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Old 3rd August 2009, 11:17 PM   #1
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Question 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
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Old 3rd August 2009, 11:42 PM   #2
jcx is offline jcx  United States
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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
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Old 3rd August 2009, 11:45 PM   #3
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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.



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Old 4th August 2009, 12:24 AM   #4
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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.

Click the image to open in full size.
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Old 4th August 2009, 08:39 AM   #5
Elvee is offline Elvee  Belgium
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Simply using a comparator will be faster, simpler, and more accurate:
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Old 4th August 2009, 11:34 AM   #6
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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.

Click the image to open in full size.
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?
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Old 4th August 2009, 12:13 PM   #7
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Anatoliy,

Is this for a Blomley circuit?

jd
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Old 4th August 2009, 04:30 PM   #8
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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.
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Old 4th August 2009, 05:15 PM   #9
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Blomley is Peter Blomley.
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Old 4th August 2009, 06:50 PM   #10
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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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