Data Acquisition PCB Design for Industrial Systems

A data acquisition PCB in an industrial setting has to measure signals that arrive from sensors tens or hundreds of metres away, in an environment full of motors, contactors and variable-frequency drives. The measurement chain has to survive that environment and still resolve the small changes the process control depends on.

This article covers the front end of an industrial DAQ board, the isolation and grounding measures that make the measurement valid, and the design decisions that determine the noise floor.

What a DAQ Board Has to Do

The board accepts several channels from sensors, conditions each signal, converts it to digital form and passes it to a controller or a host. Depending on the application the channels may be voltage or current inputs, thermocouples, resistance temperature detectors or strain gauges, and each type brings its own conditioning requirement.

The board also has to protect the inputs. A signal that arrives from a remote sensor can carry a common-mode voltage well outside the supply rails, and an industrial environment can impose transients that would destroy a converter input directly. Input protection is therefore part of the signal chain rather than an accessory to it.

Input Protection and Signal Conditioning

Protection begins with series impedance and clamping. A resistor in series with the input limits the current that reaches the clamp, and the clamp limits the voltage that reaches the amplifier. Both have to be placed so that the protected node is actually protected, which means the clamp device sits at the amplifier pin with a short return rather than at the connector.

Conditioning then adapts the signal to the input range of the converter. Instrumentation amplifiers are used where a differential signal has to be extracted from a large common-mode voltage, and their common-mode rejection is only as good as the matching of the resistors around them. Those resistors should be a matched network rather than individual parts, because the rejection depends on their ratio rather than on their absolute value.

Channel Isolation

Where the sensors are grounded remotely, or where their common-mode voltage is unknown, the channels have to be isolated from each other and from the controller. Isolation prevents a ground potential difference between two sensors from driving a current through the measurement ground, which would otherwise appear as an error or as damage.

There are several ways to achieve it: an isolated amplifier per channel, a digital isolator after the converter, or a multiplexed front end with a single isolated converter. The choice affects the cost, the channel density and the achievable sample rate, and it also affects the layout because the isolation barrier has to be maintained across the board with the required creepage and clearance.

Noise Floor and Grounding

The noise floor of an industrial DAQ board is usually set by the grounding rather than by the amplifiers. A ground that carries the return current of a switching supply, a relay coil or a motor driver will develop a voltage across its impedance, and that voltage appears in every measurement that shares the same reference.

The remedy is separation and a single reference point. The analogue section should have its own ground area, joined to the system ground at one place, and the return currents from the digital and power sections should be kept out of it. Where a guard ring is used to protect a high-impedance node, it should be driven from a low-impedance source at the node potential and routed so that it surrounds the node completely. The mixed-signal layout rules describe the arrangement of the returns in detail.

Sampling and Timing

Industrial DAQ systems often sample several channels at once, because the relationships between channels matter as much as the individual values. Simultaneous sampling requires a converter per channel or a set of sample-and-hold circuits that capture at the same instant, and the timing of those captures has to be controlled by a common clock.

The clock itself must be clean and its distribution must be matched, because a skew of a few nanoseconds between channels translates into a phase error in whatever the controller computes from the data. Where the sample rate is modest this is easy; where the system is measuring vibration or electrical power it becomes a significant part of the design.

The Industrial Environment

The board has to keep working while motors start and stop, contactors open and close, and variable-frequency drives operate nearby. Those events produce both conducted and radiated interference, and the board has to tolerate them without producing a wrong measurement.

Filtering at every external connection is the primary measure, combined with a low-inductance connection from each cable shield to the chassis. Coating the assembly improves humidity performance and reduces the leakage that degrades high-impedance measurements, and the coating guidance covers the materials and their application. The supply paths themselves should follow the trace width and current calculation with the derating that an industrial ambient requires.

Calibration and Drift

An industrial instrument is normally calibrated periodically, so the design has to hold its accuracy between calibrations. The main contributors to drift are the reference, the gain network and the temperature gradient across the analogue section.

Using a reference with a low temperature coefficient and gain resistors with matched coefficients removes most of the drift, and placing the critical components close together and away from heat sources removes the rest. Where the board has multiple channels, the channels should be arranged so that they see a similar thermal environment, because otherwise they drift differently and the relationship between them changes.

Mechanical and Environmental Design

An industrial board is often mounted in a DIN rail enclosure or a control cabinet, and the mechanical interface matters as much as the electrical one. Terminal blocks and connectors have to be reachable and labelled, the board has to survive the vibration of the cabinet, and the components have to stay within their temperature range in an ambient that may reach sixty degrees without any airflow.

Terminal blocks deserve particular attention because they are the interface between the board and the field wiring. A screw terminal that loosens under vibration produces an intermittent connection that is very difficult to diagnose, and the layout should allow for retaining features and for a strain relief that keeps the cable weight off the solder joints. Where the wiring carries a signal from a distant sensor, the pair should be terminated so that the shield reaches the chassis through a low-inductance path at the point where the cable enters the enclosure.

FAQ

How many channels can share one converter? Multiplexing is limited by the settling time of the input amplifier and by the sample rate required per channel. Where the channels have to be measured simultaneously, multiplexing is not an option and each channel needs its own converter or sample-and-hold stage.

Is isolation necessary on every channel? Only where the sensors are grounded remotely or where the common-mode voltage is outside the range of the amplifier. Where all the sensors share a common and well-defined reference, a non-isolated front end with good protection is simpler and cheaper.

Why does the reading change when a motor starts? The motor start produces a current transient that develops a voltage across the shared ground impedance. Separating the analogue return from the power return, and filtering at the input, is the correction.