AVR-Chat

Hi everyone,

May get ideas on how to create clean signal or reduce overshoot/undershoot. I want to create signals with variable amplitude from 1-5V, frequency is up to 20MHz and can drive up to 200mA. I have a programmable clock generator (CDCE925 from TI)which is controlled by my AVR. The freq gen output is connected to a buffer and inverter driving 2 mosfet connected together. One to switch to VIH (variable amplitude) and another to switch to GND. When i used scope to check the output, i can see overshoot and undershoot which are sometimes as big as the signal amplitude. Aside from using schottky diode, any ideas how to make reduce overshoot? or any ideas how to buffer high freq signals? 

Regards,
mago

Mago,
You need to match your output driver to the impedance of the load.

Regards,


Tim Gilbert
JEM Innovation Inc.
303-926-9053 (office)
303-437-4342 (cell)
720-890-8582 (fax)
www.jeminnovation.com
www.pdksolutions.com

  ----- Original Message ----- 
  From: magzky02 
  To: AVR-Chat@yahoogroups.com 
  Sent: Wednesday, December 30, 2009 6:45 PM
  Subject: [AVR-Chat] reducing signal overshoot/undershoot


    
  Hi everyone,

  May get ideas on how to create clean signal or reduce overshoot/undershoot. I want to create signals with variable amplitude from 1-5V, frequency is up to 20MHz and can drive up to 200mA. I have a programmable clock generator (CDCE925 from TI)which is controlled by my AVR. The freq gen output is connected to a buffer and inverter driving 2 mosfet connected together. One to switch to VIH (variable amplitude) and another to switch to GND. When i used scope to check the output, i can see overshoot and undershoot which are sometimes as big as the signal amplitude. Aside from using schottky diode, any ideas how to make reduce overshoot? or any ideas how to buffer high freq signals? 

  Regards,
  mago



  

[Non-text portions of this message have been removed]

On Wed, Dec 30, 2009 at 6:53 PM, tim gilbert <tim@jeminnovation.com> wrote:
> Mago,
> You need to match your output driver to the impedance of the load.
>

Yup.  And maybe reduce drive current.

The over and undershoot is caused by reflections.

OK, i'll do research on how to do that driver-load impedance matching.
 
Thanks

--- On Thu, 12/31/09, David VanHorn <microbrix@gmail.com> wrote:

From: David VanHorn <microbrix@gmail.com>
Subject: Re: [AVR-Chat] reducing signal overshoot/undershoot
To: AVR-Chat@yahoogroups.com
Date: Thursday, December 31, 2009, 10:20 AM


  



On Wed, Dec 30, 2009 at 6:53 PM, tim gilbert <tim@jeminnovation. com> wrote:
> Mago,
> You need to match your output driver to the impedance of the load.
>

Yup. And maybe reduce drive current.

The over and undershoot is caused by reflections.








      

[Non-text portions of this message have been removed]

Overshoot can also be caused by mismatched rise and fall times in the  
drivers.

You can also have capacitive coupling from the gate to the source and  
if the Vcc level of the top transistor is smaller than the driving  
signal, this can be quite serious.

I suggest that you look at commercial driver ICs. One problem is that  
most do not allow the input logic level to exceed the driver's Vcc.

Jim

On Dec 30, 2009, at 7:28 PM, mago Umandam wrote:

> OK, i'll do research on how to do that driver-load impedance matching.
>
> Thanks
>
> --- On Thu, 12/31/09, David VanHorn <microbrix@gmail.com> wrote:
>
> From: David VanHorn <microbrix@gmail.com>
> Subject: Re: [AVR-Chat] reducing signal overshoot/undershoot
> To: AVR-Chat@yahoogroupscom
> Date: Thursday, December 31, 2009, 10:20 AM
>
>
>
> On Wed, Dec 30, 2009 at 6:53 PM, tim gilbert <tim@jeminnovation.  
> com> wrote:
> > Mago,
> > You need to match your output driver to the impedance of the load.
> >
>
> Yup. And maybe reduce drive current.
>
> The over and undershoot is caused by reflections.
>
> [Non-text portions of this message have been removed]
>
>
> 



[Non-text portions of this message have been removed]

----- Original Message ----- 

From: "mago Umandam" <magzky02@yahoo.com>
To: <AVR-Chat@yahoogroups.com>
Sent: Thursday, December 31, 2009 3:28 AM
Subject: Re: [AVR-Chat] reducing signal overshoot/undershoot


OK, i'll do research on how to do that driver-load impedance matching.


Poor PCB design can cause that sort of problem, as well.

Leon

>When i used scope to check the output, i can see overshoot and undershoot
which are sometimes as big as the signal amplitude.

When was the last time you compensated your scope probes?  Do you have a
proper scope ground, ie. not a pigtail ground clip?

See Linear Tech. AN92 for one example:
http://cds.linear.com/docs/Application%20Note/an92f.pdf

Analog Devices has some good information in their seminar series of PDFs as
well,
on proper measurement techniques.


-- 
http://www.wearablesmartsensors.com/
http://www.softwaresafety.net/
http://www.designer-iii.com/
http://www.unusualresearch.com/


[Non-text portions of this message have been removed]

>>When i used scope to check the output, i can see overshoot and undershoot
> which are sometimes as big as the signal amplitude.
>
> When was the last time you compensated your scope probes?  Do you have a
> proper scope ground, ie. not a pigtail ground clip?
>
> See Linear Tech. AN92 for one example:
> http://cds.linear.com/docs/Application%20Note/an92f.pdf
>
> Analog Devices has some good information in their seminar series of PDFs
> as
> well,
> on proper measurement techniques.
>
>
> --
> http://www.wearablesmartsensors.com/
> http://www.softwaresafety.net/
> http://www.designer-iii.com/
> http://www.unusualresearch.com/
>
>
> [Non-text portions of this message have been removed]
>
>
I highly recommend the following Linear Tech application notes:

Linear Circuits for Digital Systems:
http://cds.linear.com/docs/Application%20Note/an31.pdf

High Speed Amplifier Techniques (very highly recommended)
http://cds.linear.com/docs/Application%20Note/an47fa.pdf

Jim Wagner
Oregon Research Electronics

Thanks for all the advices. I appreciate them especially the very helpful link from Jim & Bob.

--- On Fri, 1/1/10, wagnerj@proaxis.com <wagnerj@proaxis.com> wrote:

From: wagnerj@proaxis.com <wagnerj@proaxis.com>
Subject: Re: [AVR-Chat] reducing signal overshoot/undershoot
To: AVR-Chat@yahoogroups.com
Date: Friday, January 1, 2010, 1:18 AM


  



>>When i used scope to check the output, i can see overshoot and undershoot
> which are sometimes as big as the signal amplitude.
>
> When was the last time you compensated your scope probes? Do you have a
> proper scope ground, ie. not a pigtail ground clip?
>
> See Linear Tech. AN92 for one example:
> http://cds.linear. com/docs/ Application% 20Note/an92f. pdf
>
> Analog Devices has some good information in their seminar series of PDFs
> as
> well,
> on proper measurement techniques.
>
>
> --
> http://www.wearable smartsensors. com/
> http://www.software safety.net/
> http://www.designer -iii.com/
> http://www.unusualr esearch.com/
>
>
> [Non-text portions of this message have been removed]
>
>
I highly recommend the following Linear Tech application notes:

Linear Circuits for Digital Systems:
http://cds.linear. com/docs/ Application% 20Note/an31. pdf

High Speed Amplifier Techniques (very highly recommended)
http://cds.linear. com/docs/ Application% 20Note/an47fa. pdf

Jim Wagner
Oregon Research Electronics









      

[Non-text portions of this message have been removed]