Si5351a Voltage not turning my Line Driver ON - Page 1
Forum section: electronics / rf-microwave

author: Jeff Weinmann
author_rank: Contributor
timestamp: June 18, 2021, 12:20:38 am
content: Hello,I've been working on amplifying the clock signal of the Si5351A clock generator.Observing on an Oscilloscope I see that it constantly puts out 3.3 Volts Peak to Peak square wave with:+1.67 Volts at the top, and -1.67 Volts at the base.I've connected it to an Octal Buffer/Line Driver, this one in particular:https://www.digikey.com/en/products/detail/on-semiconductor/74AC244SC/149106I just cannot get the line driver to turn on. Input Voltage on the Datasheet says -.5 to Vcc to +.5V.The datasheet also states that the ACT244 has TTL-compatible inputs.So, is 1.67 Volts not a TTL level voltage? The goal is to power the Line driver at 5 Volts and enable the line driver just by using the clock voltage.If I cannot get the clock generator to turn this line driver on, can anyone recommend a Line driver that will?attached is my schematic on how I'm hooking it up.thanks to all

author: MIS42N
author_rank: Frequent Contributor
timestamp: June 18, 2021, 01:47:46 am
content: You say you are using an AC part, not ACT. ACT are the TTL compatible ones. The AC on voltage is Vcc dependent, probably 2.5V for a 5V Vcc.

author: mon2
author_rank: Frequent Contributor
timestamp: June 18, 2021, 02:21:14 am
content: https://www.adafruit.com/product/1787

author: oPossum
author_rank: Super Contributor
timestamp: June 18, 2021, 02:52:50 am
content: Quote from: Jeff Weinmann on June 18, 2021, 12:20:38 amThe datasheet will have minimum high levels specified.Typically...2.0V for TTL 80% of Vcc for CMOS

author: radiolistener
author_rank: Super Contributor
timestamp: June 18, 2021, 04:50:13 am
content: Quote from: Jeff Weinmann on June 18, 2021, 12:20:38 amIt looks like you forgot to switch from AC coupling to DC coupling on oscilloscope input

author: Jeff Weinmann
author_rank: Contributor
timestamp: June 18, 2021, 01:05:11 pm
content: Quote from: MIS42N on June 18, 2021, 01:47:46 amThanks so much for the info - I missed that!I will be ordering my ACT line drivers to test.

author: Jeff Weinmann
author_rank: Contributor
timestamp: June 18, 2021, 01:08:32 pm
content: DC couping still shows a +1.67 to -1.67 swing. but the problem may have been solved - I have a AC part not an ACT (TTL compatible) part

author: MIS42N
author_rank: Frequent Contributor
timestamp: June 19, 2021, 03:27:15 am
content: You may get away with using the AC part, by sending the output of the Si5351a through a capacitor and resistor in series to the input of the 74AC244. The spec sheet implies that the inputs are bypassed by clamp diodes to limit the -ve and +ve excursions to -0.5V and Vcc+0.5V. So when the Si5351a goes to -1.67 it will charge the capacitor to around 1.17V via the clamp diode. Then the +ve 1.67 from the Si5351a will be raised to 2.84V at the input to the 74AC244 which may be enough to be seen as an 'on' voltage. The series resistor is to limit the clamp diode current to less than 20mA - 100Ω should do. The capacitor size depends on the input leakage of the 74AC244 and the pulse duration. An 0.1μF (100nF) could be overkill.

author: fourfathom
author_rank: Super Contributor
timestamp: June 19, 2021, 03:09:11 pm
content: Quote from: Jeff Weinmann on June 18, 2021, 01:08:32 pmThis still makes no sense. When the Si5351 is powered at 3.3V the output (when lightly loaded) will swing between 0V and 3.3V. The spec sheet says 0.6V to VDD-0.6V. I am using the '5335 in a design, where it directly drives a dual buffer (a NL27WZ17, 3.3v VDD).Are you using a '5351 breakout board perhaps, with a series capacitor on the output? That's the only way I see you getting a ground-centered output signal.