Not sure what a fake ground..

Discussion in 'Homework Help' started by woon_h88, Apr 9, 2014.

  1. woon_h88

    Thread Starter Active Member

    Mar 25, 2009
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    hi,
    Currently im trying to make an addition circuit with allow me to use only +15V inside of + - 15V. I using the circuit as shown on the SSM2018T datasheet as shown below..

    [​IMG]

    I compare both OP176 and OP284 datasheet and decided to use the circuit in SSM2018T datasheet but OP284 was used instead of OP176. Today i went to test on the OP284 circuit, The output wave was a DC with 7.5V ( till to the Voltage divider from the 2 100K resister..) Then i connect the output to pin 13 as shown on the above picture. Originally, SSM2018T required a +-15V, if not the ic will start to get very hot because its cant sweep on negative part/positive part.after connecting pin 16 to gnd. When i switch on the power, i touch the IC to make sure that its not getting too hot..After a few sec, the IC didnt heat up so i think that its ok to continue my test. I pump in a 300Hz, amp of 0.05 to pin 6...I check on the output of SSM2018T (pin14) i get a DC line with a few MV (sorry forgot the value)..and when i try to set some value (using labview) to the Vcontrol. The output remain the same.. At 1st i tot that the op284 will some how create a negative prt by offset the DC to the middle of +15V (7.5v)..So positive swing from 7.5V to 15V and negative swing from 7.5V to 0V..But after seeing the output of SSM2018T, i got lost and not sure what to do next..Is it that the way of my thinking is wrong, or the circuit shdn't be design this way? Sorry for all the top wordy...
     
    Last edited: Apr 9, 2014
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi 88,
    A fake ground is usually referred to a 'virtual ground'.

    You can create a virtual ground by using a OPA and if required transistor buffers, this will convert a single supply of say 30V into +/-15V, with the VG as the reference common.

    The OP176 in your circuit is doing just that
    E

    EDIT:
    Would you please post a marked image of the original posted image showing how you have connected the 15V supply, your description is not clear.
     
    Last edited: Apr 9, 2014
  3. woon_h88

    Thread Starter Active Member

    Mar 25, 2009
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    hi ericgibbs,
    But is it possible to use OP284 instead of OP176?
     
  4. ericgibbs

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    Jan 29, 2010
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    Analog Devices say that the OP176 is obsolete and the a OP184 is a replacement, so it should be OK. The OP284 is just a dual version of the OP184.

    Show us how you have connected the 15V supply line.

    E
     
  5. woon_h88

    Thread Starter Active Member

    Mar 25, 2009
    46
    2
    hi,

    [​IMG]

    This is how i connect..Tested but some how the output is just 1 line..
     
  6. ericgibbs

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    hi,
    Is the pin #14, output at +7.5V, ie half the supply,?
     
  7. woon_h88

    Thread Starter Active Member

    Mar 25, 2009
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    hi,

    Nope...Its show abt 400+ mV..by right when i put a sine wave to pin 6, with a Vc of 0 (gain 1), i shd get a output sine wave with almost the same as input...But the output of OP284 have 7.5V...
     
    Last edited: Apr 9, 2014
  8. ericgibbs

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    Jan 29, 2010
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    OK,
    Does the output of the OPA184 measure +7.5V with respect to the Common 0V.??

    Your sine wave when we get it working should be sitting on an offset voltage of +7.5V
     
  9. woon_h88

    Thread Starter Active Member

    Mar 25, 2009
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    Yup...have 7.5V...the scope show a 7.5 Vrms..but not sure why the output of the SSM2018T didnt get a sine wave..i tried back by removing OP284 and replace pin 16 to -15 and pin 13 to gnd..The output was correct...
     
  10. ericgibbs

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    Jan 29, 2010
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    OK,
    Where are you referencing the Control voltage to on your circuit.?

    EDIT:

    It should be between pins 11 and 13

    If you are referencing with respect to the -V of your power supply, you will be applying -7.5V as a Control voltage
    The Control voltage has to use the virtual ground as its reference.
     
    Last edited: Apr 9, 2014
  11. woon_h88

    Thread Starter Active Member

    Mar 25, 2009
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    Its on pin 11, pin 11 will connect to a device call NI ELVIS, and can be control by a software call Labview..
     
  12. ericgibbs

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    Jan 29, 2010
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    If you are referencing with respect to the -V of your power supply, you will be applying -7.5V as a Control voltage
    The Control voltage has to use the virtual ground as its reference.

    Use a DVM and measure the voltage between Control pin#11 the Virtual Ground, what is it.?
     
    Last edited: Apr 9, 2014
  13. woon_h88

    Thread Starter Active Member

    Mar 25, 2009
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    is a positive 7.5V..Also the circuit control range is cap at a range of -1.3V to 3V..anything above will have lot of noise and distortion..
     
  14. ericgibbs

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    You should not have a voltage between Control pin#11 and the Virtual Ground [the output of the OP184] of +7.5V.

    The Control Voltage range as you say is -1.3V thru +3V, but you have +7.5V so the SSM is driven into saturation...
     
  15. woon_h88

    Thread Starter Active Member

    Mar 25, 2009
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    o...u mean measure pin 11 with pin 13 for the voltage...didnt try that when im in the lab..will try that tomorrow when i got myself a space..also must all the ground at the SSM be connected to the Virtual ground?
     
  16. ericgibbs

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    Hi
    When you create a Vgnd dual supply from a single voltage source, you should not use the single source voltage 0V as a Common for any other connected circuits.

    Use the VGnd as the Common for external circuitry.
    Be aware that the OP148 will have limitations on how much current it can supply as a VGnd. +/-V supply.

    If you post the SSM image with all the external connected circuits shown, we can talk you thru it.

    E
     
    Last edited: Apr 9, 2014
  17. woon_h88

    Thread Starter Active Member

    Mar 25, 2009
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    hi...This the actual circuitwith + - 15V..

    [​IMG]

    This the circuit i try..15V only

    [​IMG]

    I try measure the voltage..its give me a 7.5V...I try to change the IC but its seem the same..And the output voltage give dc line with 7.6mV...And the VCA output 10uF is to connect to other circuit which do not need -15V..forgot to remove the Capacitor...Sorry~
     
    Last edited: Apr 10, 2014
  18. ericgibbs

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    Where is the DAQ 0v/Common connected to on SSM.??
     
    Last edited: Apr 10, 2014
  19. woon_h88

    Thread Starter Active Member

    Mar 25, 2009
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    DAQ go to a device call NI elvis which can adjust voltage and 0/gnd connect together with OP284 gnd..
     
    Last edited: Apr 10, 2014
  20. ericgibbs

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    Jan 29, 2010
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    hi,


    Look at the image I posted as an edit in my last post.

    It shows how the PSU must be connected, do not use the same PSU for the SSM and the DAQ.
     
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