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…
Dear Scott (and other Analog stuff):
as I mentioned, I am about to test the circuit. I've been reading a lot of datasheets, application notes and articles from Analog Dialogue and Planetanalog.com.
I have prepared final schematic of the ECG/EEG amplifier so have few questions now.
Schematic is divided into sections:
-four EEG/ECG channels with filters and regulated gain
-cable shield driver & Right Leg Driver
-Vmid votlage driver.
Used amplifiers symbols: A1-A4 is AD4692-4, D1(x4 channels) is AD4692-4, B1(x4 channels) is AD4692-4 and C is AD8541.
P1-P6 is digital potentiometer - AD5206
Switch S1 is for selecting the bandwidth of inverting amplifier
Switch S2 is for selecting Low/High Bandwidth mode (I prefer Low - I need gain=100 and bandwidth = 1kHz)
Switch S3 is for changing the reference voltage - 2.5V from ADC or 3V3 from LDO (VCC-GND).
1) I've decided to put lowpass filter before AD8237 (10kOhm + 1nF) as mentioned in datasheet (fig. 69) - does the filter affects the input impedance of the AD8237? I'd like to have big input impedance. This filter is also part of the patient-amplifier protection (10kOhm in series) although whole circuit is battery powered. What is more - 10kOhm in series on both inputs makes electrodes impedance 'more even' - it is good, isn't it? On the other hand it is 0,4uV (RMS) Johnson's noise... What do you think about it?
2) Capacitors parallel with resistors in voltage divides - I used ceramic 1uF, but have also 10uF - which are better?
3) Does the cable shield driver and Right Leg Driver look correct? It is similar like here: Planet Analog - Articles - SIGNAL CHAIN BASICS #58: Analyze the RL drive in an ECG front end using SPICE and fig. 77 in AD8237's datasheet. I used 100R resistor in series with cable shield for stability. I use voltage followers (A3,A4 in order to have big input impedance. The gain (and bandwidth) of RLD may be changed with potentiometer - it is for the fact, that different electrode cables and different patients may have different capacitance/resistance so it is good to have the possibility to change the RLD's parameters. I have chosen AD8541 (C) as it has big phase margin, so hope it will be all working stable.
4) Can I treat the inverting integrator (B1) connected to REF as a highpass filter, so I do not need more highpass filtering (and DC-removing) before D1 amplifier (gain 1~255)? As for ECG I need highpass <0.025Hz, for EEG <0.25Hz and for SSVEP-based EEG <2.5Hz I used S1 switch here. As 0.025Hz needs big R*C constant I have to use X7R capacitors that are poor for filters. Also, I have to use big (1M) resistor that may contribute noise - how to calculate the noise contributed by this resistor (that is further connected to REF)? How to deal with the fact that X7R-type capacitor may change capacitance with voltage and temperature? I used another 1MOhm resistor to compensate input bias current of (B1) amplifier.
5) Due to fig. 66 and 67 of the AD8237's datasheet I decided to place the 10kOhm resistor in series with REF terminal (as 1M || 10k is ca 10k) and 1M || 10k = 10k (that is placed in series at +/- inputs of AD8237). Is it right? Do the gain-setting resistors contribute noise as the 1MOhm is connected parallel with 1uF (or 10uF)?
* I did not draw power pins decoupling at the schematic (I make it like fig. 69 in AD8237's datasheet).
PS: I promise to tell you if the circuit is working well after I make PCB for it