EMG-Main amplifier & interface unit
This (large) main unit supplies power to the electrodes, receives and conditions the EMG signals providing eight analog outputs.
The (small) belt-mounted interface unit is attached to the main amplifier by a cable. Each channel is labeled in correspondence with main amplifier channels and all signals are measured with respect to the reference input lead.
There are three amplification settings that provide an overall system gain of 100 (40dB), 1000 (60dB) or 10000 (80dB); these gains include the pre-amplification gain of 10 within the electrode itself. The AUX setting provides a fixed overall gain of 1000 but with a wider operating bandwidth (20 – 2000 Hz). At a gain setting of 0 the channel input is forced to be at the reference potential. When channels are not in use their gain should be set to 0.
Two LEDs indicate signal saturation (±4.8V) and line interference (60Hz). The main unit contains a tone switch which enables an audible tone when either condiiton occurs.
The power supply provides a regulated 12V output and conforms to the IEC 60601-1 safety standard.
An in-house EMG Analysis was performed in February 2004, which verified the frequency response for each channel & the gain settings for the 4-channel unit which was used at the time.
Subject Safety
[The following information was provided by Delsys.] Subject safety is insured by the use of an isolated medical grade power supply that conforms to the medical IEC 601 standards. The issue of electrical safety with the power supply is addressed through various mechanisms. The many technical points to consider are spelled out in the IEC 601-1 family of standards for medical electrical devices.
Isolation is achieved with an isolated transformer.There are many considerations for how to build this transformer - the windings must be isolated a certain way, the creepage and clearances must meet minimum specifications, the leakage current must be controlled, etc.
There is a double fuse inside the power supply, and current is limited with specially designed regulators. This wall mounted supply has leakage currents less than 10 uA and is safety isolated to 3750 VRMS.
The power suppply is also designed and tested to ensure that it conforms with EMC (electro-magnetic compatibilty) requirements. Also the enclosure must have certain mechanical characteristics and the power supply cord must be a certain type.
The issue of subject safety is not limited to only the power supply but is really considered for the device as a whole. For example, the Bagnoli main amplifiers also have isolated power to protect the subject from the output side (the power supply protects the subject from the input side.) In this particular case, the output isolation is achieved by isolation transformers or in more recent Delsys products by optical isolation.