Knock out the noise! Create a cleaner PLC design


Used in factory automation, programmable logic controllers (PLCs) are basic necessities of any industrial automation design. Simply put, they are industrial computers specialized to control machines and processes and are designed to work in rugged industrial environments.


Programmable Logic Controllers

PLCs come in all shapes and sizes but can mainly be segmented by their input/output (I/O) count: Some come in fixed casing with a predetermined I/O count, while others are modular with extendable I/O capability. Regardless of type, a PLC mainly consists of:

  • The central processing unit (CPU), which controls logic, monitoring and communications.
  • The power supply, which supplies regulated DC power (from line power or 24V DC) for the CPU and I/O modules as needed.
  • The I/O modules, which measure and collect data and control motors and actuators. The selection and building of I/O modules varies based on the type of operation needed.

In this blog post, I’ll focus on the analog I/O modules of the PLC and the use of high-performance low-dropout regulator (LDO) solutions to deliver optimum performance.

Analog input modules convert voltage or current signals to a digital number for use by the CPU; these modules can measure signals like pressure, flow and temperature etc. Analog output modules convert digital information from the CPU into voltage or current signals that can control actuators, motors, relays, flow, etc.

The I/O modules use 4-20mA current loops. These current loops are standard for transmitting signals over some distance with little signal loss because signal accuracy is not affected by any voltage drop in wiring. The current-loop transmitter maintains the proper amount of current up to its maximum voltage capability.

Voltage inputs in I/O modules can typically range up from a few millivolts to +/-10V, depending on sensor types. PLC applications can use a variety of data converters (analog-to-digital [A/D] and digital-to-analog [D/A]) ranging from 12 bits to 24 bits, depending on accuracy and linearity requirements. Along with data converters, instrumentation amplifiers (INAs) and precision operational amplifiers are also used to maintain low offsets when extracting small signals from sensors like thermocouples.

 

Figure 1: Block Diagram: Analog Input and Output Module

As these I/O modules interface with the field, they have stringent safety requirements for electrostatic discharge, electrical fast transients or surge events. You must have isolation between the field side and control side using a DC/DC converter and discrete or isolated DC/DC modules.

Because the I/O modules have high-resolution and accuracy requirements, you will have to confirm that power-supply noise from the DC/DC is not deteriorating signal-chain performance. Any noise on the analog reference will cause a change to the digitally converted value, causing a high-resolution data converter to have reduced performance. Figure 1 shows a typical input (left) and output (right) module block diagram with isolated power, low noise power to clean the supply, and signal-chain components.

In general, when powering sensitive analog circuits, using a high power-supply rejection ratio (PSRR) low-noise LDO (see the example below with the TPS7A49, TPS7A30) for post-regulation will help clean the switching ripple and noise, provide clean power to the signal-chain circuits, and improve measurement performance. As a post-regulator, the high PSRR LDOs attenuate the DC/DC converter’s switching frequency ripple and any superimposed harmonics, and eliminate the need for bulky passive filtering. The LDOs also maintain output regulation from light- to full-load conditions. You can use the NR/SS pin to reduce the intrinsic noise of the LDO and provide soft start at the output. You can also use the enable pin and soft start for sequencing the supply rails based on system requirements.

 

Figure 2: Low Noise Power for Precison Analog using TPS54060, TPS7A49, TPS7A30

TI offers several high-voltage, high-performance LDOs to generate clean bipolar supplies. As shown in Figure 2, the TPS7A49 paired with a negative-voltage TPS7A30 can provide a 150mA/channel solution and features high PSRR performance with low noise. For applications with requirements greater than 150mA, consider a TPS7A47 paired with a TPS7A33.

Try one of these reference designs: