I was looking at a design and started to wonder, how to analyze and verify the design.

A digital microphone has the output (SD) specified with a maximum Load of 150pF.

How do I translate this information, if I want to connect the SD output to a 50 ohm transmission line, using source termination ?


150pF is maximum capacitive load the part is specified to drive. If you look at the top of page 14, it confirms this. There is no specification to drive a resistive load, like 50ohms source termination + 50ohms line. This is the max load, it can drive lower loads.

A source terminated transmission line is, by its very nature, a capacitance at low frequency, it does not have any resistive termination at the far end, plus the input series resistor.

This means that if you want to drive through a 50ohm resistor into an unterminated line, that's OK, as long as the capacitance of the unterminated line is less than 150pF. IIRC, 50ohm line is in the ballpark of 100pF/m, so less than 1m and you will be OK.

As the part is not specified to drive 100ohms, (a) don't expect the series termination to 'work' as it's supposed to at the far end to give crisp transitions and (b) don't expect the part to deliver crisp transitions into that load anyway. Design your receiver to cope with sloppy transitions according to the setup and hold times specified in the data sheet. Hint, the lower the line's capacitance, the crisper will be the transitions and the easier the receiver's job.


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