op amp buffer Vo not right

Discussion in 'General Electronics Chat' started by Gibson486, Dec 5, 2013.

  1. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    I have an op amp buffer with an MCP601. The issue I am having is that the output is off by 200mV. The input on the positive side is simply from a divider that downscales the input by 9/10.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Need a circuit diagram.
     
  3. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    pic attached
     
  4. LvW

    Active Member

    Jun 13, 2013
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    Are yu sure that the "offset" is not caused by resistor tolerances?
    What means "off"? With respect to which expected voltage?
     
  5. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    When I say off, I mean the op amp is getting 4.4V at the input, but the output is 4.2V
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    That could be op amp itself. Check the datasheet. See how close you can get to the rail. Your rail is 5 volts, but real physical op amps never reach their rail.

    For example 741 op amp, I think, can only get withing 2 volts of the rail. So if you set a rail at 5 volts, the maximum output will be 3 volts, you will never get any higher. This is for 741.

    What op amp are you using? Put the datasheet up.
     
  7. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    It's a rail to rail op amp and even so, the input is 4.2V, so I am far from teh rail anyways....
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    The opamp says it is rail to rail, so that should not be the issue. Are you sure you have it wired correctly?
     
  9. shteii01

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    Feb 19, 2010
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    You don't understand. I am looking at Figure 4-3. it says:
    VOUT must be kept below VDD–1.2V for correct operation.



    Your VDD is 5 volts. The maximum VOUT=5-1.2=3.8 volts, that is max, you actually want to stay slightly below that.

    If you can raise the supply (VDD) to 6 volts, then you will be in range where 4.4 volts works.
     
  10. shteii01

    AAC Fanatic!

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    I see 5 volts supply and ground. That is single supply configuration. His rails are 0 and 5 volts.

    What rail to rail you are talking about?
     
  11. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    You made a small mistake the MCP601 is rail to rail output not the input one. The Common Mode Input voltage range is equal Vdd - 1.2V = 5V - 1.2 = 3.8V.
    Output voltage can reach Vdd - 20mV because MCP601 is a rail to rail output opamp.
     
  12. Gibson486

    Thread Starter Member

    Jul 20, 2012
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    Awww man...that sucks...I could have sworn I used this same design for a 0-5V buffer before without an issue. I could have sworn it worked in the past....know any op amps that can do what I want it to do? Is this common of all opamps that are rail to rail?
     
  13. kubeek

    AAC Fanatic!

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    Oops missed that, the output is rail to rail but the inputs are not.
     
  14. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Well, according to Jonny130 I am making a mistake somewhere.

    So far we know that when you put in 4.4 volts, you get 4.2 volts out.

    Try the formula I listed above. Put in 3 volts, see if you get 3 volts out.
     
  15. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Try using TS912
     
  16. Jony130

    AAC Fanatic!

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    You simply mixed up Common Mode Input Range with Maximum Output Voltage Swing.
     
  17. BobTPH

    Active Member

    Jun 5, 2013
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    Most of the MCP opamps are rail-to-rail input and output. You just happened to choose one of the few that does not.

    Bob
     
  18. crutschow

    Expert

    Mar 14, 2008
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    True. But for a follower connection as this is, the output can go no higher than the input maximum common-mode range.

    As a work-around you could configure the op amp with an inverting gain of say 2, and double the input attenuation. That will allow a full-scale output of 5V with the same overall gain.
     
  19. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    We don't even need to do that. The opamp is rated for 6 volt supply. So power it with 6 volts instead of 5 volts.
     
    Last edited: Dec 5, 2013
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