How to short the opamp's inputs together

  • Thread starter Minh Thanh
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  • #1
Minh Thanh

Homework Statement


How to short the opamp 's inputs together. Below is schematic of circuit.



Homework Equations




The Attempt at a Solution


This is first time that I heard " Short Inputs " .It 's terrible to me .I dont know how to short it . Could you help me about this, ?. Please you illustrate by picture so that I can understand it clearly .
 

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Answers and Replies

  • #2
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I have no idea why you would want the short the inputs together. Opamps will need negative feedback for operation, and you can't get that if the inputs are shorted together. If you short the inputs, the output might be anything.
The picture seems incomplete. A piece of the circuit on the right is missing.
 
  • #3
CWatters
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This is first time that I heard " Short Inputs "

Are you asking what "short inputs" means? It means connect them together electrically (eg with a wire).

Do you mean..
a) short the OP Amp inputs
or
b) short the PH Probe Inputs?

Sometimes you short Inputs together temporarily to allow you to measure the DC offset at the output.

It would be best to post the whole circuit.
 
  • #4
berkeman
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Sometimes you short Inputs together temporarily to allow you to measure the DC offset at the output.
That would be my guess as well, but we'll have to wait for the OP to show up again to clarify his question...
 
  • #5
Minh Thanh
Are you asking what "short inputs" means? It means connect them together electrically (eg with a wire).

Do you mean..
a) short the OP Amp inputs
or
b) short the PH Probe Inputs?

Sometimes you short Inputs together temporarily to allow you to measure the DC offset at the output.

It would be best to post the whole circuit.
I mean is that short the OP Amp Inputs . However, I did it but I dont receive expected offset voltage is 0mV at LM351 ' output . LM351 ' output just display two value are -6.01 V and 8.40V . May you help me about this ?
 

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  • #6
berkeman
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I mean is that short the OP Amp Inputs . However, I did it but I dont receive expected offset voltage is 0mV at LM351 ' output . LM351 ' output just display two value are -6.01 V and 8.40V . May you help me about this ?
Sorry, but your posts are making no sense at all. Please answer the following questions to help us out:

1) How did you "short the inputs"? Is this in a simulation, or on a circuit board?

2) How did you get two different results when you shorted the inputs? Were the results from Monday and Wednesday? Or did the output oscillate between those two output values?

Please post much more information about your question and circuit. Please.
 
  • #7
Minh Thanh
Sorry, but your posts are making no sense at all. Please answer the following questions to help us out:

1) How did you "short the inputs"? Is this in a simulation, or on a circuit board?

2) How did you get two different results when you shorted the inputs? Were the results from Monday and Wednesday? Or did the output oscillate between those two output values?

Please post much more information about your question and circuit. Please.
Well , I am sorry. My answer is :
In circuit board , I want to calibrate the circuit so that DC offset can be 0mV .I connected to pin 2 and 3 together at Opamp' inputs . I took pH probe out from my circuit then I supplied power and adjusted potentiometer to hope LM351 ' output is 0mV. . However, The output didn't oscillate . It just displayed either -6 V or 8.40 even though I was trying to adjust really slowly potentiometer . These results was on Wednesday.
 
  • #8
NascentOxygen
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To adjust the offset, I think you leave the OP-AMP output open (i.e., not connected back to its input) and connect both inputs to ground (inputs need to get a tiny bit of current, so you can't leave them connected nowhere). With this arrangement, then adjust the offset pot while you monitor the OP-AMP's output.
 
  • #9
CWatters
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+1

If you just short them together and nothing else they can float up and down creating a common mode input signal. OP Amps will normally reject common mode inputs but only within a certain voltage range. Typically things go wrong if the inputs float up/down outside that range (eg to the supply rails). So short them together and to the 0V rail while setting the offset.
 
  • #11
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The requirement is to short out the input to the circuit, not the inputs to the op-amp. So you need to connect pin 3 to ground then adjust the offset pot, not connect pins 2 and 3 together.
 
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  • #12
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The requirement is to short out the input to the circuit, not the inputs to the op-amp. So you need to connect pin 3 to ground then adjust the offset pot, not connect pins 2 and 3 together.
Agreed. The other replies are for an op-amp alone. But in the circuit given, the op-amp output is tied to the inverting input (it's a buffer), so shorting the inputs together and to ground shorts the output to ground. Nothing to measure.

In this context, it certainly means to short the input probe connector (grounding the non-inverting input of the op-amp), and adjust the pot for zero offset. Ahh, a quick google came up with the full info (not sure why OP did not post this):

http://www.edn.com/design/test-and-measurement/4340776/Convert-your-DMM-to-a-pH-meter
http://m.eet.com/media/1141038/18702-figure_3.pdf
18702-figure_3.pdf
 

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