Biopotential Amplifiers Flashcards
Requirements of biopotential amplifiers
- High input impedance (Ri) - at least an order higher than the source impedance to avoid signal loss due to voltage division; bioelectric signal sources usually have an high impedance: 10^3 - 10^7 ohm
- Low output impedance (Ro) - at least an order lower than the load impedance (RL)
- High gain - because biopotentials are usually small in magnitude
- low inherent noise
- quick calibration
- Isolation and protection circuitry: the current can be kept at safe levels; induced artifacts can be minimized.
- Differential amplifier is often used: To amplify the differences between the 2 electrodes (biopotentials) while rejecting the common signals which are irrelevant. When differential amplifier is used, high common mode-rejection ratio (CMRR) is required
Problems and solutions in biopotential recording
- Frequency distortion: relevant frequency components in the signal is filtered out
Solution: adjust filtering circuitry to set appropriate recording bandwidth - Saturation or cutoff distortion: due to too large signal or too high amplifier gain
Solution: lower the amplification gain to avoid - Ground loop that may exist between 2 machines connected to the patient: Current flows from the ground of one machine through the patient to the ground of another machine, causing safety issue and producing common mode voltage
Solution: use isolation and protection circult - Random noises (e.g. thermal noise):
Solution: use filter circuit - Interferences (artifacts):
Solution: use proper shielding and filtering
Noises in Biopotential recording
- Intrinsic electronic noises of the amplifier itself
- Thermal (voltage) noise from the electrode impedance: Solution: lower electrode impedance -> lower the thermal noise from the EEI
- Aliasing noise due to a too low sampling rate: Solution: higher sampling rate must be greater than twice the highest frequency component that has non-negligible power (Nyquist theorem)
- Quantization noise due to limited resolution of analogue-to-digital converts:
Solution: appropriate gain setting so that the dynamic range of the signal matches the full voltage range of AD converter to increase signal-to-quantization noise ratio
Interferences in Biopotential recording
- Unstable electrode half-potentials
- Motion artifacts
=> solutions: use electrode with stable and small half-potential for both 1. and 2. ; stable interface with body surface e.g. applying electrolyte gel; using suction for 1. and 2. ; high-pass filter to eliminate low-frequency drift caused by motion artifacts for 2. - Artifacts from electric shocks: e.g. electrical shock from defibrillator; discharge of buildup of static charge in body; etc.
Solution: using isolation and protection circuit - Electric interference within body itself (other electrogenic sources): e.g. EMG interferences ECG recording
Solution: using proper filtering (proper recording bandwidth) - Electromagnetic interferences (EMI) from power-line or nearby electric devices, through capacitive coupling and/or inductive coupling (magnetic induction)
Solution: proper filtering (e.g. Notch filter), shielding and isolation, use differential amplifier
When is right-leg drive circuit used
Used in a negative feedback configuration to reduce common-mode interference (e.g. from 60 Hz interference)
What is a high-pass filter
- Passes high frequencies; attenuates low frequencies
- Used to eliminate noises or interferences below the frequency range of the detecting signal
- Eliminates DC component that may saturate the amplifier
What is a low-pass filter
- Passes low frequencies; attenuates high frequencies
- Is used to eliminate noises and interferences above the frequency range of the detecting signal
What is a band-pass filter
A series combination of the low-pass and high-pass filter, which amplifies frequencies over a desired range (the frequency range of the detecting signal) and attenuates higher/lower frequencies
What is a band stop filter (notch filter)
A filter that passes all frequencies except those in a stop band centered on a center frequency
Often used to filter out 50 or 60 Hz interference noise
What is single-channel current amplifier for
It is specially designed to be able to measure tiny single-channel current which is in an order of pA. It is vv important to carefully minimize the noises in order to achieve sufficient s/n ratio. Proper shielding is needed to avoid EMI (recording is made in Faraday cage).
Single-Channel current amplifier: Thermal (current) noise in the feedback resistor
Places a lower limit on the noise level
Single-Channel current amplifier: Thermal (Johnson) noise
Noise generated by the random thermal motion of the charge carriers (e.g. electrons) inside a conductor
voltage power density spectrum of the thermal noise
see notes
current power density spectrum
see notes
current noise variance
see notes
