Re: Resistor pulse handling?

Chris Carlen wrote:
> Mike Monett wrote:
>> Chris Carlen wrote:
>>> Greetings:
>>>
>>> I wish to sample a raw 75V 125kHz H-bridge output. The two H
>>> outputs will go to a differential amplifer/attenuator to create a
>>> single ended, low-level sample of the output voltage. Trouble is,
>>> I don't want the raw signals running along my PCB from the high
>>> powered department to the low level zone and radiating spikes all
>>> over the place. Thus, I'd like to slow the edges just a bit before
>>> routing them long distances.
>>
>> [...]
>>
>>> Comments appreciated.
>>>
>>> Good day!
>>>
>>> Christopher R. Carlen
>>> Principal Laser/Optical Technologist
>>> Sandia National Laboratories CA USA
>>> crcarleRemoveThis@xxxxxxxxxxxxxxx
>>> NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to
>>> reply.
>>
>> Chris,
>>
>> Why not use a passive divider to get down to 50 ohms, and skip the
>> diff amp? You might miss some important stuff with a low-pass filter.
>>
>> This allows you to look at either output individually to see if one
>> side has a problem such as distortion, oscillation, risetime, etc. You
>> can connect both outputs to the scope and use the diff amp in
>> the scope and add them together.
>
> Perhaps I should have explained more. I agree with you that a 10k:50
> ohm divider would provide a nice diagnostic port. But the purpose of
> the diff amp is specifically differential to single ended conversion
> in order to implement a voltage mode control loop around the PWM
> amplifier H bridge.
>
>> A 10k resistor into 50 ohms will give plenty of bandwidth to see
>> oscillations well into the VHF region. With 50% duty cycle, you
>> would only need 1/2 watt.
>
> Indeed.
>
>> I find every time I throw information away, it turns out to be just
>> the piece I need to solve a problem:)
>
>
>
> Thanks for the input Mike.
>
> Good day!

I would recommend the THS4502 or AD8132 true differential amplifiers. I used
one in a similar application and they are fast enough to follow the rise and
fall times of a H-Bridge.
The inverted output can come very handy for delta-sigma or integrator stages
of higher order or any other analog signal processing. The differential
output can also drive a high-speed ADC directly.
--
ciao Ban
Bordighera, Italy


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