TDC7200: What are the pulse requirements for the START and STOP pulses?

Hey Gregor,

Thanks for posting to the sensing forum!  Just trying to make sure I got everything but you are  basically looking to answer the following questions:

1.Minimum Rise/Fall Time requirements for START/STOP- some notes below

2.Amplitudes required for START/STOP pulses- answered below

3. Device behavior in the scenario that the pulse length for START is still high when the STOP pulse rises.

 If you could please just confirm that I got and understood all your questions correctly that would be very helpful on my end.

My thought process is that only the rise or the fall time would matter and not both,  since the device is specifically configured to look at either the rising or the falling edge of the signal. I have not tried various rise or fall times but I should be able to check a couple for you to see if they make a difference.

As far as amplitudes are concerned they are dependent on the devices input supply.  Noted under section 7.3 of the datasheet. The input high and low are 0.7* VDD up to 3.6V and for an input low it would be 0 to 0.3*VDD V. I attached a screenshot of the datasheet below.

If your voltages are in the order of millivolts then the device would not be able to recognize that it received a START or STOP pulse, so some sort of conditioning would be required for your input signals.

If you could clarify question number 3 to make sure that I understood that correctly. 

Best,

Isaac

Hello Isaac,

Your question summary is correct.

Q1:  FYI, we are most interested in the rising edge which is ~ 1 us.

Q2:  I must've missed the 0.7*VDD spec; thanks.  We would have to either amplify or use a comparator, etc. to get the right pulse amplitudes.

Q3:  Yes, the long fall time means that the pulses could overlap if the TOF is shorter than the pulse width.  If only rising edge matters, then this may not be an issue, but I need to know.

Thanks,

Greg

Hey Gregor,

Thanks for the clarification. I should be able to check this in the lab for you. I haven't looked at the fall time issue much before so its mainly vetting that one. As far as the rise time I believe this shouldnt be a problem but I can confirm this on the bench as well.

Best,

Isaac

That would be helpful.  Thanks.

I should be able to check this tomorrow and send you an update then. 

Best,

Isaac

Hey Gregor,

I appreciate the patience. I was able to test the remaining two questions you had regarding the TDC7200.

1.Minimum Rise/Fall Time requirements for START/STOP, for your situation specifically 1us

I tested this out by using a slow triangle ramp as a START pulse, the slowest ramp I tested was using a 2us/1v ramp which might be well below what your system might produce and the device will recognize it as a high regardless of the ramp time as long as it reaches the appropriate voltage level. The main thing to note here is that the device will begin to time your measurement as soon as that START pulse meets the required threshold for the system to identify it as a high and will stop the timer as soon as the threshold is meet as a low, the issue with this is that if you have an extremely low ramp it can skew the accuracy of your results. So I think if you are trying to obtain as much accuracy as possible having a fast ramp time would be beneficial. If you ramp time is steady you might be able to calibrate it out afterwards but you are also dependent on part to part variation of when the voltage is considered high and the device START/STOPs the measurements.

3. Device behavior in the scenario that the pulse length for START is still high when the STOP pulse rises.

This had no issue, since the device is configured to look at the rising edges even if the START pulse time exceeds the STOP pulse you will still obtain a good measurement. The only issue to note here is that you will not be able to commence a new measurement until the START pulse pulls falls.

I hope this helps let me know if you have any other questions!

Best,

Isaac