Can pressure measurement solutions achieve high precision, stability and low power?

Pressure transmitters are critical precision instruments used throughout industrial process control and factory automation to monitor hydraulic presses, pneumatic systems, steam lines, and pipeline pressure control. These devices measure the force exerted by fluids (liquids or gases) and must deliver accurate, stable measurements across wide temperature ranges and harsh industrial environments. Transmitters that operate on a 4-20 mA loop must also have low power consumption.

A common pressure sensor technology is a strain-gage in the form of a Wheatstone bridge. A typical measurement signal chain for this type of sensor is shown below.

Figure 1. Strain-based Pressure measurement signal chain

As the applied pressure modifies the strain on the bridge elements, the differential voltage varies proportionally.

The signal chain consists of the following functional blocks:

1. Pressure Sensor Front-End – The strain gage sensor(s) resistance may vary from +/- 1.5% to +/- 20%, generating mV-range differential signals, requiring a significant amount of gain to occupy most of the ADC’s input range.
2. Signal Conditioning – The signal conditioning includes a MUX, PGA, and excitation (voltage or current).
3. ADC – The ADC is high resolution, typically 24-bits, often sigma-delta.
4. Voltage Reference – High-stability, low-drift reference for ratio-metric operation
5. Isolation – Power and data isolation, if needed
6. Microcontroller – Linearization, compensation algorithms, and possibly 4-20mA loop control

System Requirements

Pressure transmitter signal chains impose demanding performance criteria:

DC Accuracy and Stability

DC accuracy and stability are key requirements. They are impacted by the amplifier’s initial input offset, offset drift, gain error, gain drift, and 1/f noise. For high impedance strain gages, bias current may also be an error source.

Ratio-metric Configuration

Ratio-metric operation is essential for pressure transmitters, where both the sensor excitation and ADC reference are derived from the same precision voltage source. This provides inherent rejection of supply voltage variations and drift, improving long-term accuracy and avoiding frequent system-level calibration.

Configurable Gain and Sensitivity

Programmable gain allows the signal chain to accommodate sensors with varying output sensitivities without external amplification. This reduces BOM cost, board area, and power consumption.

Power Constraints

Loop-powered transmitters operate from the 4-20 mA current loop itself, with typical quiescent current allocation of 4 mA. To meet this constraint, the ADC must consume <250 µA, necessitating aggressive power management strategies including FIFO buffering, duty cycling, and low-side power switches.

Analog Devices Low Power High Precision Pressure Measurement Solution

The ultra-low power AD4130-4 is a compact, complete signal chain solution in a small package with integrated signal conditioning (PGA, FIFO, current sources, bias generation) that reduces external component count by >50% versus discrete solutions.

Figure 2. AD4130-4 Pressure Measurement solution

A buffered precision voltage reference that is capable of being externally routed to provide sensor excitation, supporting ratio-metric operation and achieves 2 ppm/°C drift (typ.).

A key feature of the AD4130-4 is its built-in calibration capability that enables either device level or system level calibration. With this feature, static offset and gain errors can be reduced to the level of RMS noise.

The AD4130-4 active current is 32 µA, consuming 0.096 mW @ 3V, making it ideal for 4mA loop-powered transmitters. The integrated FIFO enables efficient burst-mode data capture, while duty cycling further reduces average power by powering down between conversions.

Other key features include:

• Integrated PGA with gain of 1 to 128
• With adjustable output data rates, selectable digital filtering, and programmable gains, the effective resolution can range up to 20+ bits.
• AIN buffers to minimize sensor loading
• Low-side power switch for sensor power management

To meet even more demanding stability over temperature, the AD4130-4 can be paired with the ADR4525 voltage reference. It offers high initial accuracy (±0.02% max.) and temperature drift as low as 2 ppm/°C for the industrial grade temperature range.

To achieve ratiometric measurements, the external reference can be used as excitation for the strain sensors, buffered by the LTC2050 low noise zero-drift op amp. For use cases where isolation is required, the ADuM341E is used to isolate the digital interface.

 The Signal Chain Designer tool link below illustrates a solution that combines the ADA4254 instrumentation amplifier with the AD4130-4 to achieve more gain. However, the ADA4254 can be omitted if the additional gain isn’t needed.

Pressure measurement signal chain with additional gain.