Given the diversity of silicon photonics architectures, AFE integration levels, and cost considerations, a wide variety of AFE solutions exist in the market. In cases where the number of built-in MUXIN inputs of an AFE is insufficient to cover all required analog signal channels, external analog switches can complement the design. By combining a cost-effective 12-channel analog input AFE with external 8:1 expansion, the total analog sampling capacity can be extended to up to 40 channels.

Use case #1: In a 1.6T (8-Lane) silicon photonics module, 8 silicon photonic modulators generate a total of 32 MPD/RSSI signals (24 MPDs + 8 RSSIs). Using TMUX1108/TMUX1104 to assist the AFE, the analog signal sampling system can support up to 40 current/voltage signal inputs, offering a flexible architecture that can be adapted to various MCU/AFE platform combinations.

Use case #2: In an 800G (4-Lane) silicon photonics module, 4 silicon photonic modulators generate 16 MPD/RSSI signals (12 MPDs + 4 RSSIs). In this case, a single TMUX1104 paired with a 12-channel AFE is sufficient to achieve full analog signal sampling and channel selection coverage.

As illustrated by both scenarios, using an External analog multiplexer to expand analog input channels offers a compelling advantage in terms of cost efficiency and resource utilization compared to adopting a higher-integration AFE or MCU. Furthermore, analog multiplexers provide greater design flexibility when developing 800G, 1.6T, and even 3.2T optical modules.

Design Note: Leakage current is a critical specification for multiplexers in this applications. MPD and RSSI signals being switched are weak current signals (typically in the µA to mA range), the primary selection criterion for the analog switch is its leakage current in both on and off states, ensuring no significant impact on signal integrity. Taking TMUX1108 as an example, its 3pA leakage current represents an excellent specification — significantly better than the internal switch leakage of most AFEs. Its compact 2.6mm × 1.8mm package makes it highly suitable for multi-channel, low-current sampling applications, including standalone use independent of the AFE.