AD8232 for EMG Signal Conditioning

Hello,

I'm a ME student but I'm interested in EE and I have some circuits knowledge so please feel free to take the time to educate or correct me throughout the discussion.  My grandma recently suffered a stroke and I wasn't happy with the passive rehabilitation she was getting so I set out to design a more active solution.  I designed a myoelectric controlled orthosis.  It’s a motorized arm brace that assists through the range of motion and it senses and uses the muscle contractions as the trigger.  I designed the EMG amp/filter/rectifier circuit with components from texas instruments and it works fine but it requires a +/- supply which is currently two 9 volt batteries in series with ground between them.  The microcontroller (arduino nano) requires a 7-12V supply and the motor requires 12V.  Everything is done and everything works except the integration of the power supply for these three components is too confusing and TI isn’t much help.  Everything needs to be compact and fit into the orthosis so I want a single battery solution. 

In doing research on power supply circuits I came across the AD8232 and it seemed like an easy way to solve the power issue since it was designed to be powered directly from a 3v coin battery or I can use the arduino nano’s built in 3.3v LDO which can source up to 50mA.  I also decided to go with two 6v servo motors instead of one 12v.  That said, the power supply issues have been resolved… but only if the AD8232 will work for EMG signal conditioning.  EMG signals are around 1.5mV and from 10Hz-1000Hz with most of the signal power around 150Hz.  It seems like it would work just fine but I want to make sure.  If it will work, I have a list of specific questions pulled from the data sheet that I’d like to discuss.

Thank you,

Chad K

  • Hello Chad,

    It is difficult to say whether it will work without further knowledge of your particular design goals. AD8232 was mainly designed to acquire ECG signals, but as with most instrumentation amplifiers, it is not restricted to this application. Please feel free to ask any questions regarding the specifications on the data sheet.

    In addition to AD8232, we have other instrumentation amplifiers that may be worth considering for biopotential acquisition. The following examples are specific for low-power, single supply applications: AD8420, AD8237, AD8235, AD8236.

    Good luck!

    Gustavo

  • Hi Gustavo,

    Thank you for clarifying that for me!

    Do you have the specs on the evaluation board, AD8232-EVALZ.  I want to

    know the configuration of the evaluation board such as any filters and

    their cutoff frequencies and what types of connectors the board has.

    The arduino nano microcontroller board has a FTDI FT232RL chip which has an

    internal LDO providing a regulated 3.3v output @ < 50mA.  Do you see any

    issues with using this 3.3v supply?  Would I still need to use the 0.1µF

    bypass capacitors?

    Questions from the datasheet:

    1. REFERENCE BUFFER, "The signals present at the out-put of the

    instrumentation amplifier are referenced around this voltage."  I've

    already designed a working EMG amp circuit with a full wave rectifier in

    the last stage.  With dual supply op-amps the output goes positive and

    negative with respect to the 0v ground which means a rectifier is needed

    because the microcontroller can't accept AC.  The AD8232 is outputting a DC

    signal correct, and if there is no difference at the inputs, the output

    voltage = the reference voltage, say 1.5v.  So I don't need to use a

    rectifier with the AD8232, right?

    Do I need to even use a voltage divider at the REFIN pin or can I just

    leave it unconnected?  If the REFIN pin requires the voltage divider, can I

    leave the REFOUT pin unconnected if it isn't needed?

    2. INPUT REFERRED OFFSETS, I found a definition for this term which

    basically says, input referred offset is a voltage applied to the + input

    so the output is zero.  Is this related to the stored potential in the

    electrodes?  Would I want to use the DC blocking amp as an integrator then?

    3. LAYOUT RECOMMENDATIONS, it suggests a ground plane but doesn't recommend

    any layering order.  Is there a recommended layering order for power,

    ground, signal planes?

    I really appreciate your help on this!

    Thank you,

    Chad K

    On Tue, Sep 18, 2012 at 2:20 PM, Gustavo.Castro <

  • Hello Chad,

    The evaluation board configuration may change, but the default is similar to the circuit in figure 62 of the data sheet. I don't see any problem if you decide to use the supply provided by the FTDI chip, as long as it doesn't exceed the maximum rating of 3.6V. The current consumption from the AD8232 is less than 170uA, so you won't have a problem. However, it is a good idea to have provisions for bypass capacitors, such as 0.1uF. However, if you have other things on that rail that require 3.3V and you already have bypass capacitors, you may get away with that.

    1. Reference buffer. Yes. Think of it as a "raised ground level" for all your signals. We usually set it to mid supply, but you want to set it a mid-range of the ADC. That is, if your ADC internal reference is at 2.5V, you should set the reference buffer to 1.25V.

    2. Input referred offsets. Yes, it is best to use it as an integrator. This is a special type of instrumentation amplifier. There are signal inputs and a feedback input (HPSENSE). If you use the DC blocking feature on the feedback, it will reject all the offsets (from the amplifier inputs AND the electrodes). The only offset left is from the HPSENSE input to REFOUT. If you decide to say, use it as a follower, it won't reject any offsets and to the contrary, it will amplify them. Therefore, it is best to use it as an integrator.

    3. Layout is very simple for this chip. There are no high speed signals and there is not a specific layering order, although I would recommend you use the supply ground right below the signal layer. So, for a two-layer board use SIG-GND and for a four-layer board I would recommend SIG-GND-PWR-SIG. Since most of the resistors are of high value for keeping power consumption low, keep all the high impedance nodes as short as possible and close to the AD8232.

    Good luck and let me know if you have any further questions.

    Gustavo

  • Chad,

    Have you seen some of the other options, that are in a DIP package?  For example:

    http://www.analog.com/en/specialty-amplifiers/instrumentation-amplifiers/ad620/products/product.html

    This might be easier to work with than an eval board, especially with your Arduino Nano.  And, it's cheaper, considering you can free samples on analog.com. 

    -Anne

  • Gustavo,

    you mention page 62 of the datasheet? Where is this datasheet located, as I am looking for the same thing: I would like to have a look at the AD8232-EVALZ?

    I guess I could always produce a board around the AD8232 myself, but right now, a development board would be faster.

    So before I go ahead and buy a couple of these boards, I'd very much like to have a look at their datasheet. Can you help me please?