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Op amp integrator with reset circuit ...

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Chat Zone » Archive through 18 October, 2015 » Op amp integrator with reset circuit - not working properly « Previous Next »

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atferrari
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Username: atferrari

Post Number: 1688
Registered: 05-2005


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Posted on Sunday, 30 August, 2015 - 02:27 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

LTSpice

Simulation of integrator with reset-to-zero circuit for 2 Vin ramps (-V to +V and viceversa). The "chopping" signal (Vctrl) is used to verify the output at different ranges of Vin.

For Vin values above 0V, the output gets hard limited (?!). The reason obviously resides on the MOSFET whose source sits on the summing point (always at 0V).

A sanity check shows the integrator itself, (no reset circuit), working OK.

For anyone interested I attached the .asc file plus the subckt file for the NMOS 2N7000 (also used the BS170 with similar results).

I stripped out any .INCLUDE or .LIB directive since I am using now full paths to my bag of models and can forget copying models / subckts here and there every time.

(Please delete those .txt extensions)





text/plain
Integrator 07 with windup reset by NMOS no path.asc.txt (5.9 k)


text/plain
2N7000.sub.txt (1.7 k)

Agustín Tomás - Buenos Aires - Argentina
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zeitghost
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Username: zeitghost

Post Number: 1979
Registered: 01-2006

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Posted on Sunday, 30 August, 2015 - 03:44 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Don't forget there's a body diode in the 2n7000.

This may screw things up if you turn it on.

A normal jfet wouldn't have that problem.

Alternatively I was going to suggest that you could use two 2n7000 with the sources & gates connected together, with one drain going to the virtual earth & the other going to the opamp output.

But on reflection it won't work.
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atferrari
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Username: atferrari

Post Number: 1689
Registered: 05-2005


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Posted on Sunday, 30 August, 2015 - 04:17 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

YES!!! A JFET did it!

Gracias zeit!


Agustín Tomás - Buenos Aires - Argentina
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atferrari
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Username: atferrari

Post Number: 1690
Registered: 05-2005


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Posted on Sunday, 30 August, 2015 - 07:37 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hola zeit

I am conscious of the hard limit imposed by the limited voltage (-7V) to cut the J310 completely.

My circuit has just +/- 9V and as per LTSpice, I need around -12V.
Agustín Tomás - Buenos Aires - Argentina
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james
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Username: james

Post Number: 551
Registered: 02-2007

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Posted on Sunday, 30 August, 2015 - 10:19 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Agustin,

Is this any good to you?... a -9V to -12V converter. I've not built it but it simulates ok in Circuit Wizard.

-9V to -12V CONVERTER


Cheers

James
Enjoy the little things in life for some day you will realise that they were the big things.
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atferrari
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Username: atferrari

Post Number: 1691
Registered: 05-2005


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Posted on Monday, 31 August, 2015 - 01:02 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hola James!

Thanks for the concrete idea.

After posting the second time, I started to think your way but then I realized that I should accept my coinstraints and scale down everything to fit within the saturation limits. (There is life under 5V, after all...!) (Too much inlfuenced by digital myself, I guess).

Op amps and components seems to procreate quickly in my project.

For the moment, after some preliminary tests it seems I could make it without resorting to an extra voltage.

Tomorrow, if my workload allows, I will test the real thing on a breadboard.

Gracias again.
Agustín Tomás - Buenos Aires - Argentina
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zeitghost
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Username: zeitghost

Post Number: 1980
Registered: 01-2006

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Posted on Tuesday, 01 September, 2015 - 09:23 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I'd use an analogue switch instead.

Simpler to drive since it's got all the drive circuitry for the switch integrated inside it.
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atferrari
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Username: atferrari

Post Number: 1692
Registered: 05-2005


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Posted on Saturday, 05 September, 2015 - 08:06 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hola zeit,

I also considered one (CD4066) but such a big thing there just for one switch...!

Finally I managed (in LTSpice at least) to do it right by scaling things down.

Soldering this part tonight or eventually tomorrow. Luckily, there was a free op amp in my last quad.

Gracias for replying.
Agustín Tomás - Buenos Aires - Argentina
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cjay
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Username: cjay

Post Number: 247
Registered: 10-2013

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Posted on Tuesday, 08 September, 2015 - 08:11 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You can get 'single' analogue switches, the tiny SOT device I replaced is one example and there are many SPST/SPDT devices in larger packages, there are also the OP-AMPs with disable pins, the AD810 as used in the EPE vido cleaner project for instance

http://www.epemag.com/lib/free_projects/picmicro/0200%20-%20PIC%20Video%20Cleaner.pdf
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atferrari
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Username: atferrari

Post Number: 1693
Registered: 05-2005


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Posted on Tuesday, 08 September, 2015 - 10:22 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hola Clint,

Right now, I am starting a new thread because I solved my problem (yes, with the J310) and moved forward. If you like to take a look there as well, I will appreciate that.

Gracias for replying.
Agustín Tomás - Buenos Aires - Argentina

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