We are currently working on a bio medical project that is an ECG machine.There is a problem that we are unable to solve on software side is the removal of 50 Hz noise. Now we are trying to remove 50 Hz noise using analog filters. Does anybody have an idea of removing 50 Hz noise using analog components.
(the original title stated that the problem was in the signal — Steven)
(note: the question said the noise was in the signal. Seems to be the power supply; see my edit further down.)
A 50 or 60 Hz notch filter is typically done as a twin-T filter. A passive twin-T has a poor Q-factor however, meaning that neighboring frequencies also will be attenuated, which can cause the ECG profile to be distorted.
An active notch filter may look like this:
Don't forget to recalculate the resistor values for 50 Hz; R1 will be 11.8 MΩ. Any opamp will do. The difference with the passive filter is shown in this graph:
Maybe not so clear, but the active filter's graph is the vertical line at 60 Hz. Much better than the passive filter.
edit re your comment
If the 50 Hz is on the power supply like you say it needs decoupling. First the power supply itself. A good regulator won't have a 50 Hz ripple on its output, so maybe the smoothing capacitors on the inputs are too small. (On second thought that ripple should be 100 Hz. A defective diode in the rectifier bridge would explain both 50 Hz and a too high ripple.) If the input ripple is much too large it may go below the minimum input voltage of the regulator. Can you post a schematic of the power supply with component values? You can also place a 100 µF capacitor at the output.
Also use a 100 µF in parallel with a 1 µF on the power supply inputs of ICs. If they're low power you can place a 10 Ω to 100 Ω resistor in series with the supply line, before the capacitors. So the capacitors are directly to the IC's pins. Note that the resistors will cause a voltage drop on the 5 V, so only use the 100 Ω if the supply current is less than 1 mA. Up to 10 mA you can use the 10 Ω. Higher is better, you'll have to see what you can afford. 10 mA through a 10 Ω resistor will cause a 100 mV drop, that's 2 %, which is probably acceptable.
As a student, we designed and tested our own discrete Instrument Amplifier (IA), using a standard 3x Op. Amp. for Biomedical Engineering lab experiments in 1974. We used it for ECG, EEG signal and prosthetic control. As well we applied electrodes to my temple and used it to monitor eye movement. It created a sawtooth with lateral eye movement, which grabbed the attention monitoring girls walking by and some stopped to volunteer for ECG tests. (with electrodes applied to chest) Once we understood the design requirements for CMRR, the 50/60Hz hum disappeared.
Here is my debug checklist for you;
When a 100V E-field ingress is superimposed on a 100uV signal and if you have a good common mode rejection ratio of 120 dB, you get a 100uV noise level.
Ways to Improve CMMR related 50/60 HZ hum are:
1. Use a high quality design Instrument Amp (IA) ( but very low cost)
2. Guard signal with “right leg drive” technique. (known as analog guarding method) where you buffer common mode signal to make a low impedance common mode reference on the leg that is still floating but suppresses the high E fields voltage of 50/60Hz by impedance ratio.
Shield probe wires
Use higher CMMR designed Instrument Amp >130dB
Use an active tuneable Notch filter with Q=100 ( such as reported earlier )
Use CM Ferrite choke around cables. ( high permeability type sleeve)
Ensure V+ supply is noise free with Linear regulator, low ESR cap on in and output. and use short leads between V+ and amp.
My preferences in bold
Trying to filter out the 50 Hz noise should be the last resort only, in part because your valid signal frequency range includes 50 Hz. Anything you do to reduce 50 Hz will distort your desired signal too.
The best answer is to design the analog front end to minimize the line frequency pickup in the first place. The 50 Hz is coming from capacitive coupling of the power line, which is all around the room. However, you are measuring the difference between voltages at several electrodes on the body, and the 50 Hz power line hum will be largely a common mode signal.
ECG front ends need to be extra squeaky clean about common mode elimination. This means full differential signal handling to well past 50 Hz, making sure each leg has the same impedance, using instrumentation amps with good common mode rejection, absolutely no ground reference for one side of the measurement, etc. The power line common mode noise can be many times the amplitude of the signals you are trying to pick up, so you really really have to wake up and pay attention to this issue.
Another thing most ECG systems do is to put a electrode on the leg opposite the heart, which is usually the right leg. This is used purely to pick up the common mode signal, amplified, and then it becomes sortof a floating ground reference for the first stage differential circuits until the differential signal can be amplified and its impedance lowered.
If you do all that right and you still have too much power line noise, then you can consider power line frequency reduction of the final signal. However, this is best done in software so that you can make the filter tight without running into analog component tolerances. That also allows you to measure the power line and make a filter synchronous to it. The resulting very tight notch will have less impact on the real signal than a analog filter with affordable parts. The analog filter has to be broader due to part tolerances alone to guarantee enough 50 Hz attenuation even if the center frequency is off a bit.
So in summary, in order of precedence you should attack the problem by
