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Issue with Comparator circuit

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alok1982

Member
Hi,


I made a comparator circuit in which according to different input resistance I am giving indication using LED.


While testing I am facing one problem;

Whenever I bring my hand near to IC LT1707 the LED automatically starts to glow.

Even when I touch multimeter probe to pin 2 and pin 5, the LED becomes bright otherwise it glows less.


How to solve this issue.

Attached circuit is for your reference.


Thanks!
 

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alec_t

Well-Known Member
Most Helpful Member
With such high value resistors on the opamp inputs the circuit is sensitive to interference. Your body acts as an antenna for picking up interference which can be coupled capacitively to the opamp inputs.
Is the circuit inside a screened enclosure? If not, it should be.
 

alok1982

Member
Hi Alec,
Thanks for reply!
What type of enclosure I can use?

Can any grounding issue is there or can we modify ground to solve this?

Thanks!
 

alec_t

Well-Known Member
Most Helpful Member

atferrari

Well-Known Member
With such high value resistors on the opamp inputs the circuit is sensitive to interference. Your body acts as an antenna for picking up interference which can be coupled capacitively to the opamp inputs.
Is the circuit inside a screened enclosure? If not, it should be.
Are they justified?
 

alok1982

Member
What would be reason for, " by touching multimeter probe to pin 2 and pin 5, the LED becomes bright otherwise it glows less"?

I am still thinking any grounding issue.
 

ci139

Active Member
probe to pin 2 and pin 5
the comparator is !basically! an open loop op amp make your schematic first with these (inexpensive ones) see how it acts
the schematic to work fast takes biasing of near minimum allowed resistance values also minimum not to "distort" your input (by dc bias e.g. if your comparator input biasing/compensation network is 100 x MΩ range but your source is 1 x GΩ and the ΔVin is small it might have no NRG to drive the detector or to drive it properly)

if your input is high impedance you likely want to buffer(filter/alter) it for the same reason about impedance converters https://en.wikipedia.org/wiki/Buffer_amplifier to match your preferred biasing/compensation for comparator -- the drawback is you introduce an additional time delay and error-function to the input chain
◄ this is when you use op amp for comparator or to get the hint what's going on in biasing/feed-back network (as the difference of referencing your multimeter's COM to variety of potentials avail around your test circuit or modifying it's probes with series resistors or j-Fet input translator/buffer)

notice the "Problem 6.3 - Follower" + "Problem 6.7 - Even More Virtues of Feedback - I" & "Problem 6.8 - Even More Virtues of Feedback - II" in http://instrumentationlab.berkeley.edu/Lab6
 
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