A signal is an electric current or electromagnetic field used to convey data from one place to another. Signal converters are electronic devices that receive one type of signal and output another type of signal. They can be found in many industrial and commercial applications.

Types of Signal Converters

There are several types of signal converters.

• Analog-to-digital converters (ADCs) sample analog signals and convert them to a series of digital values.
• Digital-to-analog converters (DACs) convert digital numbers into corresponding voltage or current levels.
• Frequency converters or translators convert or scale input frequencies to specific output frequencies.
• Voltage converters accept voltage inputs and provide scaled voltage outputs by using conversion types such as voltage doubling and inversion.
• Frequency-to-voltage converters provide analog voltage outputs that are a function of input signal frequency.
• Voltage-to-frequency converters provide sine wave, pulse train, or pulse width modulation (PWM) outputs that are a function of input signal voltage.
• Current-to-voltage converters produce output signal voltages that are a function of the input current.
• Current loop converters convert an analog or digital signal to a current loop output such as 4 – 20 mA or 0 – 20 mA
• Charge converters are devices that convert the charge output from piezoelectric or other charge-producing sensors to signals such as analog voltage or current.

Performance Specifications

Performance specifications for signal converters include channel specifications, accuracy, resolution, signal specifications, mounting options, and the user interface.

Input Channel Specifications

Signal converters can have either single-ended or differential inputs. The signal converter device can have several channels for these types of inputs.

Analog signals are a wave signal which means that the value changes steadily over time and can have any value in a range. Signal converters with analog inputs can have multiple channels. Channels are either single-end or differential.

Single-end inputs have only one low wire shared by all inputs. For example a board could have 2 single-end inputs, there will be two input lines and one ground line. Single-end inputs are less expensive and allow for twice the number of inputs in the same size wiring connector since they only require one analog input and one ground input which is shared by all the inputs. They save space and are easier to install. When single-ended outputs are available, suppliers often specify the maximum number of analog channel outputs as twice the number of differential outputs.

Differential channelshave two inputs. The voltage is the signal processed between the two inputs. The board will have one signal and one ground pin for each input to allow for measurement voltage difference between two signals tied to the same ground. Differential channels provide excellent common-mode noise rejection. This type of input should be used when EMI, RIF or noise is present.

Digital signals do not have 'in between' values. They are an on or off signal producing a square wave. Digital signals break down the information into binary code, which is a series of 1sand 0s. The data receiver reassembles the code into useful information. Digital signal allows users to send more information in a smaller space.

Analog versus digital signal shapes. Image Credit: mit.gove.in

Accuracy

Accuracy is defined as the difference (error) between the true value and the indication expressed as percent of the span. Accuracy, which is represented as a percentage of a full- scale measurement range, depends on signal conditioning linearity, hysteresis, and temperature considerations. It includes the combined effects of method, observer, apparatus and environment.

Static accuracy is the combined effects of Linearity, Hysteresis, and Repeatability. It is expressed as +/- percentage of full scale output. The static error band is a good measure of the accuracy that can be expected at constant temperature.

Linearity is the deviation of a calibration curve from a specified straight line. One way to measure linearity is to use the least squares method, which gives a best fit straight line. The best straight line (BSL) is a line between two parallel lines that enclose all output vs. pressure values on the calibration curve.

Repeatability is the ability of a transducer to reproduce output readings when the same pressure is applied to the transducer repeatedly, under the same conditions and in the same direction.

Hysteresis is the maximum difference in output at any pressure within the specified range, when the value is first approached with increasing and then with decreasing pressure. Temperatures hysteresis is the sensor's ability to give the same output at a given temperature before and after a temperature cycle.

 Hysteresis is a sensor's ability to give the same output at a given temperature before and after a temperature cycle.

Image Credit: sensorsmag

Resolution

Resolution refers to the degree of fineness of the digital word representing the analog value. The resolution for an analog-to-digital converter is the number of bits used to represent the analog input signal. The greater the resolution, the more accurate the analog signal can be replicated. Resolution is similar but reversed for digital- to-analog converters. They increment the code to a higher resolution DAC which produces smaller step sizes in the analog output.

Signal Specifications

The input signal can have a variety of specifications as it enters the converter. The type of converter selected depends largely on the type of input signal from the system and the desired output signal. The input signal could have properties such as

• DC type voltage and/or current
• AC type voltage and/or current
• Frequency waveforms for varying frequency, pulse or specialized waveforms.
• Charge which comes from a piezoelectric device and usually requires conditioning.

Mounting Options

There are a variety of form factors for signal converters. Devices can be mounted on:

• Standard Deutsches Institute for Normung (DIN) rails are a German standard.
• Printed circuit boards(PCBs) that attach to enclosures or plug into computer backplanes.
• Walls, cabinets, enclosures, or panels with bolts.
• Rack units that fit inside a standard 19” telecommunications rack.
• Modular styles units include stackable units that dock in bays, slots, or boxes.
• Benchtops or freestanding that often feature full casings or cabinets and integral interfaces.

User Interface

Signal converters have several user interfaces available that allow the user to make adjustments to the system.

• A front panel is a local interface with integral controls, a keypad, and/or display on the panel of the unit
• Computer programmable converters are interfaced with a separate supervisory or host computer.
• Touch screens have a visual display which interacts with the user through touch. The user can directly put in information through the contact-sensitive screen.
• Remote and handheld devices can be mobile while the user enters program parameters.
• Devices with no user interface for input or programming are used for storage. The downloading and processing is done in another location.

Resources

TechTarget - Attenuation Defintion

Austerlitz, Howard, and Howard Austerlitz. Data Acquisition Techniques Using PCs. San Diego, CA: Academic, 2003. Print.

Single-Ended vs Differential Inputs

Analog. Digital. What's the Difference?

Maxim Integrated - ADC and DAC Glossary

Image credits:

Advantech Corp. | mit.gove.in | Sensorsmag