SN74LVC1G17: SN74LVC1G17

Owen,

Thank you for replying

In my circuit, VCC=1.8V, so if we use the 2RC rule to calculate the output rising edge, then (1.8-1.26)/6mA = 90 ohms, where 1.26V is 70% of 1.8V and 6 mA appears to be a sensible value if Vcc-1.8V

rising edge rate = 2RC = 180*50pF = 9ns and my requirement is <=10 nS

If I capture this edge using a high-speed scope triggering in single-shot mode, the resulting rise time from the SN74LVC3G17DCTR is around 400 uS which is too slow. 

Comments?

400 μs is not correct unless there is huge capacitance. How is your oscilloscope input configured?

AUC devices are optimized for lower voltages, and increase their drive strength while switching. Try the SN74LVC1G17 SN74AUC1G17; it certainly has a lower propagation delay.

Hi Rajan

As Clemens said that is a very unusual number for us to see. Do you have any more information on the setup? perhaps the actual capacitance in system?

You could try an AUC device, but if you are still seeing transition times at that speed the problem likely doesn't reside with our part, and with the system as a whole.

Regards,

Owen

Hi Owen,

I have asked Rohde & Schwarz for the input capacitance of their high-speed, RT-ZM modular probe system, which is a high-quality, low-loading probe. 

I will repeat the measurement and try to solder the two tiny probe ends to the output of the schmitt trigger ...

Rajan

I now have the capacitive loading information for the probe which ranges from 279 to 520 fF, so, it's low!

Therefore, rising edge rate will range from 2RC = 180*279fF = 50 pS to 120*520fF = 93.6 pS

How do these number reconcile with your parts?

Hi Rajan,

Are you testing this part within a system or is a standalone part. These numbers are very irregular for one of these parts. 

You can see in our IBIS model for the SN74LVC3G17DCTR typical delays rising waveforms. Even worst case we expect the rise time to be within ns time range.

It would help debug if we could see a schematic, and scope shots of this switching.

Regards,

Owen

The 74LVC3G17DCTR is being used within a system and I am using a single buffer within two of these chips, i/p = pin 1 and o/p = pin 7. Vcc = 1.8V connected to pin 8 and GND to pin 4.

I have had to add the two schmit triggers to a PCB that has already been fabricated as several of the original reset signals were too slow. The way I have fitted these parts isn't pretty, but they are working and I have little wires connecting pins 1 and 7 to pads on the PCB. At the moment I am using two separate 74LVC3G17DCTR chips, one of which has speeded up the rise time of the power-on-reset signal from ms to ~400 us. However, it needs to be <= 10 ns! Rohde & Schwarz has asked me to repeat the measurement, but this time solder their probe tips to the output of the 74LVC3G17DCTR. Previously I manually pressed the probe tip across the output!

Hi Rajan,

We really need a schematic or a drawing on paper to better understand your application. Could you provide that?

Regards,

Owen

And what probes are you actually using? Are there 1:1/1:10 switches? Is the oscilloscope input termination configured as 1 MΩ or 50 Ω?

My scope probes are 50 Ω, I was expecting a faster (nS) rising edge output from the schmit trigger which is connected to the power-on-reset input of a microcontroller

would adding an external pull-up resistor to the output of the schmit trigger speed the rising edge?

i'm having difficulty uploading an image to this discussion, it's effectively an opamp voltage follower connected to your schmitt trigger

It will be hard to drive a 50 Ω load with a logic device.

Is that 50 Ω load the same as in your actual application? 50 Ω inputs are designed to be used with a function generator, not with logic devices.

Clemens,

I may have got that wrong, the input coupling on the oscilloscope is 50 Ω, but its input capacitance is low, ranging from 279 to 520 fF: I measured a rise time of around 400 us from the output of the schmit trigger, but need and was expecting <= 10 ns.

Ordinarily, this output would go to a power-on-reset input of a microcontroller which has a input capacitance from 10 to 15 pF.

Hi Rajan,

Would it be possible to take your device out of the system, and test it on it's own?

If we could see the scope shots at Y that would be helpful to ensure the device is operating as expected. Then we can try to figure out what is happening in your system that is causing the output to be rising at such slow speeds.

how to add image to post

Regards,

Owen

The microcontroller input has an impedance of several megaohms. Don't use the 50 Ω setting.

I might have one spare device and could connect and test it?

Would adding an external pull-up resistor speed up output's rising edge?

The SN74LVC1G17's output is much stronger than a pull-up resistor; this would have almost no effect.

I disconnected the schmitt trigger output from the mcu input and probed the output using the high-speed, high-performance oscilloscope: the plots below show the rise time of the schmitt trigger output when connected (loaded) to the MCU and when floating - I would have expected a faster rise time!

You are still using the 50 Ω setting. Use the MΩ setting instead.