What are the uses of a preamplifier

Biosignals - recording devices

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Requirements for the recording device of the bio-signals

  • Record the bio signal without distortion.
  • Amplify the biosignal (amplification factor 1,000 ... 50,000) in the ranges 1 ... 10 volts.
  • To remove unwanted components from the biosignal if possible.

The derived useful signal usually has a number of undesirable signal components. Therefore, the suppression of the interfering components must be started at the same time as the initial amplification of the biosignal.


Block diagram of a recording chain

  • Preamplifier
  • High pass filter
  • Level unit
  • Isolation stage (galvanic isolation!)
  • Low pass filter

Preamplifier


Equivalent circuit diagram of the preamplifier:

  • Offset sources (U0, I.0)
  • Noise sources (UR., I.R.)
  • Input impedance (R || C)

Tasks and interpretation

  • The preamplifier must amplify the bio-signal without significant distortion. It should be the highest quality component in the uptake chain.
  • Designed as a differential amplifier in order not to distort the signal.
  • The input resistance should be around 1,000 times greater than the source resistance R over the entire bandwidth of the biosignalQ.
  • R.Q is a few MW in the range of 10 ... 100Hz (absolute maximum), so the input resistance of the preamplifier must be 109W.
  • For larger electrodes, R isQ approx. 109W (absolute minimum).
  • R.Q (normal) approx. 100kW.
  • So that a large, in-phase interference voltage can be suppressed with the aid of the differential amplifier, the common-mode rejection should be 50 Hz> 100 dB.
  • The maximum amplitude of the current and voltage sources I.R. and UR. should be in the range of pA (10-12A) or nV (10-9V).

Improvement measures

  • Choosing a large-area electrode (this reduces RQ)
  • Use of an electrolyte paste (so that RÜ).
  • Using a low noise amplifier.




Noise behavior of the preamplifier

with (BI) and FET semiconductors as a function of the source resistance RQ.

High and low pass

The high-pass or low-pass filter suppresses different signal components that are either derived from the human body with the useful signal or that arise in the amplifier itself.
The suppression must not affect the useful signal in terms of its phase position or its amplitude.
Passive analog filters (RC elements) or active filters that are phase-linear, such as Bessel filters, are used.

Separation stages

The galvanic separation of the receiving device and the patient ensures patient safety. It prevents dangerous currents from the device power supply from reaching the patient.

This can be achieved by:

Interference fields

Sources of interference in the bio-signal in the recording chain

Measures against electrical interference fields

  • Increasing the distance between the source of interference and the patient.
  • Separation stage in the recording chain.
  • Separate grounding of the patient (is problematic!).
  • The lead of the electrodes from the patient to the amplifier is shielded and earthed on one side.

Measures against magnetic interference fields

  • Increasing the distance between the source of interference and the patient.
  • Twisting the supply lines, especially the supply lines of devices with high power consumption.
  • Shielding the magnetic fields (expensive).

Coupling of the mains-frequency electromagnetic interference

Motion artifacts

Movement artifacts of the lead electrode

Any movement at the point of contact changes the contact voltage. Moving cables also cause movement artifacts. With cheap cables they can be up to 100mV.
Movement artifacts cause interference signals.

Measures to reduce motion artifacts

  • Conductor electrode with low contact voltage (small voltage drop on the electrode) → non-polarizable electrode.
  • Reduction of the contact resistance RÜ.
  • Suitable preamplifier.
  • Avoid patient movements as much as possible, e.g. by briefly holding your breath.
  • Constructively integrate the preamplifier into the lead electrode (problem, since it is usually a single-use item).

→ Continue to "Constructive details / standards" ...

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