure 8-5 shows the bandwidth of the LMH32401 device when its input is sinking the current. The input current range of the LMH32401 device is reduced when it 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 device when the input current is increased past the amplifier linear input range. When the input is sinking current, a soft clamp will aid in fast recovery; however, the pulse will stretch 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 can be used to determine the approximate input current even under overrange situations that can occur due to the presence of retro-reflectors in the environment. As shown in Figure 7-1, each half of the differential output pulse will only swing above or below the VOCM voltage and the resulting maximum differential output swing is 0.75 V_PP since VOD is set to 0 V. Consequently only half of the total ADC range is utilized in this photodiode configuration.