Plug-in data acquisition boards: D/A boards

D/A boards convert digital signals from a host computer into an analog format for use by external devices such as actuators in controlling or stimulating a system or process. The principle component of all D/A boards is the digital-to-analog converter (D/A Converter).

D/A boards fall into two main categories:

• Waveform generation boards

• Analog output boards

Waveform generation D/A boards:

As their name would imply waveform generation D/A boards are used in the high speed generation of analog waveforms, typically in a laboratory environment for the reproduction or simulation of noise, audio signals, power line signals and also for many other control applications.

The functional diagram of a waveform generation D/A board is shown in ill. F.24 and comprises the following main components:

• D/A converter (DAC)

• Output amplifier/buffer

• FIFO buffer

• Timing system

• Expansion bus interface

Each of these components plays an important role in determining the speed, accuracy, and flexibility with which the D/A board can generate analog waveforms.

ill. F.24 Functional diagram of a waveform generation D/A board

Analog output D/A boards:

Unlike high speed, high resolution, waveform generation boards, more typical analog output D/A boards, as shown in ill. F.25 and used e.g., in industrial control, are not designed for outputting precise waveforms. Instead, they maintain constant output levels unless instructed otherwise. While multi-function data acquisition boards often include two or more analog output channels, applications which require many dedicated analog outputs are most efficiently provided for by dedicated analog output boards.

Whether part of a multi-function DAC board or a dedicated analog output D/A board, the D/A conversion sub-system is straightforward in design and can be divided into two main functional components:

• D/A converter

• Output amplifier and buffer

ill. F.25 Functional diagram of an analog output D/A board

Analog output D/A boards typically have between two and sixteen dedicated output channels, each with its own D/A converter and where required output buffer/amplifier.

F.8.1 Digital to analog converters

Digital to analog converters (D/A converters or DACs) accept an n-bit parallel digital code as input and provide an analog current or voltage as output. The primary output value is a current, however, this is easily converted to a voltage using an operational amplifier. A D/A converter consists principally of a network of analog switches, controlled by the input code, and a network of precision weighted resistors. The switches control currents or voltages derived from a precise reference voltage and provide an analog output current or voltage. The output current/voltage represents the ratio of the input code to the full-scale voltage of the reference source. The main types of current output DACs and their specific important parameters are discussed in the following sections.

Weighted-current source D/A converters

The weighted-current source method of implementing a D/A converter is shown in ill. F.26.

ill. F.26 N-bit weighted-current source D/A converter

This method creates an output current, IT, which is the summation of the weighted currents from each of the parallel transistor sources; the current contributed by each transistor set by the resistances R, 2 R, 4 R, 8 R, etc. The selection of the currents to be summed is determined by the digital code appearing at the input. e.g., if the digital voltage at the MSB is logic low, current will flow through the forward biased diode rather than through the collector of the transistor, and the transistor will remain off.

When the digital voltage at the MSB is logic high, the current flowing through the collector and emitter of the transistor is equal to VREF/R. A stable reference voltage with suitable temperature compensation (base-to-emitter for each transistor) ensures that each transistor produces a constant emitter current inversely proportional to the collector resistance.

Since the output from the inverting summing amplifier is V0 = - IT R/2 the output voltage is directly proportional to the voltage reference according to the equation

V0 = VREF ( B02 -1 + B12 -2 … + Bn-12 n-1 ) Weighted codes other than straight binary can be converted by proper choice of the weighting resistors.

R-2R ladder D/A converters:

A D/A converter which uses resistors of only two values, R and 2 R, is shown in ill. F.27.

ill. F.27 N-bit R-2R ladder D/A converter

Like the weighted-current source network, this DAC produces an output current IT, proportional to the input code and the voltage reference source. The principle of operation of the ladder network relies on the binary divisions of the current as it flows through the ladder resistance network. A simple resistance calculation at point A shows that the resistance to the right adds up to 2 R, and the resistance to the left is 2 R. Hence the current flowing in the resistive leg of the MSB is I0 = VREF/ 3 R. At node A, this current splits, half flowing to the left of node A and half flowing into node B. At node B the current splits in half again, half flowing into node C and half flowing to ground through the resistance 2R in the leg of this, the next most significant bit. This continues, with the current from the LSB being divided by 2 n when it reaches the summing junction of the operational amplifier. The same analysis can be applied for each switch that connects the voltage reference source to the ladder network, with the current contributed by each finally being added at the operational amplifier's summing junction.

The main advantages, which make this type of DAC popular, are the easy matching of resistances (R or 2 R), the constant input resistance for the output amplifier, and the fact that low resistor values can be used, thus ensuring high-speed operations.

F.8.2 Parameters of D/A converters

Most of the performance parameters and errors associated with A/D converters are applicable to D/A converters. In addition, several specifications for D/A converters determine the quality of the output signal produced. These are settling time, slew rate, and resolution.

Resolution:

This is a measure of the size of the output step associated with a change of 1 LSB at the input.

A greater number of bits, in the digital input code generating the analog output, reduce the magnitude of each output voltage increment. This allows the D/A converter to generate a more smoothly changing output signal for applications, where there is a wide dynamic output range.

Output range:

Output from a D/A converter can be in two forms, current, and voltage. If a DAC produces a current output where the application requires an output voltage, an external operational amplifier is required.

The feedback resistors that would be used to set the offset, gain, and therefore range of the output, are usually provided within the D/A converter. Internal resistors are provided which track the temperature characteristics of the internal resistors of the ladder network. This eliminates the need to use an external resistance, which may introduce tracking errors. If more than one feedback resistor is provided, a choice of analog output ranges is available.

Bipolar output voltage ranges are usually obtained by simply on-setting the unipolar offset voltage, with an internal bipolar offset resistor.

The selection and range of a unipolar or bipolar output range is commonly made with jumper connections.

Input data codes:

There are a number of ways in which the digital data can be presented to D/A converters. The type of coding (i.e. binary, binary offset, two's complement, BCD, arbitrary etc), and its sense (positive true and negative true) must be applicable to the D/A converter used.

Settling time:

In a practical D/A converter, there is a limit to the rate at which the converter can acquire new analog output values, because the analog output signal produced takes a finite time to settle to a new value, in response to a change in digital input. The settling time is defined as the time required for the output to reach, and remain, within a given error margin of the final value, usually a percentage of full-scale or ± 1/2 LSB, following a prescribed change at the input (usually a full-scale change). This figure takes into account all internal factors affecting the settling time, i.e. turning the switches on and off; current changes within the resistor network, and the time required by the op-amp or buffer outputs to settle within their error bands.

The settling time of the D/A converter, especially of high speed DACs, is prolonged by the occurrence of sometimes-large transient errors in the output. Glitches are spikes in the output of a D/A converter that may result when, due to the occurrence of an intermediate state, the output is driven toward a value opposite to its final value. An intermediate state is the result of one or more switches in the DAC being faster than the others are. As an example, consider the most major transition of a DAC, when the input changes from 100…000 to 011...111. If the MSB switch changes faster, an intermediate state of 000...000 could occur, momentarily driving the output of the DAC to 0 V before returning the correct value. This is shown in ill. F.28.

ill. F.28 Glitch occurring at the output of a DAC during the major transition

The better matched the switching times and the faster the switches, the smaller will be the energy contained in the glitch. As the size of the glitch is not proportional to the signal change, linear filtering may be unsuccessful and can in fact make matters worse. De-glitchers, in the form of a sample and hold circuitry, are often included as part of the D/A converter, holding the outputs constant at the previous value until the switches reach equilibrium, then sampling and holding the new value. The de-glitcher circuitry, though cleaning up the output, will result in a reduction of the update rate.

Slew rate:

The slew rate is the maximum rate of change that the DAC can produce on the output signal, usually limited by the slew rate of the amplifier used at its output.

Update rate:

The speed or update rate of a DAC is a function of both the settling time and the slew rate and is critical in determining the maximum frequency of an output waveform that can be produced. Therefore, a DAC with a small settling time and high slew rate can generate high frequency signals, because little time is needed to accurately change the output to a new voltage level.

The generation of high frequency signals in the audio range is one application where a high slew rate and small settling time are required to reduce over-tones and interference generated as the output stabilizes. In motion control applications, where the system responds more slowly to the output voltage, the settling time and slew rate are less critical. The motor acts like a damper and reduces the effect of the oscillating output. Another application that does not require fast D/A conversion is a voltage source, which controls a heater, since the heater responds relatively slowly to a voltage change.

F.8.3 Functional characteristics of D/A boards

The important functional characteristics of D/A Boards are given below.

Double buffered input:

Double buffering of input latches internal to the D/A converter allows a complete data word to be stored in the first register buffer, and then transferred to the second buffer for con-version.

This prevents invalid partial data from reaching the DAC and generating spurious output, especially when updating a 12-bit DAC via an 8-bit data bus.

Simultaneous update:

Double buffering of the inputs of DACs used commonly on D/A boards allows the simultaneous update of the outputs of the DAC of each channel. When programmed for simultaneous update, the data written to the registers of the D/A converters has no effect on the output value until the board is commanded to update the output of all channels simultaneously.

Remote sensing:

A remote sensing facility allows the D/A converter of each channel to compensate for voltage drops over long output leads.

Offset and gain adjustment:

Where the output operational amplifier is not provided on the DAC itself, an external amplifier is used. As for instrumentation amplifiers, the settling time and slew rate are the most important parameters to consider, since it's these that affect the performance (update rate) of the analog outputs.

Offset and gain errors from the D/A converter are most commonly adjusted using the offset and gain trims of the output amplifier.

F.8.4 Memory (FIFO) buffer

One of the features that differentiate a waveform generator board from an analog output board is the inclusion of on-board memory, or I/O in the form of a FIFO (first in first out) buffer. This ranges in size from 1024 bytes to 64 Kbytes.

The FIFO buffer(s) form a fast temporary memory area, addressed as I/O that holds a pre-defined array of data points. This gives great flexibility in creating arbitrary waveforms. Once stored in the FIFO, a single cycle of the waveform can be output, or the waveform can be continuously repeated without intervention from the PC. This allows full processor power to be dedicated to other tasks, including calculation of waveform data. It takes only a few milliseconds to load a modified or alternate waveform from memory. In continuous cycling mode, a delay can be programmed between cycles.

Waveform generation boards with more than one channel to be output, either simultaneously or sequentially, require that the digital information stored in the FIFO must also include the address of the channel to which the data is to be output.

F.8.5 Timing circuitry

Analog output boards, which don't have FIFOs don't require timing control circuitry.

Output of any generated wave-shapes is performed by polled I/O. Using this method, to out-put waveforms, does not guarantee strict or accurate timing between the update of the outputs. The maximum update rate of the waveform output is determined by the maximum transfer rate of data to the D/A converters on the board.

Where accurate frequency and amplitude control are required, the update rate of the board must be very accurate and well controlled. High-speed waveform generation boards are provided with either on-board programmable high-speed counter/timer circuitry, to generate precise high-speed conversion probe signals, or facilities at least, to utilize an external signal as the pacer clock. Clocking circuits are made up of a frequency source, which is either an on-board oscillator between 400 kHz and 10 MHz or an external user supplied signal, and a pre-scaler/divider network, typically a counter/timer chip, that slows the clock signal down to more usable values. The clock frequency can be as DC Hz or up to the maximum update rate of the board.

D/A conversions can be initiated by triggers - either by a software trigger (writing to the D/A converter directly) or an external hardware trigger. Data conversions can be synchronized with external events with the use of external clock frequency sources and external triggers. The external trigger event is usually in the form of a digital or analog signal and will begin the acquisition depending on the active edge if the trigger is a digital signal, or the level and slope, if the trigger is an analog signal.

F.8.6 Output amplifier buffer

Operational amplifiers connected to the output of D/A converters are most commonly used where the on-board D/A converter produces a current output and the application requires an output voltage. An operational amplifier connected in the configuration shown in ill. F.26 and ill. F.27 can be used to convert the current to a voltage. Operational amplifiers are also used at the output of D/A converters to provide alternative voltage output ranges or higher current output. The feedback resistors that would be used to set the offset, gain, and therefore range of the output, are usually provided within the D/A converter. This allows accurate tracking of the temperature characteristics of the internal resistors of the ladder network of the DAC and eliminates the need to use an external resistance that may introduce tracking errors. If more than one feedback resistor is provided, a choice of analog output ranges is available. Bipolar output voltage ranges are usually obtained by simply offsetting the unipolar offset voltage, with a bipolar offset resistor internal to the D/A converter.

F.8.7 Expansion bus interface

The expansion bus interface provides the control circuitry and signals used to transfer data from the PC's memory, either directly to the D/A converter or the on-board FIFO, for sending configuration information (e.g. number of times to repeat a waveform or setting the clock source and frequency) or other commands (e.g. software triggers) to the board.

It includes:

• The plug-in connector, which provides the hardware interface for connecting all control and data signals, to the expansion bus (e.g. ISA, EISA or PCI) of the host computer

• The circuitry, which determines the base address of the board - this is usually a selectable DIP switch and defines the addresses of each memory and I/O location on the D/A board

• The source and level of interrupt signals generated. Interrupt signals can be programmed to occur at the end of a single conversion or at the end of one cycle of a waveform - the configuration of the interrupt levels used is commonly selected by on-board links

• The DMA control signals and the configuration of the DMA level(s) used - the configuration of the DMA levels used is typically selected by on-board links

• Normal I/O to and from I/O, address locations on the board

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