Blood Pressure Monitor PCB Design: Signal Chain and Layout

A blood pressure monitor is a good example of a product where the board is the instrument. The measurement depends on resolving a small pressure oscillation that sits on top of a much larger static pressure, and the difference between the two is extracted in the analogue front end before any software sees it. Layout decisions that would be harmless on a logic board directly affect the reading.

What the Device Has to Do

The measurement cycle is mechanical before it is electrical. A pump inflates a cuff wrapped around the arm, a valve releases the pressure at a controlled rate, and a pressure sensor converts the pressure in the cuff into a voltage. As the cuff deflates, the artery opens and closes with each heartbeat, and that produces a small oscillation superimposed on the falling static pressure. The amplitude of the oscillation peaks at a particular static pressure, which is the mean arterial pressure, and the systolic and diastolic values are derived from the shape of the envelope around that peak.

Two signals therefore have to be recovered from one sensor output. The static pressure needs a slow, accurate measurement, and the oscillation needs a gain of several hundred with the offset removed. Once the design is described that way, the front end almost designs itself, and the layout requirements follow from the signal amplitudes involved.

The Sensor Front End

The sensor is excited by a constant current, typically in the range of 0.3 to 1 milliampere, because a current drive produces an output proportional to pressure without the temperature coefficient that a voltage drive would introduce. The first stage after the sensor is an instrumentation amplifier, chosen for a high input impedance so that it does not disturb the constant current source, and for the gain it provides on a signal of a few millivolts. A programmable gain instrumentation amplifier is the common choice, because a single device can provide several gain steps – for example 16, 32, 64 and 128 on one channel and about 1.3 to 2.6 on a second – giving the design a wide range without adding parts.

The sensor and the first stage together form the most sensitive node on the board, and the layout has to treat them accordingly. The traces from the sensor to the amplifier are short and are kept away from the pump, the valve and any switching node. The reference and bias pins are decoupled at the pin. And the sensor supply and the amplifier supply are filtered separately, because the pump is a mechanical device driven by a motor and the disturbance it produces travels through the supply as easily as through the air.

Amplification and Filtering

The oscillation signal and the static pressure are separated by a bandpass filter. The static component is removed because it carries no information about the heartbeat, and the high frequency content is removed because it is noise. The sources of that noise are specific and worth naming: mains hum at the supply frequency, electrical noise from the pump motor and the valve solenoid, and friction between the cuff and the skin as the cuff moves.

A filter placed in the analogue chain before the converter is what makes the software’s job possible. If pump noise reaches the converter, no digital filter can separate it from the oscillation, because the two overlap in frequency. The filter is therefore a layout concern as well as a schematic one: its capacitors and resistors sit close to the amplifier pins, its traces are short, and the ground it references is the analogue ground rather than a shared digital return.

Grounding the Analogue Section

An analogue ground region defined inside the digital ground is the standard arrangement, and its purpose is to keep the return currents of the digital circuitry out of the path the measurement uses. The two regions connect at a single point, so that they sit at the same potential without sharing a conductor that carries both currents.

The converter is the device that straddles the boundary, and how it is treated decides whether the arrangement works. It is placed on the analogue side, with its analogue inputs and its reference referred to the analogue ground, and its digital interface referred to the digital side. The digital return current then flows back to the digital ground rather than across the analogue region. The principles behind that separation are the ones described in mixed-signal PCB design guidelines, and the way ground and supply conductors are planned is covered in ground routing and power trace planning.

Layout of the Pump, Valve and Display

The mechanical parts are the largest loads on the board and the largest sources of disturbance. The pump motor and the valve are driven by switching devices, and the current those devices interrupt is the current the whole system draws. Three measures keep that disturbance away from the measurement.

First, the driver circuitry is placed as far from the analogue front end as the board allows, ideally on the opposite side. Second, the loop formed by the switching device, the motor or valve and the supply is kept as small as possible, because that loop is the antenna. Third, the supply for the mechanical section is filtered at its own entry point rather than downstream of the analogue supply, so that the disturbance is not carried along a shared conductor. The liquid crystal display is a smaller concern electrically, but its driver should still be placed away from the analogue input, and its segments are driven by waveforms rather than static levels, which makes the display a low level source of noise on its own.

Manufacturing and Calibration

A device that measures pressure has to be calibrated, and the cost of calibration is set at the design stage. Where the amplifier gain and the reference are set by internal registers rather than by external trimmer resistors, the calibration can be performed by the software against a reference pressure and no manual adjustment is needed on the production line. That removes a labour step from every unit and removes the variability that a hand adjusted trimmer introduces.

The rest of the manufacturing requirements are ordinary. A conformal coating over the analogue section protects against the moisture and handling a medical device sees, and the coating options are compared in conformal coating for board protection. Test points on the analogue nodes let the front end be measured during production rather than inferred from the display, and the connector for the cuff should be keyed so that it cannot be mated the wrong way round.

FAQ

Why does the sensor need a constant current source? Because a current drive produces an output proportional to pressure with less temperature dependence than a voltage drive, and a high input impedance amplifier then reads that output without disturbing it.

Why is bandpass filtering done in hardware? Because the pump and valve noise overlaps the heartbeat signal in frequency. Once that noise reaches the converter it cannot be separated digitally, so it has to be removed before the conversion.

How should the analogue and digital grounds be arranged? As separate regions joined at a single point. The converter sits on the analogue side, with its digital interface returned to the digital ground rather than across the analogue region.

How is the unit calibrated in production? By software, against a reference pressure, using gain and offset registers inside the signal chain. That avoids a trimmer adjustment on every unit.