TIDA-00663: Problem with the TPL0202-10MRTER output and Comparators

Joaquim Santos

Part Number: TIDA-00663

Hi,

I've been implementing the TIDA-00663 Reference design and I have it all working up to the Comparator parts.

I've measured the laser output pulse using DET10A/M of ThorLabs and the result is according to the laser datasheet (an 80ns clean pulse). The signal measured in the oscilloscope is in annex. Furthermore, I've also detected the return pulse with the on-board detector and the signal after the TIA is also annexed.

However, the problem are the comparators. When I measure the signal "laser_on" in TP4, I've obtained a really strange oscillating signal. Plus, the DC voltage controlled through the digital potentiometer seems to be affected by the laser_on signal, having a strong correlation with it. The idea was for the threshold to be constant and, whenever the "laser_on" signal was superior to it, a pulse would be generated to STOP the TDC.

With the return signal, I have a similar problem, with the thresholding oscillating with approximately 1V amplitude whenever the pulse is detected.

When I try to measure both DC thresholds without feeding the laser, I can obtain a constant signal at the potentiometer outputs.

Can anyone please help me find the source of this problem.

Does anyone ever successfully implemented the full TIDA-00663 design?

Thanks in advance

Joaquim Santos

over 5 years ago

0 Giovanni Campanella over 5 years ago

TI__Intellectual 1560 points

Hi Joaquim,

Nice to see that you have moved forward with the evaluation of the TIDA-00663.
Are these tests done on a PCB or on a breadboard?
I ask because the signals you are measuring are high speed ones and a non optimized layout could led into these kind of problems.

Thanks
Gio

0 Joaquim Santos over 5 years ago in reply to Giovanni Campanella

Intellectual 430 points

Hi Giovanni,

The tests are done directly on the PCB with the exact layout provided. However, instead of welding directly the laser on the PCB, I've soldered three wires in order to mount the laser on an optical setup. The wires are approximately 20cm long. Images below:

Thanks

0 Joaquim Santos over 5 years ago in reply to Joaquim Santos

Intellectual 430 points

Meanwhile, I've soldered directly the laser on the PCB and the result persists.
These oscillations occur at every point in the circuit and simultaneously with the laser pulse emission.

0 Giovanni Campanella over 5 years ago in reply to Joaquim Santos

TI__Intellectual 1560 points

Hi Joaquim,

Are you using the same laser that we have recommended? The SPL LL90_3 from OSRAM has built-in components to reduce parasitic influence. Keep in mind that even the laser diode package will introduce some parasitic inductance. So if you have some parasitic caps somewhere this could make it all oscillate.

Thanks
Gio

0 Joaquim Santos over 5 years ago in reply to Giovanni Campanella

Intellectual 430 points

Hi,

All the components are the ones you've recommended.

0 Giovanni Campanella over 5 years ago in reply to Joaquim Santos

TI__Intellectual 1560 points

Hi Joaquim,

We need to further investigate on this.

We will get back to you as soon as possible.

Best regards
Giovanni

0 Joaquim Santos over 5 years ago in reply to Giovanni Campanella

Intellectual 430 points

Hi Giovanni,

First of all, thanks for all the attention provided.

I've just noticed that in the Laser Driver (UCC27231DR) datasheet a 0.1uF decoupling capacitor is recommended. However, in the TIDA-00663 design, the power supply Vcc_laser is bypassed to ground via a 4.7uF capacitor (almost 50 times larger). Can this difference affect the oscillations? From what I've measured, I can almost 100% assure these oscillations are introduced in the Laser Driver.

Thanks again.

Best regards,

Joaquim

0 Joaquim Santos over 5 years ago in reply to Joaquim Santos

Intellectual 430 points

Also, the Schmitt Trigger does not have any decoupling capacitor.

This is the PWM outputted at the microcontroller:

And this is the signal after the Schmitt Trigger (Driver input):

0 Giovanni Campanella over 5 years ago in reply to Joaquim Santos

TI__Intellectual 1560 points

Hi Joaquim,

Could you please check the signal at the driver output? Is that signal oscillating as well?

If yes, this might be the reason why laser on is oscillating.

Thanks
Gio

0 Joaquim Santos over 5 years ago in reply to Giovanni Campanella

Intellectual 430 points

Hey,

The signal at the Driver output is also oscillating. In the image below, the yellow signal is the Schmitt-trigger output and the purple is the Laser driver output. Both signals are oscillating.

So, in this situation, what do you recommend me to do?

Thanks

0 Giovanni Campanella over 5 years ago in reply to Joaquim Santos

TI__Intellectual 1560 points

Hi Joaquim,

Did you try to substitute the 4.7uF decoupling cap with the 0.1uF? Do you still see the oscillations?

Thanks

Gio

0 Joaquim Santos over 5 years ago in reply to Giovanni Campanella

Intellectual 430 points

Hi,

I've already replaced the 4.7uF by a 0.1uF capacitor and the oscillations persist at the leading-edge transition as shown below for a Vcc_laser=12V.

However, in the trailing edge the response is cleaner:

0 Joaquim Santos over 5 years ago in reply to Joaquim Santos

Intellectual 430 points

Hey,

I've noticed another problem related to the oscilloscope probes.

This is the threshold signal (TPL020-10MRTR) measured directly on the test point. Some oscillations occur in the order of 50mV.

When I try to measure the Schmitt trigger output and the threshold simultaneously, the oscillations increase significantly to the other several hundred mV, increasing with an increase in Vcc_laser.

0 Giovanni Campanella over 5 years ago in reply to Joaquim Santos

TI__Intellectual 1560 points

Hi Joaquim,

You should use the bare probe with short ground leads in order to reduce the ringing you are seeing on the measurements.

Best regards
Giovanni

Simulation, hardware & system design tools

Simulation, hardware & system design tools forum

TIDA-00663: Problem with the TPL0202-10MRTER output and Comparators