NM20216A-F
11
Technical data
1
1.3.2.
MEG preamplifiers
• Preamplifiers for MEG channels inside the shielded room on top of the
dewar
• SQUID tuning: by an automatic program or manually
• 12 channels on each preamplifier board, connected to a preamplifier
motherboard on top of the Dewar top flange
• Flux locked loop operation, controller on main electronics board
• MEG preamplifier based on amplifier noise cancellation
• Integrated heater (detrapping) control in preamplifiers
• Control bus for setting the operating points of the front-end electronics
• User-controlled parameters: bias, offset, amplifier noise cancellation
• No digital traffic during measurement
• Powering: from a single power supply inside the main electronics
cabinet via control/power feedthrough unit
1.3.3.
EEG preamplifiers
• 8 channels/board, all channels having internal differential input
• 4 preamplifier boards connected to one preamplifier frontplane, 32
channels/frontplane. First 32 channels are always connected as unipolar
(also referred as single-ended), using common buffered reference from
reference electrode connected to minus input of the preamplifier. Each
following group of 32 channels has 28 channels connected as unipolar
(single-ended) and 4 channels that can either be connected as bipolar (also
referred as differential) or unipolar (requires corresponding cabling in
electrode interface)
• 8 boards connected to a preamplifier backplane
• For 64 channels one and for 128 channels two backplanes are utilized,
both accommodated by a single subrack
• Noise < 0.4
µ
Vrms (0.1 – 100 Hz), measured with a 10 k
Ω
impedance
across the input
• Input impedance > 100 M
Ω
• Software-controlled gain 30 /100/1000 of the preamplifier itself,
selectable individually for each channel. The optoisolation/feedthrough
filter (see 1.3.5) has an additional fixed gain of 5, making the total gain
150/500/5000.
• Common-mode rejection ratio 100 dB (with active ground)
• Amplifier stable for electrode impedances up to 50 k
Ω.
However, to
minimize low-frequency noise, electrode impedances below 20 k
Ω
(below 10 k
Ω
for optimum performance) are recommended.
