Can multiple HMC542BLP4E be daisy-chained together? From the description of the Serial Output pin and the Latch Enable pin, this seems to be an intended feature but it's not explicitly described in the datasheet.
The serial output is served as a monitor/read, with 8 clock cycle delayed from the serial input. The HMC542B is design targeted for cascading the HMC542B with Dattn out of pre-stage to the Dattn in of the cascaded stage, with LE to control the Dattn setting for each HMC542B.
Below is the functional diagram of our serial control circuitry used in the HMC542BLP4E, as shown in the datasheet, and our suggested serial control configuration for cascaded application:
For others reading this, MN and I discussed this some more offline. The part can support daisy-chaining the serial interface, but you have to be careful with the serial timing. The datasheet's serial input data setup time is for a single part operating by itself and the data going from the serial input to the attenuators. When daisy-chaining multiple parts, though, each part adds more delay for data to propagate through to get to the last part in the chain. So the serial data setup time for the last part in the chain is longer than for any of the previous parts in the chain and sets the minimum that can be used by the system
The datasheet's serial data setup time of 20ns minimum for a single part is a result of a pair of inverters on the SDI input. When adding a second part to the chain, the data has to go through a second pair of inverters that's on the SDO output (shown in the figure above) and a third pair on the second HMC542's input. So each part added to the chain adds an additional 40ns to the minimum setup time. In equation form this is Tlsup = [2N - 1] x 20ns where N is the number of parts in the chain.
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