PE2-3.3 Biomedical Amplifiers, Telemetry & Imaging

What this is and why it exists

The amplifier requirements in this subject are not a matter of preference. They follow arithmetically from the numbers in the first topic.

A signal of a few microvolts must be recovered while interference hundreds or thousands of times larger sits on the same wires. That single sentence produces every requirement below.

The vocabulary

• Common mode rejection — how well an amplifier ignores a signal present equally on both inputs.
• Input impedance — how little current the amplifier draws from the source.
• Drift — slow, unwanted movement of an amplifier's output over time.
• Chopper amplifier — one that switches its input rapidly, moving the signal away from where drift lives.
• Carrier amplifier — one that modulates the measurement onto a carrier before amplifying.
• Phase sensitive detector — a detector that multiplies by a reference and filters, recovering only what matches it.
• Conduction velocity — how fast an excitation travels along a nerve or muscle, computed from timing.
• Biotelemetry — sending a measurement without a wire, including from inside the body.
• Isolation — an electrical barrier preventing current flowing from equipment into a patient.

The mental model

Three requirements, each following from the numbers.

The signal is tiny, so the gain must be enormous. The interference appears almost equally on both electrodes. The amplifier must reject what is common and keep what differs, which means very high common mode rejection. The electrode-tissue interface has a high and variable impedance. The amplifier must draw almost no current from it, which means very high input impedance. None of the three is optional, and together they make this the hardest analog problem in the subject.

A differential amplifier meets the second requirement directly, which is why it is the basic structure here.

Drift is the problem the other two structures exist to solve. At these gains, a slow movement of a fraction of a microvolt at the input becomes a large movement at the output. Amplifier offsets drift with temperature and with time, and that drift sits at very low frequencies, exactly where several of these signals live.

The trick is to move the signal away from where the drift is. A chopper amplifier switches its input rapidly between the signal and a reference. The wanted information is then carried at the switching rate, rather than near zero frequency. Amplify at that rate, where the drift is not, and then bring it back down. The drift is left behind, because it was never modulated.

A carrier amplifier applies the same idea to a passive transducer, exciting it with a carrier so its changing property modulates that carrier.

Bringing the signal back down is what the phase sensitive detector does. Multiply by a reference at the same frequency and phase, then filter. Anything matching the reference produces a steady output. Anything at another frequency, or at the same frequency but the wrong phase, averages away. So the detector is extremely selective, and it is how these amplifiers achieve their noise rejection.

Recording practicalities follow. Scalp electrode placement is standardised, which is what makes recordings from different laboratories comparable. Muscle recordings can be made at the surface, which is painless and picks up many fibres at once. A needle is invasive and sees a small region clearly. That is the usual trade between comfort and specificity.

Conduction velocity is a clinically useful number computed from timing alone. Stimulate at one point, record at another, measure the delay, divide the distance by it. A slowed velocity is evidence about the nerve.

Respiration is a different physical quantity again, measured mechanically by the volume moved or by the flow rate, so it needs transducers unlike any above.

Biotelemetry sends the measurement without a wire. For an implanted unit, power and safety dominate every other consideration, because the battery cannot be changed easily and the device sits inside a person.

Isolation is why connecting a computer to medical equipment is not ordinary interfacing. A patient connected to an instrument is connected to whatever that instrument is connected to. A fault elsewhere must not be able to drive current through the patient, so a deliberate electrical barrier is placed in the path.

The imaging survey closes the topic, and the useful way to hold it is by what physical property each method measures. One measures how much different tissues absorb radiation as it passes through. Another measures how the body's own nuclei respond to a magnetic field and return to rest. A third follows an injected tracer. Knowing which property each reads explains what each is good at far better than the machine details do.

What you should now be able to explain or do

Derive the three amplifier requirements from the signal and interference amplitudes. Explain drift and why it is worst at low frequencies. Explain how a chopper amplifier moves the signal away from the drift. Describe what a phase sensitive detector rejects and what it keeps. Compute a conduction velocity, and say why isolation is required when connecting a computer.

Check yourself