Analog I/O basic knowledge

This section offers basic knowledge and glossary terms related to analog I/O, A/D conversion, and D/A conversion in an easy-to-understand way that includes illustrations. The content offered here is recommended knowledge for anyone looking to perform measurement control using a computer as well as anyone considering automating data logging or measurements. It may also prove a useful resource for individuals already using CONTEC analog I/O devices.

Contents

What is analog I/O?

The signals from sensors that measure surrounding natural factors such as temperature, pressure, and flow rate are often analog signals, and most control actuators move according to analog signals. On the other hand, only digital signals can be handled by computers. For this reason, in order to input a signal from a sensor using a computer, or to output a signal to an actuator, it's necessary to have a device that can bridge the analog signal and the digital signal handled by the computer. That bridge is called an analog I/O interface.

Analog I/O device classification

Analog input device(A/D conversion)

This device is responsible for converting analog signals from external devices to digital signals that can be processed by a computer.

Analog output device(D/A conversion)

This device converts the digital data from a computer to an analog signal before outputting that signal to an external device.

Analog I/O device(A/D, D/A conversion)

Analog I/O devices are devices with both an A/D conversion function and a D/A conversion function.

Analog to digital, digital to analog.

When inputting an external analog quantity into a computer, in order to display the quantity properly digitally, an infinite number of digits is need for the corresponding digital quantity. This is not possible with a computer capable of handling only a limited number of digits, even from the viewpoint of the circuit technology that makes up the converter.

The most effective way to solve this problem is to keep the number of digits within the acceptable range by rounding or using either the floor or the ceiling. In other words, this means replacing an amount within a certain range with a representative value. This is referred to as quantization.

When quantizing analog values represented by a solid line, you get a stepped line. This makes it possible to express any analog signal using a finite value. This technology is active in things many people are familiar with, for example, in cellular phones. Cell phones make calls by converting voices (analog) to digital sounds.

For example, the following figure shows a staircase in decimal numbers with the first step as 1, as well as the result if the decimal numbers are replaced with binary numbers. In this way, an analog quantity can be digitalized at 4 bits. This is the basic idea behind quantizing analog quantities.

Analog I/O device isolation types

Analog I/O devices can be broadly classified into two categories: isolated and non-isolated. Let's look at the features of two types of isolation. Note that non-isolated types do not employ an isolation element.

Bus isolation

Using a photocoupler, the computer and external I/O circuits are isolated. Because it's possible to prevent electrical interference from being introduced, this method allows for worry-free usability even if noise is easily generated in the wiring or if there is a concern of the computer malfunctioning or becoming damaged.

Independent (channel-to-channel) isolation

In addition to bus isolation, independent isolation uses a photocoupler and an isolation amplifier to add isolation between each input/output channel. This allows interference between channels to be prevented and allows for accurate sampling even if the equipment connected to the various channels have different ground levels.

What is a photocoupler?

A photocoupler is a device that contains a light-emitting diode coupled to a phototransistor. The light-emitting diode outputs light when exposed to an electrical current (about 10 mA). The phototransistor receives this light, which turns the phototransistor ON, which results in an electrical current. Part of this optical signal is electrically insulated from the outside.

What is No. of I/O channels?

Input/output channels refer to the number of signals that can be input or output for a single device. In other words, this shows how many sensors (signal sources) or actuators (controlled objects) can be connected. In the specifications found in catalogs and the like, you may find expressions like X number of single-ended channels or X number of differential channels.

What is single-ended input?

Single-ended input, which uses a signal wire and a ground wire for a two-wire connection, is a method of measuring the voltage of a signal source using the potential difference from ground. For analog input, this is the most common input method and has the advantage of using only two lines for any one signal source. Compared with a differential input, a disadvantage of single-ended input is that it is easily affected by noise.

What is differential input?

Differential input uses two signal wires and a ground wire for a total of three wires for measuring the voltage of a signal source. The signal source potential (A–B) is measured by taking the difference in potential between ground and point A and between ground and point B. In this way, the noise from the ground is cancelled for A–B, which gives differential input the advantage of less susceptibility to noise compared to single-ended input. However, one disadvantage is that three wires are needed for one signal source, meaning that the number of channels that can be used is half that compared with single-ended input.

What is resolution?

Resolution refers to the extent of fineness that the analog signal can be digitally represented (approximately). A high resolution means that the voltage range can be finely divided, resulting in a more accurate conversion to digital values.

Let's look at some example considerations for selecting a device with the optimal resolution from among the wide variety of analog I/O devices.

For example, if "measuring between 0°C and 100°C" ...

Example 1: To measure at units of 1°C
An accuracy of 1/100 is necessary. A device with 8 bits of resolution (2^8 = 256 segments) is sufficient.
Example 2: To measure at units of 0.1°C
An accuracy of 1/1,000 is necessary. A device with 12 bits of resolution (2^12 = 4,096 segments) is required.
Example 3: To measure at units of 0.01°C
An accuracy of 1/10,000 is necessary. A device with 16 bits of resolution (2^16 = 65,536 segments) is required.

What is I/O range?

This is the range of analog voltage or current that can be input or output. A bipolar range covers, for example, from -10 V to +10 V, while a unipolar range would cover only 0 to +10 V. The input/output range should be similar to the output from the sensor and the input to the actuator, or you could choose a device that offers a slightly wider range.

For example, let's assume that we are using a sensor that converts an analog amount to 0 to 5 volts. So, between a device with an input range of 0 to 10 V and one with 0 to 5 V, which is effective? If both have a resolution of 12 bits, the smallest divisible voltage for the 0 to 10 V board is about 2.44 mV (10 ÷ 4096 = about 2.44). With the 0 to 5 V device, the equation becomes 5÷4096, making the smallest divisible voltage 1.22 mV. Because the sensor only outputs 0 to 5 V, the device with an input range of 0 to 5 V will provide the opportunity for finest judgment.

What is gain?

Gain refers to magnification. Some analog input devices are equipped with a function that can amplify the input signal. For example, with an external signal of 0 to 2.5 V, if the input range of an analog input device is 0 to 10 V, amplifying the external signal (the signal being input) by a factor of 4 and then converting the resulting 0 to 10 V signal rather than converting it as is will allow for conversion with higher precision.

What is the conversion rate (sampling cycle)?

The conversion rate refers to the fineness at which an analog signal can be converted to a digital signal over a specific interval of time. It also shows how much time it takes for digital data to be output as analog data. Higher conversion speeds mean conversions with higher repeatability are possible.

Sampling theorem

If sampling cannot be performed with a sampling cycle twice the input/measurement frequency, accurate waveform measurements will not be possible.

What is conversion accuracy?

Conversion accuracy refers to the error range produced when performing A/D conversions or D/A conversions. Errors are represented in units of 1 LSB. For example, the smallest resoluble unit for an A/D conversion device with a resolution of 12 bits set to an input range of ±10 V is 20 ÷ 4,096 ≈ 4.88 mV (1 LSB). If that A/D conversion board's conversion accuracy is noted as ±2 LSB, it means that there is a possibility that errors produced will be about 4.88 × 2 ≈ ±9.76 mV.

  • *
    LSB is an abbreviation of least significant bit and signifies the least significant bit of binary data.

Relationship between binary data and voltage (with 16-bit resolution)

A/D conversion data input from a 16-bit analog input device becomes binary with 16 digits (hexadecimal = 4 digits), and with an analog output device, the D/A conversion data to set is handled with 16-digit binary (hexadecimal = 4 digits). As shown in the following figure, if "0000" is used at "-10 V", then "FFFF" is used at "9.99970 V".

With a 16-bit resolution device, the voltage between -10 V and 0 V can be expressed as 32,768 data segments, where -10 V is "0000H" and 0 V is "8000H." As such, 0 V to 10 V becomes "8000H" to the max. "FFFFH", which must then be expressed as 32,767. For this reason, the maximum value is referred to as "+10 V -1 LSB".

These relationships are the same for any analog I/O device. For example, with analog output, to output a signal externally, "FFFF" is set to a device with 16-bit resolution, but the maximum outputtable voltage is "+10 V -1 LSB".

Voltage meaning Analog voltage (V) DATA (hexadecimal) Binary representation
  • MSB
  • LSB
FSR-1LSB 9.99970 FFFF 1111 1111 1111 1111
FSR-2LSB 9.99938 FFFE 1111 1111 1111 1110




+1LSB 0.00030 8001 1000 0000 0000 0001
0.00000 8000 1000 0000 0000 0000




-1/2FSR+1LSB -9.99970 0001 0000 0000 0000 0001
-1/2FSR -10.00000 0000 0000 0000 0000 0000

Abbreviation (unit) meanings

LSB

Abbreviation of least significant bit. Signifies the least significant bit of binary data.

MSB

Abbreviation of most significant bit. Signifies the most significant bit of binary data.

FSR

Abbreviation of full scale range. In the ±10 V range, FSR becomes "20".

Expressing analog values as digital values

Data converted (quantized) through an analog input (A/D conversion) and data set to an analog output (D/A conversion) can be represented using the following unique code systems. The methods for displaying these systems are as follows.

Straight binary

A voltage of 0 V is given a digital value of 0, and the digital value increases in proportion to the increase in voltage. A unipolar form is used.

Offset binary

The maximum (lowest) value of negative voltage is given a digital value of 0. In this way, 0 V is assigned a digital value in the middle, and the largest positive voltage value is given the largest digital value. A bipolar form is used.

Complement binary(Two's complement)

To obtain the two's complement, the most significant bit of the offset binary code is reversed. A two's complement expression is an easy-to-use code for calculations on a computer. This data format can be found in bipolar forms.

What is the sampling method?

When performing sampling over multiple channels, a multiplexer method using multiplexers (switching units) or a simultaneous sampling method is used.

The multiplexer method performs sampling while switching the multiplexer, meaning that simultaneous conversion of more than one channel is impossible (because time is needed to change between channels).

With the simultaneous sampling method, there are two types: where each channel is equipped with an A/D converter, and where a sample/hold amplifier is equipped. Either type is capable of performing simultaneous conversion over multiple channels.

Conversion rates and Nos. of channels

In a system where the analog input channels are switched through a multiplexer method, to perform sampling over multiple channels, the settable sampling period must maintain the following relationship. Conversion rate × sampling cycle ≤ sampling cycle"

What is a clock?

A clock indicates the timing at which conversion operations of an analog I/O device can be synchronized. For sampling clocks that determine sampling cycles, the following main methods are used.

Internal clock

A timer element capable of setting the sampling period is installed in the device. As a clock source, this is a method for performing periodic conversions. Internal clocks are useful for time-series processing at precise and rapid cycles.

External clock

External clocks can be used for devices equipped with external clock input terminals. Conversion is performed in synchronization with a pulse signal or other signal input from an external source. This method is useful for synchronizing with external devices.

Software clock

A software clock is a method of performing periodic conversions by synchronizing with the system timer on a PC and sending a start command from the software. However, because errors with VisualBasic's timer control and other functions are large, this method is not suitable for systems requiring fast and accurate cycles.

What is a trigger?

A trigger is a source for determining the timing at which to start or stop a conversion. Starting and stopping can both be set independently. The main triggers are as follows.

Software trigger

A software trigger starts/stops conversion operations using commands from software.

External trigger

An external trigger starts/stops conversion operations using an external signal (digital signal). Conversion operation starts or stops when the pre-set edge movement (rising or falling) is input from the external control signal.

Level comparison (conversion data comparison) trigger

Conversion is started or stopped according to the signal changes for a specified channel. A pre-set comparison level is compared with the magnitude of the analog signal for a specified channel, and if the criteria match, the conversion operation starts/stops.

What is buffer memory?

Buffer memory is where conversion data is temporarily stored. Such memory not only enables fast and high-performance analog input processing but also significantly reduces the load on the computer. Depending on the application, buffer memory can employ either the FIFO method or the ring method.

FIFO method

With the FIFO (First In, First Out) method, converted data is stored in the buffer memory in a first-come, first-served behavior with data written to the buffer memory first being read first (in chronological order). The converted data read from within the memory is delivered sequentially, with reading of the oldest conversion data remaining in the buffer memory always readable. Data that exceeds the FIFO memory capacity will be discarded and not written, and data that has been read will be discarded from the buffer memory.

Ring method

The ring method arranges the storage area in the buffer memory like a ring. Conversion data is written sequentially, and when storing beyond the memory capacity, areas where prior conversion data is stored are overwritten. Ring memory is useful when data is not normally obtained but data near a conversion operation stop due to some event should be obtained. With the ring method, once data is captured, it can be read multiple times before it is overwritten.

What is bus master transfer function?

This function is a DMA (direct memory access) transfer that uses a PCI bus master function.

  • Without applying a load on the PC's CPU, data can be transferred from the device directly to the PC memory at a speed of 80 MB/sec (maximum 133 MB/sec).
  • Because the CPU's capacity is not reduced due to data transfer processes and the like, it is able to perform other processes, thus reducing the impact on other applications.
  • The necessary settings for input/output are already set to the device, and because the board performs processing based on that information, it's possible to construct a more efficient system than possible with normal input/output processing.

Normal processing

With normal input/output processing, after the CPU reads the data from the device, it stores the information in the main unit's memory. This intermediate CPU cannot be used for other processes. In the following figure, other processes are only possible after (4) and (5) are finished.

Bus mastering

During bus mastering, the CPU instructs bus master processing for the device, allowing data to be sent to the main unit's memory directly from the device (without going through the CPU). In the following figure, other processes can be performed while (2) and (3) are processing.

What is interruption?

This function generates priority processing externally by connecting a certain input terminal to an IRQ (interrupt request line) on the computer. By detecting changes in external devices, interruption can be used, for example, in applications that perform specific processing and for processing emergency high-priority external commands, to name a few.

What is current consumption?

To operate a device, power is needed, but current consumption indicates how much current that board consumes. This power is usually supplied from the computers expansion bus connector.

This means that the total maximum current consumption of the board should not be more than the rated power capacity of the computer (the maximum current that can be supplied to the expansion slot).

If the rated power capacity is exceeded, the computer's power supply voltage will be reduced, which could result in such trouble as runaway. For this reason, it's necessary to take appropriate countermeasures, such as extending the computer's slots by using an "expansion unit".

Example: The power supply capacity the computer can supply to the expansion slot is 3.6 A.

When mounting two 1.2 A external interface boards, the maximum current consumption is as follows:
1.2 (A) × 2 (boards) = 2.4 A, which is lower than the power capacity of the computer (3.6 A). [Acceptable]
When mounting six 0.8 A external interface boards, the maximum current consumption becomes:
0.8 (A) × 6 (boards) = 4.8 A, higher than the power capacity of the computer (3.6 A). [Unacceptable]

Types of noise and applicable countermeasures

Noise can be roughly divided into the following two types. Unlike electrical testing, various sources of noise are present on-site, resulting in many cases that do not go as expected in theory. In such cases, many causes of unexpected inaccuracy are due to noise.

External noise

  • Noise that is transferred through the air from outside the signal transmission line.
  • Noise that exists around wiring for movement system devices such as motors, and noise that results from wiring located in the vicinity.

Internal noise

  • Noise caused by connection of analog input/output circuits.
  • Noise and offset voltages caused by ground potential differences between devices.
  • Interference noise and crosstalk caused by the wiring material.

Solution

The general rule, particularly when performing measurement, is that noise should not affect the measurement target. To ensure this, it's necessary to take care that impedance, ground levels, and the like, are matching. Familiarizing yourself with solutions is not difficult, but it could be said that failing to consider these solutions can have a large impact. The table below lists some countermeasures against noise.

Hard

Countermeasure Specific example Effect Points to remember
Strengthen signal level Amplification of the measurement point of a weak signal General noise removal Small effect on PC side
Elimination through wiring method Using shielded cables Elimination of incoming noise
Elimination through wiring method Using twisted-pair cables Elimination of crosstalk
Elimination through wiring method Separate wiring for measurement and control system from the power system (power source, GND and piping) Elimination of incoming noise
Elimination by incorporating a filter circuit EMI filter and CR filter incorporation General noise removal Effective when noise frequency is identifiable
Elimination by input/output format Connect with differential input/output Elimination of incoming noise Effective against common mode noise only
Elimination through grounding Connect each device to ground Removal of potential differences between devices Some grounds instead become sources of noise

Soft

Countermeasure Specific example Effect Points to remember
Elimination through averaging calculation(multiple readings) Noise smoothing by moving average Elimination of high-frequency noise Responses become worse as a result of change
Elimination through averaging calculation(multiple readings) Elimination of noise through block averaging Elimination of high-frequency noise Sampling rates become lower
Elimination through software filters Elimination of noise through filter functions Elimination of high-frequency noise Not suitable for real-time processing

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