DRV8814: DRV8814 : Jitter on outputs

Hi Sam,

Can you provide waveforms showing the jitter?

I'm not sure if what you are observing is related to the issue in the linked E2E thread.

Regards,

Pablo

Hi Pablo,

Thanks for following up.

Attached is an oscilloscope grab with persistence hold.

• Blue trace, motor output terminals, ~374ns max jitter output
• Yellow trace, corresponding channel enable input, ~2ns max jitter input

Regards,

Sam.

I was having the same output jitter issue for the DRV8701, and reviewing the chip logic block diagram doesn't show the inputs were re-clocked by the internal clock, but at least the datasheet does say input signals are de-glitched. I checked the 8814 datasheet and it says nothing about de-glitched inputs.

Brian

Hi Samuel and Brian,

When the device is not in current regulation mode, the output will follow the input PWM signal and is not affected by the digital clock frequency jittering. However, when the driver is in current regulation mode, the control inputs (INx) are ignored and the driver itself controls the states of the outputs. In this case, digital clock affects the jittering at the output. 

Samuel,

Is the driver in current regulation?

Regards,

Pablo Armet

Pablo Armet said:

Is the driver in current regulation?

Hi Pablo,

This is easy to check by just drive the load with small pwm pulse, say 10% to avoid any kind of current regulation. 

In my case with the 8701, I'm sure there was no current load regulation when the jitter happened.

Brian

Brian,

Brian Dang said:

In my case with the 8701, I'm sure there was no current load regulation when the jitter happened.

In your case, did you measure similar jittering of 374ns as Samuel's? And did you try it with a purely resistive load or only with a motor?

Pablo

Pablo,

Pablo Armet said:

In your case, did you measure similar jittering of 374ns as Samuel's? And did you try it with a purely resistive load or only with a motor?

In my case, the output pwm has about 130ns jitter with or without motor connected. 

Brian

An off topic question: how do I find my old threads? I had tried under my Activities and Questions but it only shows posts up to 7 months old. 

Brian,

The Jitter may differ from part to part due to clock frequency variations from part to part.

Brian Dang said:

An off topic question: how do I find my old threads? I had tried under my Activities and Questions but it only shows posts up to 7 months old.

Go to your profile and click on "View all of Brian Dang's posts" and you can browse through your post history since you started. Congratulations on becoming Genius rank by the way. 

Regards,

Pablo Armet

Hi Pablo,

I have re-checked without any load attached, I'm still getting similar jitter readings ~347ns. So it would seem unlikely that PWM current regulation is being applied.

Also note that I have a zero ohm resistor in the ISEN path for this channel

Hi Samuel,

I'm not sure how PWM regulation is related to the jitter. They are independent of each other. Have you tried using a different DRV to check if the jitter is the same?

Regards,

Pablo Armet

Pablo, thanks for your rapid reply!

You previously asked me if current regulation was being applied:

Pablo Armet said:

Is the driver in current regulation?

This IC employs PWM current (chopping) regulation (datasheet s7.4.2) based on an external resistor as connected to the IC's ISEN pin(s). It appears that in my case this is unlikely given that I have repeated the result with an open circuit load.

However you now mention:

Pablo Armet said:

I'm not sure how PWM regulation is related to the jitter. They are independent of each other.

Noting that you previously stated:

Pablo Armet said:

When the device is not in current regulation mode, the output will follow the input PWM signal and is not affected by the digital clock frequency

Seems to imply to me that there could be a link between the IC's PWM current regulation mode and jitter.

Anyway, in response to trying different DRV ICs, I have confirmed the same issue on a number of different drivers ICs / and channels. Similar jitter on all.

Hi Samuel,

Apologies for the confusion. I'm supporting other threads and often times mix and confuse them.

Anyways,

Samuel Bolton said:

I'm not sure how PWM regulation is related to the jitter. They are independent of each other.

Please disregard this comment.

Samuel Bolton said:

Anyway, in response to trying different DRV ICs, I have confirmed the same issue on a number of different drivers ICs / and channels. Similar jitter on all.

Do you measure a great difference in the jitter from device to device? Some jitter is expected at the outputs due to clock frequency variations. But is the jitter you are measuring a point of concern for you? 

Regards,

Pablo Armet

Ok, thanks for clarifying.

No, I’m not measuring a great deal of jittter variance device to device. Without applying any proper statistical analysis, they seem to be in the ‘same ballpark’.

The good news is that this level of jitter is not currently adversely affecting my systems’ performance.

I just want to know if this level of jitter could be classified as ‘normal’ for this IC? It seems that this is the case for other TI DRV series IC’s such as the DRV8872 and possibly others.

Samuel Bolton said:

The good news is that this level of jitter is not currently adversely affecting my systems’ performance.

For my application of driving cine lenses, the jittering pwm output causing an audible noise and the audio recorder can pick it up. 

Brian,

This is concerning. How were you able to resolve the audible noise? 

No, not with the DRV8701's jitter. We have to use other drivers that have no jittering output for the next revision design.

Brian