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
magnitude of current noise
see notes
Capacitive current noise
- generated by voltage thermal noise
- thermal voltage noise of the amplifier acts on all the capacitance associated with the amplifier input and thus produces capacitive current noise whose power spectrum rises in proportional to f^2
The overall capacitance associated with the amplifier input includes
- The gate-to-source and the gate-to-drain capacitance within the FET (10-15pF)
- All stray capacitance (1-2pF)
- The capacitance of the feedback capacitor (1-2pF)
- Capacitance from the pipette and pipette holder (1-1.6pF)
What is shot noise
generated when charges flow across a potential barrier
due to the quantal nature of the charges carrying particles (e.g. individual electrons/ions)
Shot noise increase with?
increases in proportional to the mean current and the charge of the current carrier
Shot noise sources
- Associated with the gate current in the FET
- Depends on the DC current through the patch-seal combination and may be minimized by a high seal resistance
- from the ion channel current (which is usually negligible)
Calculate the SD of total noise level (noise sources are uncorrelated)
see notes
