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EMG Design HELP!!!!!

Pierre Chao
Prodigy 10 points
Other Parts Discussed in Thread: INA116, OPA4209

Hey

Our group is working on our final year engineering and have ran into a lot of issues one of which is overheating of our instrumentation amp.

Our system is operating on a +/- 6 V source with reference mapped to ground. The design is comprised of two INA116, to amplify and remove crosstalk and bio-noise, and two OPA4209, working as filters. We have two silver electrodes that are feed into the inputs of the first INA116 which outputs into the second INA116 with the reference electrode [comprised of a single INA116 at the same gain value of the first one]. The final output is mapped to the input of the OPA4209.

The tests we ran proved that the INA116 are operating as expected with controlled voltage inputs. What we noticed is that when there is no input, we are reading voltage on the input of the same magnitude as the power source... [Question 1] Is the power leaking out of the input terminals of the INAs?

The main concern is that the OPA are overheating instantly when power is supplied. There are periods of where the heat is oscillating and other times that there is no heat, but most of the time it;s overheating. We believe that the current is too high but don't know where malfunctioning. The first OPA is a notch filter that removes 60hz and the second OPA is a band pass filter, passing frequencies between 20 and 500hz. When the OPA is not overheating we confirmed that the notch filter is somewhat working, but not completely removing the 60hz frequencies.

Could someone kindly look over our schematic and help us diagnose our issue of overheating. Any input on improving our filters would be kindly appreciated as well, especially the second one since it's kinda funny....

Thanks in advance.

Pierre 

  • 0
    Guru 21975 points

    Pierre,

    If your inputs are left floating (electrodes not connected to a body, ground defined potential) there will be no path for input bias current. See the discussion on page 8 of the INA116 data sheet.

    I don't see any obvious errors in your schematic. Regarding overheating and other symptoms, I encourage you to do some careful troubleshooting. Are the power supply currents higher than expected? Where is the current flowing? What are the voltages on the terminals of the overheating components? Are they what you expect? Perhaps you have made a wiring error and the circuit is not actually what you have drawn.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    TI__Expert 3780 points

    Hello Pierre,

    To add to Bruce's comments, the schematic doesn't appear to have any obvious errors for the op-amp circuit.  However, if your test board is wired exactly like the schematic you could potentially be running into power and noise issues due to lack of power supply capacitors.  There aren't any shown in your diagram, but an op-amp circuit needs to have good local decoupling and bulk capacitance on the power supplies.

    I propose adding a minimum of 10uF (you can experiment if you need more) bulk capacitiance to each your +/-V supplies on the board where the wires connect to your board.  Then make sure you add some high-frequency decoupling caps (such as .1uF) right near the op-amp supply pins.  Without these caps to store local charge and filter high frequency noise, your circuit could be subjected to all kinds of transients and noise.

    Last but not least, make sure you have a "very good" ground.  I'm not sure if your board has a ground plane or if ground is simply a bunch of wires connected together.  I highly recommend using a board with a ground plane versus just connecting a bunch of wires to the GND connections. It will be very low impedance, reduce noise signficantly and greatly enhance the circuit's performance. If your board doesn't have a GND plane you can create one using materials such as copper tape on the bottom of the board.

    Hope this helps, Todd

  • 0 in reply to ToddToporski
    Prodigy 115 points

    Hi,

    I just want to note that INA116 is obviously the best instrumentation amplifier that i've worked with last 10 years in Biomedical instrument design. But i've been little surprized that TI has not yet consider to make small version of any INA like TSSOP or uMAX or MSOP8. This is the only reason for what many has to move to Analog device ic and Maxim.

    As i'm always been a great fan of TI's components i just wonder why TI don't consider to make any small package of INA of INA116 quality? It would facilitate us to make small biological sensors and many more...

    Best Regards,

    Alam.

  • 0 in reply to Monzurul Alam
    Genius 3585 points

    ^^ I second that.