When recording electrical signals from cells (in a dish or inside a living human or animal body), one major problem is to increase the signal to noise ratio.
These signals are usually in the 10uV to 100mV range and are generated by very low power sources that can yield currents in the order of nanoAmps.
Often signals of interest fall within 1Hz-10KHz range (most often 10Hz-10KHz).
To make the matters worse usually there are lots of noise generating tools that are necessary to be around (in the clinic these are other monitoring, diagnostic and therapeutic devices in the lab these are other monitoring, scientific devices).
To reduce the impact of noise and increase the signal to noise ratio, there are a few generally applied rules like:
- If possible use a current amplifier (often called head-stage), an amplifier with very high input impedance and rather low voltage amplification or even no voltage amplification. very near to the signal source (body).
- To connect the source (recording electrodes) to the first stage amplifier (head-stage) use wires that don't have shields (to avoid capacitative distortions of the signal).
- Avoid ground loops
- When possible use differential amplifiers (to cancel the induction noise from the electromagnetic sources around).
- Always use Faraday cages and grounded shields (usually Aluminium foils) to cover the signal source and anything connected to it (body, equipment ...).
- You can't do this without proper filters (usually a 10KHz high cut and a low cut that depending on the signal may be anywhere from 1Hz to 300Hz )
- If you can't get ride of the mains noise (50Hz or 60Hz in different countries) and only if your signal covers that range you can use active filters like Humbug http://www.autom8.com/hum_bug.html
My question is: Are there any other suggestions that I missed? Is any of these suggestions flowed or wrong?
Usually people in this fields (like me) do not have formal education in electrical engineering and sometimes there are myths passing from a teacher to student generation after generation without proper evidence. This is an attempt to correct this.
EDIT:
- if possible use batteries or very well regulated power supplies in all your devices, including any pumps, microdrives, monitoring devices, even you can put filters on the mains of your computers (although this usually is not a serious issue).
Answer
Driven shield
It is possible to use shielded wires between the electrodes and the pre-amp without a lot of influence from the shield's added parasitic capacitance (your 2nd dot). The signal itself won't be hurt much because it is very small compared to the common-mode component. To understand this, imagine a tiny differential signal on top of a much, much larger common-mode signal (mostly caused by 50 Hz or 60 Hz mains voltage) and a DC-to-low-frequency component caused by the interaction of the tissue with the electrodes and the body itself. As far as I understand the issue, the interference coupled onto the signal via the cable's capacitance is much worse than having the signal itself fed through the cable capacity.
The trick is to actively drive the cable's shield with the common-mode part of the signal instead of connecting the shield to the pre-amp's ground. Some years ago, I've built such pre-amp with an active guard and was able to use shielded wires as long as 2 m between the electrodes and the first stage of the amp. The schematics can be found in this thesis (not mine, but conveniently includes the most interesting schematics of my EMG amp). Please see fig. 8.7, 8.8 and 8.9 and all the stuff around them in chapter 8. Fig. 8.12 discusses how interference is capacitively coupled onto the signal of interest. Sorry, the thesis is in German, but I hope the images and schematics are international.
A good place to pick up the common mode signal is the "middle" of the gain setting resistor of the initial InAmp (again, see the thesis linked above).
Driven right leg
The right leg is used as a reference to measure signal on left leg, left arm and right arm.
The concept of a driven shield can be extended to actively drive the patient, and the connection is made at the location used as a reference for the signals to be measunred, which is the right leg. This is known as a driven right leg (DRL); there's a good discussion about DRL amps in this article by EDN.
If your measurements are not taken from a human body but from some cells in a dish, you can probably put the DRL electrode onto the bottom or into the jelly / growth medium, close to where your reference electrode sits. This way, you use the same strategy as you would in the sense of a DRL setup.
Notch filter
Also, If the hum is really bad, you can put a notch filter at 50 Hz or 60 Hz into the signal path, but this will also hurt the signal of interest.
Very important safety note: The electrodes must not have any direct galvanic connection to protective earth (PE). This is necessary because once the patient gets connected to a potentially lethal voltage by a fault in another device around the lab, the fault current will have a very good path through the patient and via the electrodes to ground. When talking about a ground reference around the electrodes or the pre-amp, be sure to make this a ground referenced to the pre-amp only and not to the real ground usually known as PE! This usually requires an isolation amp somewhere around or just past the pre-amp, or a digital isolator if you wish to have the ADC close to the pre-amp. More about this in DIN EN 60601-1 and other relevant standards.
No comments:
Post a Comment