EMRFD Message Archive 14128

Message Date From Subject
14128 2017-07-02 13:27:44 Dana Myers Si5351A output drive waveform and observations

I connected a 54 ohm load via a 100nF capacitor to the output of
an Si5351A (one of Hans' QRP-Labs boards). The Si5351A is programmed
for 1MHz CLK0 output with Drive Strength = 8mA

(54 ohms as 2 x 27ohm resistors because they were handy, and it's
close enough)

Here's the scope capture of this:

From this, I calculated output resistance of the Si5351A as 26 ohms.
(54 ohms * (3.3V - 2.24V) / 2.24V)

The CMOS output is sourcing 20.7mA from the Vdd rail, then sinking
that 20.7mA into the ground rail. With a 50% duty cycle, you'd expect
10.3mA average DC current flowing from Vdd to ground.

I measured the current used by the Si5351A+TCXO with no load as
23.8mA and with the 54 ohm load as 33.8mA; giving 10mA as the
"average load current". Slightly less than I expected but well
within the accuracy of my measurements :-)

In this case, the measurement both shows that the CMOS output
is reasonably modeled as a Vdd source with a 26 ohm series resistor,
and that the output will source/sink ~20mA. I don't really know
how good for the long-term reliability of the output it is to run
at 20mA though :-) - I'd buffer it to drive a 50 ohm load.

Dana  K6JQ