Analog vs Digital Signals

A practical explanation of how medical equipment senses, converts, transmits, and interprets information

Medical equipment constantly moves information.

Published August 16, 2026 · Revised September 6, 2026

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What This Page Explains

This page covers:

The Simple Version

An analog signal changes continuously and can represent values such as ECG voltage, temperature, pressure, or light intensity. A digital signal uses discrete logic states or encoded data to represent and move information. Most medical devices use both: a sensor produces an analog response, conditioning circuits prepare it, and an analog-to-digital converter turns samples into numbers for software.

This distinction helps localize faults. Noise or offset before conversion can corrupt every later display and calculation, while a communication or software problem can lose correct digital data after the sensor path is working. Follow the signal from physical input to final output and identify the last point where it remains believable.

Worked Example: A Pressure Value Jumps Randomly

Compare the physical pressure with an independent reference, then inspect the analog sensor output and supply if the service procedure exposes safe test points. If the analog signal jumps too, investigate the sensor, wiring, shielding, reference, and conditioning circuit. If analog output is stable but the displayed value jumps, focus on conversion, data transfer, processing, or software.

Use proper grounding and measurement bandwidth so the test instrument does not create or hide noise. After repair, verify zero, multiple pressure points, stability, and related alarms. A clean digital display at one instant does not prove the analog path is stable.

What Is an Analog Signal?

An analog signal changes continuously over a range.

It may represent:

Example:

A pressure transducer may output a voltage that changes as pressure changes.

Higher pressure:

Higher voltage.

Lower pressure:

Lower voltage.

The exact relationship depends on the design.

Analog Signals Can Have Many Values

An analog signal is not just:

On or off.

It might be:

0.51 V

0.74 V

1.12 V

2.06 V

Every value can represent a different physical condition.

What Is a Digital Signal?

A digital signal represents information using discrete states.

At the basic electrical level, that often means:

or:

But digital communication usually combines many of those states into meaningful data.

Example:

A module may send:

Temperature = 37.1°C

as encoded digital information.

You are not measuring the temperature directly from one simple voltage level anymore.

Analog Example: Pressure Sensor

Imagine a pressure sensor.

Physical pressure changes.

Sensor output changes.

Example:

0 mmHg:

0.5 V.

100 mmHg:

2.5 V.

200 mmHg:

4.5 V.

That continuously changing voltage is analog.

The device then interprets it as pressure.

Digital Example: Module Communication

A parameter module may measure pressure internally.

Then instead of sending an analog voltage to the host monitor, it sends a digital message.

The message may include:

The host monitor receives data, not just one varying voltage.

Many Devices Use Both

A common path looks like this:

Physical Condition

↓

Analog Sensor Signal

↓

Analog-to-Digital Converter

↓

Digital Processing

↓

Display

This is extremely common in medical equipment.

Example: ECG

The patient's electrical activity is analog.

The monitor:

The signal may begin analog and become digital very early inside the device.

Sensors Often Start Analog

Examples may include:

The physical condition changes an electrical property.

That change is then measured.

But Some Sensors Are Already Digital

Modern sensors may contain internal electronics.

Instead of outputting a simple analog voltage, they may communicate digitally.

Examples can include:

That means you cannot always test them with a simple voltage measurement.

Analog-to-Digital Conversion

An analog-to-digital converter is often abbreviated:

ADC

Its job is to take an analog electrical value and convert it into a digital number.

Example:

Sensor voltage:

2.50 V.

ADC converts that into a digital value.

Software then calculates:

100 mmHg.

Why ADC Matters

If the sensor voltage is correct but the displayed value is wrong, the problem may be farther downstream.

Possible areas include:

This helps separate sensor failure from processing failure.

Digital-to-Analog Conversion

The reverse also exists.

A digital-to-analog converter:

DAC

takes digital information and creates an analog output.

This can be used in control systems and signal generation.

You may encounter the term in schematics or service documentation.

Analog Signals Are Sensitive to Noise

Because analog information is carried in a changing electrical value, unwanted electrical noise can distort it.

Possible noise sources include:

The device may still receive a signal.

It is just a bad signal.

ECG Artifact Is a Good Example

ECG waveform may become:

The cable may still have continuity.

The monitor may still detect the leads.

But the analog signal quality is poor.

That is different from complete communication loss.

Digital Signals Can Also Be Corrupted

Digital does not mean immune to problems.

Digital communication can fail because of:

The difference is that digital systems may detect corrupted information and reject it.

Result

Instead of a slightly wrong waveform, you may get:

Communication Lost.

or:

Module Not Recognized.

Analog Failure Can Be Gradual

An analog problem may produce:

The signal may still exist.

It is simply inaccurate.

Digital Failure Is Often More All-or-Nothing

Digital problems may appear as:

Not always.

But digital failures often behave more discretely than analog measurement errors.

Analog Offset

Suppose a pressure transducer should read:

0 mmHg.

But the device displays:

15 mmHg.

That may indicate an offset error.

The signal exists.

It is shifted.

Analog Gain Error

Suppose:

Expected:

100 mmHg.

Displayed:

80 mmHg.

Expected:

200 mmHg.

Displayed:

160 mmHg.

The error increases proportionally.

That may indicate a gain or scaling problem.

Drift

A measurement may begin correct and slowly move over time.

Possible causes include:

This is a classic analog-type failure pattern.

Digital Communication Errors

A digital system may report:

These messages usually mean the device detected a problem moving or interpreting digital data.

They do not automatically identify which component failed.

Serial Communication

RS-232 is a digital communication method.

Data is transmitted as a timed sequence of electrical states.

Settings such as:

must match.

A perfectly good cable can still fail if the communication settings differ.

USB

USB is also digital.

It involves:

An accessory may receive power but fail to exchange data.

The LED turning on does not prove communication works.

Ethernet

Ethernet carries digital network data.

A cable may have:

while application communication still fails.

Again, one successful layer does not prove the whole path.

Digital Identification

Some accessories contain digital identification.

The device may read:

If that digital identification fails, the accessory may show:

Not Recognized.

even if its sensing element is fine.

Smart Batteries

A smart battery may provide both:

The device might receive battery voltage correctly but lose communication with the battery controller.

That can create:

Power and communication are separate paths.

Continuity Is Not Signal Quality

This is important.

A cable may pass a continuity test and still fail in real operation.

Why?

Because continuity only tells you:

A conductive path exists.

It may not tell you:

A cable can be electrically connected but functionally bad.

Example: ECG Cable

Meter:

Continuity passes.

Clinical signal:

Heavy artifact.

Possible reason:

Shielding damaged.

The simple continuity test did not prove the cable could carry a clean analog signal.

Example: Ethernet Cable

Individual conductors may appear continuous.

But the cable may still have poor high-speed performance because of:

Digital communication has electrical requirements beyond basic continuity.

Sampling

When an analog signal becomes digital, the system measures it repeatedly over time.

This is called:

Sampling.

The sampling rate determines how frequently the signal is measured.

You usually do not need to calculate this during routine service.

But it explains how an analog waveform becomes digital data.

Resolution

An ADC has a finite number of digital steps.

More resolution means the device can distinguish smaller changes.

This matters in measurement-system design.

For basic troubleshooting, the key idea is:

A digital representation of an analog signal still depends on the analog input being correct.

Filtering

Medical equipment often filters analog signals.

Filtering may remove:

But filtering can also affect what the waveform looks like.

Different filter settings may make the same physical signal appear different.

ECG Filter Example

Staff reports:

ECG looks different on this monitor.

Possible cause may be:

before assuming hardware failure.

Configuration can affect signal processing.

Calibration

Analog measurements often require calibration.

Calibration establishes the relationship between:

Electrical signal

and:

Physical value.

Example:

Voltage from pressure sensor

becomes:

mmHg.

If calibration is wrong, the electronics may work perfectly while the displayed value is inaccurate.

Digital Data Can Carry Status Along With Value

A digital sensor may send more than:

37.0°C.

It might also send:

This allows more detailed diagnostics than a simple analog voltage.

Analog Troubleshooting Mindset

For an analog problem, ask:

You are often evaluating signal quality and magnitude.

Digital Troubleshooting Mindset

For a digital problem, ask:

You are often evaluating whether information moves successfully.

Where Does the Signal Become Wrong?

This is the key question for both.

Example:

Pressure source:

Correct.

Sensor analog output:

Correct.

ADC reading:

Wrong.

Now you know the failure appears between the analog sensor and digital processing.

Compare Raw and Displayed Data

Some service modes expose:

These can be extremely useful.

If raw input is correct but the displayed result is wrong, you can move downstream.

Signal Ground and Reference

Analog signals usually depend on a reference.

If the reference is unstable, the measured signal may be unstable.

Possible causes include:

Do not automatically blame the sensor.

Differential Signals

Some analog measurements use two signal lines and measure the difference between them.

This can help reject noise.

ECG and pressure sensing may use differential techniques.

You do not need to master the circuit design to understand that both signal paths can matter.

Digital Logic Levels

Digital circuits use voltage ranges to represent logic states.

For example, one range may represent:

Low.

Another:

High.

The exact voltages depend on the technology.

Do not assume all digital logic is 5 V.

Modern electronics commonly use lower logic voltages.

Oscilloscope Use

Some analog and digital problems cannot be fully diagnosed with a multimeter.

An oscilloscope can show:

But only use it when:

Do not probe random digital buses without documentation.

Real-World Example: Pressure Reading Wrong

Pressure analyzer:

100 mmHg.

Sensor output:

Within expected range.

Device displays:

40 mmHg.

Now investigate:

The physical sensor may not be the problem.

Real-World Example: ECG Artifact

Simulator:

Stable ECG.

Monitor:

Heavy noise.

Known-good trunk cable:

Signal becomes clean.

The analog path through the original cable was degraded.

Real-World Example: Module Not Recognized

Module powers on.

Host reports:

Communication Failed.

Known-good module works.

Original module fails on another host.

The failure is in the digital communication/module path.

Real-World Example: Battery Powers Device but Is “Not Recognized”

Battery voltage:

Normal.

Device runs from battery.

Battery percentage unavailable.

Charging disabled.

That suggests the power path works while smart-battery digital communication does not.

Real-World Example: USB Accessory Lights Up but Does Not Work

Accessory LED:

On.

Device:

Accessory not detected.

USB power is present.

Digital communication is not.

Do not treat the LED as proof the full connection works.

Common Mistakes

Treating Every Signal Like Simple Voltage

Some signals carry encoded data.

Assuming Digital Means Perfect or Broken

Digital systems can still be intermittent.

Assuming Analog Noise Means Sensor Failure

Cable, shielding, grounding, and environment matter.

Assuming Continuity Means Cable Is Good

Signal quality may still be poor.

Ignoring Calibration

Correct electronics can produce wrong displayed values if scaling is wrong.

Probing Digital Lines Without Knowing the Circuit

Use documentation and appropriate tools.

A Useful Troubleshooting Framework

First ask:

Is this function mainly analog, digital, or a combination of both?

Then:

Where does the physical condition become an electrical signal?

Then:

Where does that signal become digital data?

Then:

Where does the value first become incorrect or disappear?

That helps you choose the right test.

Another Useful Question

Ask:

Am I troubleshooting the measurement itself, or the communication of the measurement?

Those can look similar.

Example:

SpO2 value wrong:

Measurement problem.

SpO2 module not recognized:

Communication problem.

Different path.

What Did You Actually Prove?

If you measure the correct analog sensor voltage, you proved:

The expected voltage was present at that point under the tested condition.

You did not prove:

If a digital module is recognized, you proved:

Some level of communication and identification succeeded.

You did not necessarily prove its measurements are accurate.

Keep moving through the signal path.

Final Thoughts for Biomeds

Analog and digital signals are everywhere in medical equipment.

The important difference is not just vocabulary.

It changes how you troubleshoot.

Analog problems often look like:

Digital problems often look like:

And many systems use both.

A sensor may produce an analog signal.

Electronics convert it to digital data.

Software processes it.

Then a network sends it somewhere else.

So when a measurement or communication path fails, ask:

What kind of signal exists at this point?

What should it look like?

Where does it first become wrong?

Once you understand the signal path, a complicated device becomes much easier to reason through.

— Jake

Important Note

Signal types, voltage levels, communication protocols, test points, and calibration procedures vary by manufacturer and design. Follow current service documentation, approved diagnostic methods, electrical-safety procedures, and your authorized service scope when measuring internal analog or digital signals.

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