1 Application Brief

Introduction

Pressure transmitters are used to monitor weight, level, force and flow in industrial process control systems. The primary challenges encountered when designing pressure transmitters are nonlinearities and temperature dependence. Overcoming these challenges requires the proper implementation of an appropriate operational amplifier (op amp) and a deep understanding of, the precision, voltage range, and drive characteristics of the op amp.

Pressure Sensors

The four most common types of pressure sensors used in pressure transmitter applications are: Resistive, Capacitive, Silicon, and Linear Variable Differential Transformer (LVDT) with resistive strain gauges being the most commonplace. These four variants all use similar paths in the pressure transmitter signal chain. #X1799 illustrates a block diagram of a typical pressure transmitter signal path.

Figure 1-1 Common Pressure Transmitter
Block Diagram

Various sensors and interfaces call for diverse device implementations.

• Instrumentation amplifiers, also known as INAs, are used to amplify small differential signals in the presence of larger or noisy common-mode signals
• Programmable-gain amplifiers, also known as PGAs, can provide higher gain accuracy using selectable gain steps
• Capacitive, LVDT and silicon sensors require implementation of a driver amplifier, which provides the excitation required for the sensor to capture changes in pressure or displacement.
• Voltage-to-current (V/I) converters are used to interface with commonly used 4–20-mA current loops.

Instrumentation Amplifiers

The output signal from the sensing element requires amplification prior to digital conversion as sensor signals are typically very small and can commonly be in the low single millivolt range. Instrumentation amplifiers provide the necessary amplification of differential signals while providing low noise and good rejection of the common-mode signals.

The INA819 instrumentation amplifier with super-beta input transistors provides exceptionally low input offset, offset drift, noise, and input bias current to help maintain signal accuracy when interfacing sensing elements. In addition, the wide supply (4.5 V to 36 V) and input overvoltage protection up to 60 V beyond the supplies enables a wide variety of sensing elements.

Driver Amplifiers

Capacitive-, LVDT- and Silicon-based sensing elements require excitation by an op amp with sufficient drive characteristics necessary to enable the sensing element to capture minute changes in pressure and displacement. The important design parameters for these op amps are output current and capacitive load drive. The OPA196 op amp offers sufficient output current (65 mA) and high capacitive load drive (1 nF) capability making the device a good choice for this application.

Voltage-to-Current Converters

A common interface in pressure transmitter applications is the 4–20-mA current loop, which uses a precision, low-power op amp to create a measurable current range from a voltage range. The requisites of the op amp in this application are low power and high precision. The OPA391, an e-Trim™ op amp, features 45-µV maximum offset and 24 µA of quiescent current, making the device a good candidate for such requirements.

An integrated solution can also be implemented using an XTR115, XTR300, or XTR305, which are precision current output converters designed to provide signal conditioning of low level signals and translation of them to a robust, noise-immune 4–20-mA transmission.