This guide explains how to measure analog signals with LabJack devices more accurately by understanding impedance, settling time, multiplexing, resolution, noise, grounding, and signal conditioning. Learn practical ways to build reliable DAQ systems.
Practical Guidance on Impedance, Settling, Multiplexing, Resolution, and Signal Conditioning
Measuring analog signals accurately is one of the most fundamental tasks in data acquisition, yet it is also one of the easiest to get wrong. Engineers often focus on sensor specifications or nominal DAQ resolution while overlooking system-level effects such as input impedance, settling time, channel multiplexing behavior, and signal conditioning. With LabJack devices, analog inputs are extremely flexible, but that flexibility requires an understanding of how real-world signals interact with the analog front end.
This article is a practical, engineer-focused guide to measuring analog signals with LabJack devices, with emphasis on the issues that most often affect accuracy and repeatability in real applications. Rather than presenting theory in isolation, it explains how impedance, settling, multiplexing, resolution, and signal conditioning work together in practice. The goal is to help users design analog measurement systems that behave predictably and produce trustworthy data.
From hands-on experience supporting LabJack users across test, automation, and research environments, analog measurement problems are rarely caused by defective hardware. More often, they result from subtle interactions between the signal source and the DAQ input. Understanding those interactions is the key to accurate measurement. For additional guidance, the Analog Input App Note and the Analog Inputs T-Series Datasheet are great companion references as you work through this guide.
How LabJack Measures Analog Signals
LabJack devices measure analog signals by sampling voltages presented at their analog input terminals and converting those voltages into digital values using an analog-to-digital converter (ADC). While this process sounds straightforward, several intermediate steps occur internally that influence measurement behavior.
Key elements of the analog input path might include:
Input impedance and buffering
A sample-and-hold circuit
Channel multiplexing
ADC conversion
Each of these stages introduces constraints that must be respected. When analog signals are slow-moving, low-impedance, and well-conditioned, these constraints are rarely noticeable. As signal sources become higher impedance, faster changing, or more numerous, system behavior becomes more complex.
Input Impedance and Why It Matters
Input impedance describes how much the DAQ input loads the signal source. Ideally, an analog input would have infinite impedance and draw no current. In reality, all DAQ inputs sink or source some current, and that current can affect the measured signal.
High-impedance signal sources—such as voltage dividers, sensors with resistive outputs, or passive filters—are particularly sensitive to loading. If the DAQ input impedance is not sufficiently higher than the source impedance, the measured voltage may be lower than expected.
In practical terms:
Low source impedance generally produces accurate readings with minimal concern
High source impedance requires careful consideration of buffering and settling time
From real deployments, unexplained voltage offsets are often traced back to impedance mismatch rather than calibration or noise issues.
Settling Time and Sample-and-Hold Behavior
Before each ADC conversion, the LabJack’s sample-and-hold circuit must charge to the input voltage. This process takes time, known as settling time. If the circuit does not fully settle before conversion, the measured value will be inaccurate.
Settling time is influenced by:
Source impedance
Input capacitance
Channel switching behavior
High source impedance increases settling time because it limits how quickly the sample-and-hold capacitor can charge. When measuring a single channel repeatedly, this may not be noticeable. When multiplexing between channels, insufficient settling time can cause channel-to-channel interference.
In practice, engineers often encounter settling issues when:
Switching rapidly between analog channels
Measuring high-impedance sources
Using short acquisition intervals
Allowing additional settling time or reducing source impedance typically resolves these problems.
Multiplexing Effects and Channel Interaction
Most LabJack devices use a multiplexed ADC, meaning multiple input channels share the same converter. While this approach is efficient and flexible, it introduces important considerations when measuring multiple signals.
When the multiplexer switches from one channel to another:
Residual charge from the previous channel may remain
The sample-and-hold circuit must settle to the new voltage
If the previous channel voltage differs significantly from the next, and the settling time is insufficient, the second measurement may be biased toward the first. This effect is often called ghosting or channel bleed-through.
Mitigation strategies include:
Grouping channels with similar voltage levels
Adding settling delays between channel reads
Lowering source impedance
From experience, simply reordering channel scans often produces immediate improvement without hardware changes.
Resolution vs. Accuracy: Understanding the Difference
Resolution refers to the smallest voltage change the ADC can theoretically detect, often expressed in bits. Accuracy describes how close the measured value is to the true voltage. These two concepts are related but not interchangeable.
High resolution does not guarantee high accuracy. Accuracy is influenced by:
Input noise
Gain and offset errors
Reference stability
Signal conditioning quality
LabJack devices offer configurable resolution and gain options, allowing users to trade conversion speed for noise performance. Selecting the highest resolution is not always the best choice, especially if the signal is noisy or poorly conditioned.
In practice, matching resolution settings to signal characteristics yields better results than defaulting to maximum resolution.
Noise Sources in Analog Measurements
Noise is an unavoidable part of analog measurement. Understanding where noise comes from helps determine how to mitigate it effectively.
Common noise sources include:
Electrical interference from nearby equipment
Ground loops and reference issues
Thermal noise in high-impedance sources
Noise becomes more apparent as resolution increases. A high-resolution measurement of a noisy signal may look worse than a lower-resolution measurement with proper filtering.
From field experience, many “noisy” measurements are actually stable signals observed without adequate filtering or averaging.
Signal Conditioning: When and Why It’s Needed
Signal conditioning refers to modifying a signal before it reaches the DAQ to make it easier to measure accurately. While LabJack inputs are versatile, they cannot compensate for all signal deficiencies on their own.
Common signal conditioning techniques include:
Buffering with an op-amp
Voltage scaling and attenuation
Filtering unwanted frequency components
Buffering is especially important for high-impedance sources. A simple voltage follower can dramatically reduce settling time issues and improve measurement repeatability.
Filtering is useful when signals contain high-frequency noise that is not relevant to the measurement objective. Proper filtering improves effective resolution and stability.
Grounding and Reference Considerations
Analog measurements are always relative to a reference. Poor grounding and reference design are among the most common causes of unstable or incorrect readings.
Key grounding considerations include:
Avoiding ground loops
Using a consistent reference point
Minimizing shared return paths
Differential measurements can help reduce noise and offset issues, especially when measuring small signals in noisy environments. Man LabJack devices support differential configurations that improve common-mode noise rejection when used correctly.
Sampling Rate and Its Interaction with Accuracy
Sampling rate affects more than temporal resolution. It also interacts with noise, settling, and multiplexing behavior.
High sampling rates:
Reduce settling time per sample
Increase sensitivity to noise
Increase data volume
Lower sampling rates:
Allow more settling time
Enable averaging and filtering
Improve stability for slow signals
Matching the sampling rate to the physical behavior of the signal is a critical design decision. Faster is not always better.
Practical Design Workflow for Analog Measurements
A structured approach to analog measurement design reduces trial-and-error and improves results.
A practical workflow includes:
Characterizing the signal source impedance
Selecting an appropriate input configuration
Determining required accuracy and resolution
Adding signal conditioning as needed
Testing with a single channel before scaling to multiple channels helps isolate issues early.
Common Mistakes and How to Avoid Them
Some analog measurement pitfalls appear repeatedly across applications.
Common mistakes include:
Ignoring source impedance
Switching channels too quickly
Assuming higher resolution equals better data
Avoiding these mistakes usually requires slowing down, simplifying the setup, and validating assumptions with measurements.
Scaling Analog Measurement Systems
As systems grow, analog measurement challenges compound. More channels increase multiplexing effects, and longer cables introduce noise and impedance concerns.
Scaling best practices include:
Grouping similar signal types
Standardizing wiring and conditioning
Documenting channel configurations
Systems designed with scaling in mind are easier to maintain and troubleshoot.
Designing for Repeatable, Trustworthy Measurements
Reliable analog measurement is not about maximizing specifications. It is about building systems that behave consistently under real operating conditions.
LabJack devices provide the flexibility needed to measure a wide range of analog signals, but that flexibility must be paired with thoughtful design. When impedance, settling, multiplexing, resolution, and signal conditioning are addressed holistically, analog measurements become predictable and trustworthy.
Engineers who invest time in understanding these interactions are rewarded with cleaner data, fewer surprises, and systems that perform as expected over time.
Frequently Asked Questions About Measuring Analog Signals with LabJack
1. Why do my analog readings change when I add more channels?
This is often caused by multiplexing and insufficient settling time, especially with high-impedance sources. It can also be a sign that a signal is floating and not properly referred to the LabJack.
2. How do I know if my signal impedance is too high?
If readings vary with channel order or improve when sampling is slowed, impedance is likely a factor.
3. Does higher ADC resolution always improve accuracy?
No. Resolution increases sensitivity to noise. Accuracy depends on the entire measurement chain.
4. When should I use signal buffering?
Buffering is recommended for high-impedance sources or when fast channel switching is required.
5. Should I use differential or single-ended inputs?
Differential inputs are preferred for small signals or noisy environments, while single-ended works well for low-noise, low-impedance sources.
For additional technical guidance, signal integration resources, and DAQ support, contact LabJack.