Modern LiDAR systems are moving towards solid state configurations with multi-channel photodiode arrays. For optical front-end designs it is impractical to have single transimpedance amplifiers (TIA) connected to each diode output along with additional multiplexers or other switching solutions to connect to the digitizer. This approach causes increased solution size, complexity and signal degradation.

The LMH32404 resolves this problem in two ways, by providing higher integration within the device and by allowing user configured output multiplexing for independent output control.

Figure 8-9 shows a comparison of a non-integrated front end using individual amplifiers, a multiplexer, and fully-differential amplifier (FDA) to connect to the differential input ADC. In comparison, the front end using the LMH32404 is able to connect four channels per amplifier to each ADC or set of ADC differential inputs. Figure 8-9 shows how the LMH32404 improves solution size and system complexity compared to a non-integrated solution. With the additional features like input current clamps and ambient light cancellation, LMH32404 also improves system design and eliminates need for additional circuitry.

LMH32404 is a quad-channel device and each channel has an independent differential output stage and multiplexing switch. Figure 8-10 shows two common output configurations. In a four-to-four configuration the LMH32404 operates with no output multiplexing with each input and corresponding differential output active. This configuration is useful when the user needs to be able to capture data from four optical sensors simultaneously. In a four-to-one configuration, the LMH32404 internally multiplexes all four differential outputs into a single differential output. The LMH32404 outputs can be configured in any permutation such as one channel operating in one-to-one mode with the other three channels multiplexed in a three-to-one configuration. This independent control and multiplexing feature significantly increases channel density for systems that do not need to record all inputs simultaneously.

To show the front-end design integration and multiplexing capability, the LMH32404 performance was measured with the ADC12QJ1600-Q1 quad channel analog to digital converter. Figure 8-11 shows the data measured from the ADC12QJ1600-Q1 using a 10 ns electrical input pulse on a single channel of the LMH32404. These pulses are similar to outputs seen in a typical LiDAR application. Figure 8-12 shows the data from the ADC12QJ1600- Q1 when the LMH32404 outputs multiplex between different channels. The initial output shows a 10ns duration pulse train on channel 1, followed by a 20 MHz sinusoidal signal on channel 2 and then the output with all channels turned off but the input signals still present. Details on these measurements and application are discussed in the application brief, How to Increase the Channel Density of LiDAR Systems with the 4-Channel LMH32404 Transimpedance Amplifier.