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
I'm sorry for the delay in our answers. I have spoken to Jay about your questions already, but I thought I would just try to answer your questions on the forum directly.
1. The circuit that is connected to the AD8237 REF pin is for ac coupling at high gain without saturation. By inspection you can see that at low frequencies, it forces the AD8237 output voltage equal to Vmid, canceling the dc offset from the patient interface. It is not a Right Leg Drive. That is a separate circuit and it won’t interfere with the REF pin integrator circuit.
There is no question that circuits with Right Leg Drive get better common-mode rejection performance than simple grounded lead circuits like the one in the AD8237 datasheet. The only way the RLD circuit would be different for AD8237 than standard in-amps is the way you derive the common-mode voltage of the body, because with a standard in-amp architecture there are RG pins where a buffered version of the common-mode voltage is available. With AD8237, you have the choice of either using resistors between the leads as you have shown here, which reduces input impedance, or buffering each electrode and deriving the common-mode voltage from a resistor divider between the outputs of those buffers, which preserves the high input impedance but adds complexity. Then that voltage is inverted and driven into the body by the RLD amplifier. Generally, multi-lead ECG circuits use an average of several electrodes to derive the common-mode voltage to be inverted and driven back into the patient, but there is only one driven lead connected to the patient. Have a look at this article for some considerations setting the gain and compensation for the RLD amplifier, but usually the RLD loop compensation has to be determined experimentally based on the amplifier choice and the cable and instrument design.
2. I have seen this method used for shielding before. You just want to be sure the shield driver amplifier you choose is stable with the cable capacitance. The schematic that you posted shows the shield voltage derived from two electrodes, but with additional electrodes, the shield is driven to a voltage that is an average of the leads, such as the Wilson Central Terminal (average of RA, LA, and LL). WCT is a good voltage to use for the RLD as well.
3. It is not necessary for Vmid to be exactly at mid-supply. Set it to something that is convenient for driving your ADC, but make sure the input and output voltages of AD8237 and the other op amps are in range, especially for the largest expected electrode offsets. I personally wouldn’t recommend using the 3.3V from the LDO for the ADC reference though. There is a lot of delay in that loop. If the 3.3V changes, it immediately changes the ADC range, but to get back to the AD8237 output it goes through an LPF from the resistor divider, then a slow integrator before it is compensated at the output. I think it’s better to try to provide a stable low-noise voltage for both Vmid and the ADC reference, whether or not they are related.
I hope this answers your questions. Let me know if I missed something or if you need me to clarify.