ADInstruments Bio Amp, Owner'S Manual

Page: 32 / 50

Bio Amp Owner’s Guide
32
Bio Amp Operation
The Bio Amp and other ADInstruments front-ends have been designed to
integrate fully into the PowerLab system. The Bio Amp is essentially an
extension of the PowerLab’s input amplifiers. The amplification and ranges
offered in LabChart and Scope result from the combination of both pieces of
hardware. The Dual Bio Amp is an enhanced double version of the single Bio
Amp, with a common power supply and isolated ground. There are some
modifications, but it is similar to the Bio Amp overall.
The Bio Amp provides:
• full electrical isolation from power-line (mains) circuitry to guarantee
subject safety
• a low-noise, high-gain differential amplifier specifically designed for
biological signal measurements
• software-controlled low-pass, high-pass, and notch filters to remove
unwanted signal frequencies for particular uses
• audio output for use with EMG or EEG signals.
Technical Description
The PowerLab provides control and low-voltage power to front-ends through
a special expansion connector called the I
2
C (eye-squared-sea) bus. Front-
ends are also connected to the analog inputs of the PowerLab via a BNC-to-
BNC cable, through which they send the amplified and filtered signals. The
overall operation of the Bio Amp and Dual Bio Amp can be better understood
by referring to Figure A–1 and Figure A–2.
The input amplifier of the Bio Amp starts with an electrically isolated
differential amplifier. The output of this amplifier is fed into a low-noise
demodulator and then to a programmable gain stage, before being fed across
an isolation transformer to the non-isolated circuitry. Control of the isolated
gain is provided via a high-isolation-voltage optocoupler. An auto-restore
circuit monitors the level of the input signal and restores the input before the
signal produces amplifier ‘blocking’. Isolated power comes from a second
isolation transformer driven by a power oscillator circuit running at about 38
kHz.
The signal from the isolated input amplifier is synchronously demodulated
and then fed to a programmable, switched-capacitor, high-pass filter. Any
switching clock noise is filtered by the low-pass filter following this stage.