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.
- 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).