I was looking at the technical reference for the Si5345 to understand the input termination requirements for single-ended clock inputs at very low frequencies (8-10 KHz). The evaluation board capacitively coupled the inputs using 0.1uF capacitors, but I suspect this will not work well at such low speeds. The manual (section 5.2 Types of Inputs) describes both AC coupling ("Standard AC-Coupled Single-Ended (IN0-IN3)") and "DC-Coupled Pulsed CMOS only for Frequencies < 1MHz (IN0-IN3)". Initially I was going to 50 terminate and then AC couple with a 10 uF MLCC capacitor (1.3 ohm reactance nominal, probably 3-4 ohms at maximum bias voltage). However, the DC-coupled option for low frequency clocks looks like it might be an option as well.
Should I prefer one or the other? My input it a 5V TTL signal from a resonator with high Q, so the input should be a relatively narrow period square wave. I was going to attenuate it down to 0-3.3V for driving the input clock and then connect with a 50 ohm SMA cable.
Thanks for any guidance!
You may consider ask the question in timing forum, 8-bit MCU team don't have good knowledge on this device.
I did not even realize separate forums existed. The ask a question screen doesn't say where the message would go, so I just assumed there was only one forum for all messages.
I'll see if I can figure out a way to post in some other forum.
However, the DC-coupled option for low frequency clocks looks like it might be an option as well.
Should I prefer one or the other?
Since you have a TTL / CMOS signal, and very low kHz, use the DC coupled option as it is simpler.
Data sheet says this, so you only need > 0.8V for a hi, attenuate to 3v3 max seems fine.
Input Voltage VIL –0.2 — 0.4 V VIH 0.8 — — V
I'm curious what resonator with high Q outputs very low frequencies (8-10 KHz) ?