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Hot!Output of MCP6282 is saturated

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iqram
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2020/03/28 10:55:51 (permalink)
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Output of MCP6282 is saturated

Hi,
 
I am using MCP6282 op-amp to amplify Hall Sensor output from BLDC motor. The output of Hall Sensor is square wave swing of 0 to 500mV when Motor is running. I thought of using MCP6282 in non-inverting configuration with gain of 1000 and wanted to make output of MCP6282 to toggle  from 0 to 5V.
 
I have added pull-up on Hall Sensor output (Input to MCP6282). Now input of Op-Amp is also 0 to 5V square wave. I observed on Oscilloscope that input is properly toggling but output of Op-Amp is staying at 5V. I am assuming that Op-Amp is saturated and not toggling.
 
Please help me in solving this. How can I toggle MCP6282's output in this situation?
 
I have attached the snapshot. Pins 1 and 7 remain at 5V while pins 1 and 3 are toggling.
 
Thanks.!
Kind Regards,
Iqram

Attached Image(s)

#1

12 Replies Related Threads

    crosland
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    Re: Output of MCP6282 is saturated 2020/03/28 11:17:43 (permalink)
    5 (2)
    DO you really mean a gain of 1000?
     
    Have you done the maths?
     
    What will a gain of 1000 do to a signal of 500mV?
     
    What will a gain of 1000 do to a few mV of noise?
    #2
    iqram
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    Re: Output of MCP6282 is saturated 2020/03/28 11:27:24 (permalink)
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    Hi crosland,

    Thank you for your reply.

    You are right, I did not think of noise here. I only was looking for getting 0 to 5V of square wave.

    The feedback resistor is 1M. The gain formula is 1+(Rf/R2). So here gain is 1+(1M/1K) = 1001.

    Thanks,
    Kind Regards,
    Iqram
    #3
    Bob White
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    Re: Output of MCP6282 is saturated 2020/03/28 11:43:12 (permalink)
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    You completely missed crosland's main point.
     
    With an input of 500 mV and a gain of 1000 (or 1001, does not really matter) what would be the expected output voltage?  Do the math.
     
    #4
    iqram
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    Re: Output of MCP6282 is saturated 2020/03/28 11:43:17 (permalink)
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    Hi crosland,

    Now I understood your point.

    You are saying that, even 5mV of noise will get amplified. In my case, if output of Hall sansor is from 50mV to 500mV, I will see output of op-amp always at 5V.

    Thank you for pointing this. I will bypass op-amp for now and will use pull-up on Hall sensor output.

    Thank you,
    Kind Regards,
    Iqram
    #5
    Bob White
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    Re: Output of MCP6282 is saturated 2020/03/28 11:47:10 (permalink)
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    No, you do not understand crosland's point.
     
    With an input of 500 mV and a gain of 1000, what would the expected output voltage be?  Answer that question, please, and we will go from there.
    #6
    iqram
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    Re: Output of MCP6282 is saturated 2020/03/28 11:48:50 (permalink)
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    Hi Bob,

    I wanted to saturate output of Op-Amp when there is a 500mV input to Op-Amp. So, if I use high gain (in my case 1000), I should get 5V for 500mV input. The 0V input should give me 0V.

    Since I completely ignored noise, as crosland pointed out, noise will also be amplified with gain 1000. This can be the reason where both low and high cycle of Hall sensor output is amplified and I am getting continuous 5V output from Op-Amp.

    Thanks,
    Kind Regards,
    Iqram
    #7
    iqram
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    Re: Output of MCP6282 is saturated 2020/03/28 11:51:23 (permalink)
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    500mV*1000 = 500V.
    #8
    Bob White
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    Re: Output of MCP6282 is saturated 2020/03/28 11:52:32 (permalink)
    0
    Stop!
     
    500 mV × 1000 V/V = 0.5 V × 1000 V/V = 500 V!
     
    Why did you miss that basic point?
    #9
    iqram
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    Re: Output of MCP6282 is saturated 2020/03/28 11:57:16 (permalink)
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    Hi Bob,

    You are right. That is my mistake here. I should use gain of 10 to make it 5V.

    I was thinking that, the higher gain will anyway saturate it to 5V. So in case any other motor whose hall sensor output is less than this will also get amplified to 5V.

    Thanks,
    Kind Regards,
    Iqram
    #10
    Bob White
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    Re: Output of MCP6282 is saturated 2020/03/28 11:59:27 (permalink)
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    OK, glad you now seem to be on a good track....
    #11
    iqram
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    Re: Output of MCP6282 is saturated 2020/03/28 12:13:07 (permalink)
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    Thank you Bob and crosland for your quick comments and suggestions.

    I will update you after changing my gain to 10.

    Thank you,
    Kind Regards,
    Iqram
    #12
    drh
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    Re: Output of MCP6282 is saturated 2020/03/29 06:32:14 (permalink)
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    LoL: LoL

    David
    #13
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