AD 8237 in precision low cost multichannel biological amplifier (ECG, EMG, EEG) circuit - questions.

Dear colleagues,

I am currently designing circuit to measure biological signal - ECG, EMG, (and simple EEG if possible). Due to

battery powering and low cost, I am planning to use AD8237 instrumental amplifiers. I have some question I want to

ask you before ordering and designing PCB.

I read the datasheet carefully, and spotted there scheme for ECG circuit. I made some notes on the scheme:

 

 

My questions:

 

1) Why 47nF capacitor was used in parallel with gain setting resistor?

2) Why 110kOhm/22nF lowpass filter is placed on the output? Only for filtering?

3) Why a capacitor is used in parallel with resistor setting Vmid voltage. What should be its value?

4) Is it possible to use different values of the elements in lowpass integrator? F.e 3MOhm/1uF?

5) is it possible to power whole circuit with single Li-Ion 18650 (4.2-3.3V)? I want to use single battery and do

not want any switching regulators (due to noise it generates).

 

6) I want to make 3-channel universal biological amplifier. How should I connect the op-amps?

As far as I understand the scheme, electrode C (right leg) is for dealing with noise and DC component (thats why I

am using inverting integrator) - so I need only one electrode like this (C) for all the channels.

Electrode A and B are simply non-inverting and inverting input of the amplifier, so I can say, that A is a value

of the measurement, and B is the 'reference voltage' (A is measured referred to B). So if I want to have more than

one channel, I should connect all inverting inputs to electrode B and place electrodes A (A, A', A'') to points on

the body I would like to measure, am I right?

For the next channel I need only the next AD8237 + gain setting resistors?

And I connect all REF pins of all AD8237's to the output of inverting integrator?

 

7) I want to use shielded cables due to better noise immunity. Due to the discussion:

https://ez.analog.com/message/167615#167615

I assume, that I need to take the signal from 'between' the inputs and drive it to the shield? Due to the costs I

want to use one shield potential for all of the cables. I have written it on the schematic using blue color - is

it correct (the shield will be connected to the SH pin)?

About the filtering: I know what filters to use, so please focus on the questions. I will use single passive RC

highpass with tau (RC) = 3 (cutoff = 0.05 Hz, medical standards) and double passive RC lowpass (with cutoff

frequency = 15kHz) as I use sigma-delta ADC with 1MHz modulator.

I am waiting for your help as I study electronics in medicine and I am strongly interested in construction of

low-cost biological amplifier

 

best regards,

Mateusz Winkowski

 

PS: Is it possible to buy some AD8237's together with the samples? They are not expensive, but the shipping cost

is huge, and I can not get them in Poland (I can sample 2 pieces per two weeks, but I do not want AD to waste

money for the shipping it many times, so I would like to buy additional pieces and ship them together with the

samples).

  • Anyone please help me?

    I am waiting with the PCB design

  • We are closed for the Holidays until Jan 4.

    Harry

  • I am not an ADI employee and not expert in medical circuits. However, I think I can help with some of your questions from a general circuit viewpoint. Perhaps this will give you a start before ADI can answer after the holidays.

    1) A capacitor in parallel with a gain setting feedback resistor is generally used to limit circuit bandwidth. In doing so, it also reduces higher frequency noise. When you build your circuit, you can experiment with removing this capacitor and also try different capacitor values.

    2) The 110kOhm/22nF lowpass filter at the ECG output seems to be only for filtering. With the values indicated, the cutoff of this filter is about 66 Hz.

    3) The Vmid voltage is used to develop a potential that is half of the single supply voltage. This is done with two equal value resistors in series across the power supply. These resistors should be a high value to limit overall power consumption, but high value resistors have a lot of thermal (Johnson) noise. A capacitor at the junction of these two resistors to ground is used to reduce this noise.

    4) I am not sure about what is most appropriate or allowable feedback component values for the integrator. I would provide a layout with a parallel capacitor and resistor in the feedback of this amplifier

    5) Since the AD8607 is characterized for voltages from 1.8V to 5V, there should be no problem at all with powering this circuit with battery voltages of 3.3V to 4.2V.

    6) I would connect the amplifiers exactly as indicated in the schematic. Electrode (C) can be seen as a local “common”. Electrodes (A) and (B) pick up a differential signal. If you want more than one channel, you only need one (C) node, but will need nodes (A’) and (B’) with a second AD8237, (A’’) and (B’’) with yet another AD8237, etc. There is no “Ref” pin per se, unless you want to call electrode (C) a Ref pin.

    7) I suggest you go to www.google.com, click on “images” at the upper right, then search for “ecg circuit schematic” and study the text associated with some of these images for clues as to how you should connect the shields. I believe that what you propose is close to what is needed.

      Good luck!

  • Dear Harry - I am sorry about disturbing You during Your holiday, I will wait until 4th Jan for the answers, ok?

    Dear ngray - thanks for the answers. Here is my reply:

    1) OK

    2) OK

    3) I know that resistors 'make some noise' - exactly noise^2/df = 4KTR for resistor - but I did not know that capacitor helps. What should be the value? Bigger = better?

    4) I was thinking, that the values are big in order to not to interfere with ECG bandwidth (that is 0.05 Hz - for 3,3uF and 2M the cutoff frequency equals 1/(2*3.14*RC) = 0.025 Hz).

    --> question for AD (after holiday) - can I use smaller capacitor and bigger resistor, f.e. 1uF + 6.8M?

    5) Hope so, but I will probably use LDO stabiliser 3,3V.

    questions 6 and 7 (and a new one): a bit specific, so please AD answer me (after 4th):

    6) I want to make 4 channel amplifier. I would get 8 cables (4* bipolar = 8) and the 9th for the 'ground', here marked as C. In 'classic' 3-op-amp instrumental amplifier architecture electrode C is rather an active electrode (called 'Right Led Driver'). Can (and how with AD8237 architecture) I make the RLD to acquire better SNR performance? According to the datasheet:

    Normally, the low-freq. inverting amplifier (or similar) would be connected to the RLD, here is connected to the REF pin, so that is why I am asking for the driver that will not badly interfere with the REF-pin-driver.

    Maybe for the AD8237 it is not needed? I need good SNR due to hope to use the amplifier for (simple) EEG.

    Do I need one REF-pin-driver for one AD8237 amplifier, or I should connect all REF pins of all AD8237s together?

    7) About shielding waiting for answer:

    I want to use shielded cables due to better noise immunity. Due to the discussion:

    https://ez.analog.com/message/167615#167615

    I assume, that I need to take the signal from 'between' the inputs and drive it to the shield? Due to the costs I

    want to use one shield potential for all of the cables. I have written it on the schematic using blue color - is

    it correct (the shield will be connected to the SH pin)?

    8) My new question - is Vmid needed to be exactly at half-Vcc or can I adjust the Vmid to the ADC I will be using? I want to use ADC with precision 2.5V reference - and I am having a 'voltage mismatch' - 1.65 + - 1.65 vs 0 to 2.5V. I want to set Vmid to 1.25V in order to have scale 1.25V + - 1.25 (bipolar) that fits 2.5V ADC. I know, that I am limiting the  possible gain, but my question is - is this possible for AD8237 to work like this?

    Or (due to the fact that ADC and AD8237 will be powered from exactly the same 3,3 LDO) should I select an option in the ADC to use Vcc-GND as the reference? It would fit ideally, and all noise should be compensated (f.e. if 10mV drop occurs, I has place both for the ADC and the AD8237) - am I right? In biological circuit I am interested rather in the ratio between electrodes rather than the absolute exact voltage value?

    9) Is it possible to buy some AD8237's together with the samples? They are not expensive, but the shipping cost

    is huge, and I can not get them in Poland (I can sample 2 pieces per two weeks, but I do not want AD to waste

    money for the shipping it many times, so I would like to buy additional pieces and ship them together with the

    samples).

    best regards,

    Mateusz Winkowski

  • Anyone please help me?

    I am (still) waiting with the PCB design