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Si8635 RF Isolator Output
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Sparky49:
Hi

I've built a circuit using a Si8635 RF Isolator. The datasheet (https://www.silabs.com/documents/public/data-sheets/si863x-datasheet.pdf?fbclid=IwAR3oMAyNjynhvtSsHQ5jLYvBNxjVzWNcFvqIUMIj__LGdztuyqhBs6HhPPo) says that the output high Voltage should be ~5V, and the output Z is 50 ohm.

However, when I hook it up to my scope (HP54542A) I measure ~5V only when in high Z input, and ~2.5V in 50 ohm mode. This sounds like they have spec'd the 5V high on the output of the source and not including any series impedance... I have measured all supply voltages etc and they are fine. The data itself comes out ok, it's just the amplitude is very wrong.

My question is, where have I misinterpreted the datasheet? I imagined that I could simply connect the output to my 50 ohm input on the scope and see the 5V high voltage.

Thanks in advance.
Zero999:
No, 50Ω means its output stage has an output impedance of 50Ω. This means if the output is connected to a 50Ω load, the voltage will fall by half, as you've created a potential divider, with the source and load resistance.

In other words, this is exactly what you should expect to happen.
SiliconWizard:
Second that.
Its outputs have an output impedance of 50 ohm. If you load them with 50 ohm, the voltage output will be divided by 2 obviously. To compensate, many typical signal generators will multiply their output amplitude by 2 when you set them for a 50 ohm load. This is not the point of this IC, and there would be no way it could with a 5V power supply (unless it had some kind of internal step up or something, which is absolutely not the point of these isolators).

Note: these isolators are "high-speed" (150Mbps) but they are not "RF" isolators per se. They are not meant to be loaded by 50 ohm loads, nor meant to directly drive coax. cables for instance (if that was the application you had in mind).


Zero999:
A quick schematic showing the 50Ω resistance of the output stage, connected to the load.

If a 1V drop is acceptable, connect a complementary emitter follower to the output.

Sparky49:
So when I read the sepc for, say, a function generator it might be 10Vpp max, 50 ohm output. But that is spec'd for the output from the device, after the resistor in the output stage. Here the spec is 5V output high, but before the internal 50 ohm resistor. Why the difference in spec? Why would you spec the output voltage for a system that isn't terminated into the same impedance?

Thanks again.
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