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:
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:
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
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