THVD1452: driver rise and fall time

Nathan,

2 questions:

1) how much capacitive load is there on your bus? With the device unpopulated, are you able to measure the capacitance between A/B? It should be 50pF or less.

2) are you measuring the differential signal? This looks like the single ended measurement. Differential is A-B which requires you to use a math function on the scope.

-Bobby

Thanks for the quick response Bobby,

Item 1.) The capacitance on the line was less than 50pf and will need to dig up a tool to measure the capactance on the line. The TPP1000 has 3.9pf and there is about 0.5" inch of trace that makes me believe that this is less than 50pf

Item 2.) original o-scope measurement was with a single ended probe. Picture below is with TDP1500 (differential probe), that still has 9ns of falling edge slew rate, or 3ns more than the specification.

Nathan Mayes said:

Item 2.) original o-scope measurement was with a single ended probe. Picture below is with TDP1500 (differential probe), that still has 9ns of falling edge slew rate, or 3ns more than the specification.

The scopeshot looks like the high level sits arounds 1.1V and the low is -1V. So the total magnitude is 2.1V. The data sheet uses 90% and 10% threshold values (I'm not sure if the measurement function is using this threshold as well).

Can you use your cursor to manually measure the differential (Ch1 in your latest scopeshot) to +890mV and -790mV? 

Can you also verify your Vcc value? From the looks of the differential magnitude, I would of expected the output high and low to be closer to 2V. If its closer to 1V I'm assuming that your Vcc might not be 5V. Are you using the device at 3V or 3.3V? 

-Bobby

Hi Bobby,

Here are some answers to your questions.

Q1.) Can you use your cursor to manually measure the differential (Ch1 in your latest scopeshot) to +890mV and -790mV? 
A1.) Yes, the timing is 8.5ns so still 2.5ns longer than the max from the datasheet. This is without a device connected (0.5" inch trace and a 3.9pf capacitor). See figure below

This is with a MD04104-6 oscilloscope
Differential probe TDP1500 on CH1
Bandwidth set to Full (1Ghz)
Coupling set to DC

Q2.) Can you also verify your Vcc value?
A2.) I am using 3.3V and not 5V for my VCC. Looking at Figure 11 in the datasheet, I would expect that the 3.3V specification to still be less than 6ns.

Thanks,
-n8

Hi Bobby,

Here are some answers to your questions.

Q1.) Can you use your cursor to manually measure the differential (Ch1 in your latest scopeshot) to +890mV and -790mV? 
A1.) Yes, the timing is 8.5ns so still 2.5ns longer than the max from the datasheet. This is without a device connected (0.5" inch trace and a 3.9pf capacitor). See figure below

This is with a MD04104-6 oscilloscope
Differential probe TDP1500 on CH1
Bandwidth set to Full (1Ghz)
Coupling set to DC

Q2.) Can you also verify your Vcc value?
A2.) I am using 3.3V and not 5V for my VCC. Looking at Figure 11 in the datasheet, I would expect that the 3.3V specification to still be less than 6ns.

Q3.) Are you using the device at 3V or 3.3V? 
A3.) I just measured the power rail at the THVD1452 with a Agilent 34401A and it read 3.2858V. The HP34401A calibration is still good (expires 27-Jan-2027).

Thanks,
-n8

Would it be possible to know what equipment TI used to test this with? 

I suspect what might be happening for me is I am trying resolve timing down to 1ns using a oscilloscope rated for 5GS/s and a probe rated for 1.5GS/s.

My current test equipment cannot sample faster than 1.5Gs/s (0.667ns) and according to Nyquist needing to be able to at least sample 2x faster than my input frequency. This means I need a probe capable of measuring 0.5ns to know the frequency, but would need to sample faster (about 10x or 10Gs/s) to know what the waveform may even look like.

If I can find out what equipment TI used to validate this measurement with this could confirm my theory and show that the part does meet its rise / fall time.

Hi n8,

Thanks for the additional details to my questions. 

I did go ahead and check in our internal database for what values they measured when used at 3.3V and found the data showed it was around 2ns. I'm not sure what is the exact difference between your test set up and their is. 

Nathan Mayes said:

Would it be possible to know what equipment TI used to test this with? 

I would need to check. Based on the data though, they used an automated tester (maybe something called ETS88?). The team that did the validation for this is located in India so it may take some time for them to get back to me (my email may have to get juggled around until it finds the right person).

I'll try to update you on Tuesday if I can get the information by then.

EDIT: One other thing I could potentially think of is if your set up has a decoupling cap on close to the Vcc of the pin. I've sometimes seen this as an issue when devices are being validated internally. Sometimes specs can fall out of line if there is a decoupling cap close to the Vcc pin).

-Bobby