EMG design using instrumentation amplifier (INA118)

Discussion in 'The Projects Forum' started by Vanush, May 4, 2012.

Thread Status:
Not open for further replies.
  1. Vanush

    Thread Starter Active Member

    Apr 19, 2008
    46
    0
    Hello,

    I'm trying to design the input stage for an EMG amplifier. I'm using an INA118 with a split supply (5 V for V+ and 2.5 V for V-). I've used two DC sources but in the actual circuit I will probably use a rail splitter.

    I've tried to simulate the circuit in TINA-TI but the output is wrong. The input is a 5mVpp sine wave with the INA amp gain of 10 and a DC level of 1 V (This is so it exceeds the common mode requirement of 0.98 V). The output appears to be garbage.

    So there's obviously something I don't understand, or am missing. There are many parameters to take into account and the sheer number of graphs are overwhelming. If someone can point me in the right direction, it would be good.

    [​IMG]
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    1 volt DC level does not comply with the common mode requirements because this chip has supplies of +2.5V and +5V.

    1 is not between 2.5 and 5.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,030
    +1
    Is your signal ground connected to instrument ground?
     
  4. Vanush

    Thread Starter Active Member

    Apr 19, 2008
    46
    0
    I have modified my circuit to take into account your recommendations.

    [​IMG]

    This will be part of an EMG amplifier with multiple gain stages. I'm letting the in amp have a gain of 2. Since the emg signal will be upto for 50mVpp for different muscles, I've set the inputs to have a common mode voltage of 2.5 + 0.98 + 0.2.



    So this seems to work, but is it the most efficient way to design an AMP for single supplies? For example, how would I generate all those voltages in practice?

    EDIT: Is it better to set V- to ground, then the REF pin to 2.5V and the common mode voltage to 0.98 (from the datasheet) + 0.2 V?
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Why are you using +2.5V and +5V as your supplies?
     
  6. Vanush

    Thread Starter Active Member

    Apr 19, 2008
    46
    0
    I was under the impression that the inamp could not output negative voltages if V- was connected to ground.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Your schematic shows V- connected to PLUS 2.5V! Your battery symbol is upside down.
    This is the reason for the reply you got in post #2.
     
  8. Vanush

    Thread Starter Active Member

    Apr 19, 2008
    46
    0
    Apologies. I've included multiple supplies so it must be confusing.

    I'm trying to create an EMG amplifier using a single supply. I believe that a splitting the supply is necessary in order to be able to output negative voltages, as muscular contractions can produce 0-50mVpp.

    In addition to this requirement, the inputs must be 0.98 V above ground (according to the INA118 datasheet) in order to satisfy common-mode requirement.

    I've tried to design a circuit fulfilling this requirement. It works in the simulator, but I'm not sure whether it is best practice when using single supplies. Below is the circuit with V+ being a single supply (3 V battery) and a TLE4246 rail splitter, which will produce V+/2. This feeds into the REF pin and the input pins in order to satisfy the common mode requirement.


    [​IMG]
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Do you want the output to be at Vcc/2 when the differential input is zero? That is what you will get with that circuit.
    Are these muscles in a human? The common mode noise (primarily mains frequency and harmonics of same) will typically be higher than the differential signal.
    Fig. 5 in the datasheet implies that 2V is the optimum common mode voltage for a 3V supply.
    Fig. 9 shows how to properly bias the human body. If you want to use a single 3V supply, change the ground on the inverting op amp to +2V.
    I think that the common mode signal may be too large for a low supply voltage, like 3V.
    You don't need R2 and R3.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,030
    +1 The op-amp inputs will have an impedance of >>1MΩ, so putting 100K is series with that does very little.
     
  11. sanjay agarawal

    New Member

    Dec 6, 2014
    3
    0
    MY PROBLEM IS SIMILAR.

    I AM USING TWO OP-AMP BASED INST-AMP BASED ON LM-324. IC IS FED +12 VOLT WITH PIN 11 AT GND.

    ALL RESIUSTOR ARE KEPT AT 100 K.

    110 MILI-VOLT SIG IS FED BETWEEN PIN -03 AND PIN-05.
    O/P IS 10.32 VOLT.

    WERE AS O/P SHOULBE AT 220 VOLT +/- OFFSET.

    PL. ADVISE.


    SANJAY AGARAWL
    ----------------------
     
  12. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Welcome to AAC.

    Congratulations, you have practiced the arcane art of necromancy, the revival of a long dead thread. Likely the OP (Original Poster) has solved his problem in the years that has passed, or thrown it away, or something.

    PLEASE STOP DIGGING UP OLD THREADS

    Please start your own thread.
     
Loading...
Thread Status:
Not open for further replies.