problem recreating differentiator effect

yefj

New Member
Hello , I have built a basic PID circuit as shown below.
I put a pulse as input.
The integrator works good it converts the pulse into a ramp . but the differentiator is supoosed to convert the pulse into two spikes.
Why its not happening?
Ltspice file is attached.
Thanks.


 

Attachments

  • PID_section_united_T.zip
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Hello Dana,Its a very good point the vitual ground.
regarding slew rate from the formula I got my pulse slew rate much much lower then datasheet.
Two questions:
1.Where did I go wrong measuring slew rate from the plots?
2.Is there some good filter menu you reccomend where I can filter OPAMP by their properties?
Like BW slew rate power supplies.the menus in analog devices are very non informative and I need to open each datasheet to see the properties I need.
SR=27mV/19.7ns=1.37V/microsecond


 

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  • PID_section_united_T.asc
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Take a look at this discussion -



Basically SR is a large signal issue.

As far as better product selection spreadsheets usually its the vendors themselves, not
the distributor tools, that have better selection guides. You can always ask specific vendors
if they have a more extensive selection tool......
 
yefj, keep in mind all the sims forum does depend on models for the parts,
and their completeness. So sim results are not the end all arbiter of what a
circuit will do. Some models bare bones, many for a long time purposely
incomplete as they could potentially reveal a vendors proprietary processes
and design methods. But with concentrated fab houses, with essentially open
design tools, its gotten better, but not the last word. The last word is bench
testing. Years ago one would take many parts from different process runs and
test. That largely has come and gone.

Use them as a guideline, and recognize even bench work introduces parasitics,
compromises. And use generous margining for spec limits is not bad advice.

Regards, Dana.
 
Hello Dana, as you can see in the attached photo my slew rate .
I have 437mV/14ns=31V/usec which twice lower then 80 V/μs which described in the datasheet.
How Can I improve the slew rate performance in the circuit?
Thanks.
 

Attachments

  • 3.zip
    103.1 KB · Views: 7
I get the following, with a 1 nS time increment calculation for sim solver.



Note the passive response....

Otherwise start thinking about using a faster, greater BW OpAmp.
 
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Hello Dana , When I increase R2 I get bigger amplitude.
but when I increase C1 I get more ringing?
why C1 controlles ringing?

Thanks.

 
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What is output peak goal V for 100 mV square wave pulse in ?

What is your pulse width out goal for 100 mV square wave in ?

What is your goal for slew rate ?

At this point I would either and/or get on bench and experiment, and sim
various gains, R & C values.
 
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Thats the point I dont know what to expect on the output given the datasheet.
How do I inverpret the datasheet :
High speed80 MHz, −3 dB bandwidth (G = +1)
)80 V/μs slew rate (G = +2)

And see What is your pulse width out goal for 100 mV square wave in ?
What is your goal for slew rate ?
I dont have goal ,I just what to learn how to get the limitations of the outputs given the data sheet.
Thanks.
 
The datasheet gives you large signal slew rate, not small as your
current circuit is.

Datasheet doe not, usually, indicate the actual transition between
small signal response at low voltages and slew specs given. OpAmps
internal tolerance of current to slew its comp cap not given for all
possible use cases....over T and V and .......

No goals, specs typical in datasheets, no margining.....not sure where
you are headed. Constrained choice of components not fulfilling
your unknown goals.

If you are going to squeeze the last drop of performance you will have to
do so over several characterization runs of process of the 8034, lots of
testing.....

Good luck.
 
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Perform s plane analysis. Look at what poles and zeros are doing.
As they approach jw axis response becomes more oscillatory, and into
RHP become exponential.

 
yefj There is a direct relationship between % overshoot, damping ratio, (ζ) zeta, and Quality factor Q and ringing frequency fo? You may read at your leisure.\ or simulate and change values with mouse and reset.

Q is defined for series or parallel.
Qs = Reactive/Real impedance ratio or power

Thus raising C lowers the resonant frequency . In active integrator it has more gain at lower f then the ringing amplitude can increase. But for passive systems ringing just changes f if L is fixed.
 
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