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Wavebourn 3rd August 2009 11:17 PM

Fast and precision current comparator
 
Dear fellow DIYers;

I am looking for a fast and precision current comparator, to compare currents +/-20mA, up to 1 MHz. What I am thinking about, to use a video opamp in inverting connection, then another one with open loop. However it will work, but may be you can suggest some better solutions?

I will share results of the project with the DIY community, when good high-end results will be obtained.

Anatoliy

jcx 3rd August 2009 11:42 PM

if you want digital output then you really should look at monolithic comparators which handle overdrive and have hysterisis to prevent oscillations - which are problems with op amps optimized for closed loop operation and continuous analog signal

Wavebourn 3rd August 2009 11:45 PM

Quote:

Originally posted by jcx
if you want digital output then you really should look at monolithic comparators which handle overdrive and have hysterisis to prevent oscillations - which are problems with op amps optimized for closed loop operation and continuous analog signal
I can add hysteresis in opamp, no problem. Do you mean oscillations through power supply, or through parasitic capacitances?

Which monolithic comparators would you recommend?

Bi-polar output will be more convenient in my application, however it is not absolutely necessary.




Wavebourn 4th August 2009 12:24 AM

What are weak points of such a solution?

20 mA input currents that I compare is not necessary, I mean, enough of current to deal with parasitic capacitances.

http://wavebourn.com/images/audio/comparator-1.gif

Elvee 4th August 2009 08:39 AM

2 Attachment(s)
Simply using a comparator will be faster, simpler, and more accurate:

diy_audio_fo 4th August 2009 11:34 AM

Quote:

Originally posted by Wavebourn
What are weak points of such a solution?

20 mA input currents that I compare is not necessary, I mean, enough of current to deal with parasitic capacitances.

http://wavebourn.com/images/audio/comparator-1.gif

If I correctly understand your schematic, the main (not the only) weak pont is that the A1a HA5023 under certain condition (ie Current 1=20mA, Current 2=0mA) saturates suffering for delay to recover from saturation.

There is a paper from Robert Pease about the use of an OpAMP as precison voltage comparator
http://europe.elecdesign.com/Article...9973/9973.html

Two questions:

1) If I understand your goal your circuit doesn't compare if Current 1 > Current 2 but if Current 1 > - (Current 2).
Is it correct?

2) Which is the purpose of the bridge + diode connected transistor on the second OpAmp output?

jan.didden 4th August 2009 12:13 PM

Anatoliy,

Is this for a Blomley circuit?

jd

Wavebourn 4th August 2009 04:30 PM

1. Who is Blomley?

2. As I said, it is just an idea. However, no way I can allow the video opamp to saturate. Couple of fast diodes on input, for example, would prevent this, but I prefer linear input resistance close to zero, so feedback resistor would be smaller, let it be 240 Ohm (for 20 mA+20 mA = 40 MA, +/1 8.6V out).


3. The purpose of a bridge and a Zener is to stabilize threshold of hysteresis. However, it should be more sensitive, say R3 = 1 mOhm, need to be calculated against capacitances.

PMA 4th August 2009 05:15 PM

Blomley is Peter Blomley.

Wavebourn 4th August 2009 06:50 PM

Quote:

Originally posted by PMA
Blomley is Peter Blomley.
Thanks; but how he is related to my current comparator? Excuse me for my ignorance.


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