There's more to data acquisition than A/D.

Over the years, much I has been written about the specifications and usage of analog inputs in data acquisition (DAQ) systems. However, in most applications, the analog input is only a piece of the hardware puzzle. Often overlooked are the key attributes of and specifications for the other parts of the system: the analog outputs, digital I/O, and communications interfaces.

Almost all systems require one or more D/A, DIO, and COM interfaces. How they are integrated into the system often is a critical aspect of overall system functionality.

Analog Outputs

Analog or D/A outputs are used for a variety of purposes in DAQ and control systems. To properly match the D/A device to your application, it is necessary to consider a variety of specifications.

Number of Channels

Make sure you have enough outputs to get the job done. If it's possible that your application may be expanded or modified in the future, you may wish to specify a system with a few spare outputs. At the very least, be sure you can add outputs to the system later without major difficulty.

Resolution

As with A/D channels, the resolution of a D/A output is a key specification. It describes the number or range of different possible output states the system is capable of providing. This specification is almost universally provided in terms of bits where the number of output states is defined as [2.sup.(# of bits)]. For example, 8-bit resolution corresponds to a resolution of one part in [2.sup.8] or 256. Similarly, 16-bit resolution is one part in [2.sup.16] or 65,536.

When combined with the output range, the resolution determines how small a change in the output may be commanded. To determine the size of this change, simply divide the full-scale range of the output by the number of output states. A 16-bit output with a 0 to 10-V full-scale output provides 10 V/[2.sup.16] or 152.6-[micro]V resolution. A 12-bit output with a 4- to 20-mA full scale supplies 16 mA/[2.sup.12] or 3.906-[micro]A resolution.

The standard resolution of most DAQ D/A output interfaces is 16 bits, and you also will see some devices with 12-bit resolution. Although it is common now to see analog inputs with 20-bit or 24-bit resolution, D/A resolutions of greater than 16 bits are fairly rare in applications where DC accuracy is important. However, they are common in AC applications such as in the audio output world.

Accuracy

Accuracy often is equated with resolution, but they are not the same. An analog output with 1-[micro]V resolution does not necessarily mean the output is accurate to 1 [micro]V. Outside of audio outputs, D/A system accuracy typically is on the order of a few least significant bits (LSBs). However, be sure to check this specification because all D/A products are not created equal.

The primary error contributors of a D/A output are output offset, gain error, reference error, and nonlinearity, as illustrated in Figure 1. Both gain and reference are shown on a single graph because both contribute to an undesired change of slope in the output diagram. Remember that these errors are additive, and to get the overall system accuracy, you must account for the contributions from all error sources.

[FIGURE 1 OMITTED]

Additional contributing error factors that must be taken into account are more application specific than product specific. Errors may be generated by the D/A channel's output impedance as well as IR drops induced in the field wiring since neither the current flowing in the field wiring nor the resistance of the wiring is zero.

Both output impedance and IR errors manifest themselves when the D/A channel is required to drive a significant output current. Ohm's Law dictates that the error generated will be the product of the channel's output impedance, plus the resistance in the field wiring, multiplied by the current flowing. The equation for this error is

Resistance Error = (D/A Output Impedance + Field Wiring Resistance) (Current Flow)

In many applications, the device the output is driving is high impedance and the current is so low that this error is negligible. However, many D/A outputs can drive 5 mA or 10 mA or even more.

If your application requires output drive in the milliamp range or higher, check this error. D/A output impedances typically are on the order of 0.1 [ohm]. A 10-mA signal flowing through 0.1 [ohm] generates a 1-mV error signal. The resolution of a [+ or -]10-V, 16-bit, D/A output channel is 305 [micro]V so a 1-mV error actually represents an error of greater than 3 LSBs.

More insidious than the channel output impedance is the IR drop in the field wiring. While many people simply assume the resistance is low enough to have no impact, this often is not the case.

Note that 26- and 30-gauge, single-conductor copper wire have resistance of about 0.026 [ohm] per foot and 0.105 [ohm] per foot, respectively. If your output is driving 5 mA and connected by 50 feet of 30-gauge wire, you'll see an IR drop in the field wiring of about 53 mV.

In our typical case of a 16-bit D/A output with a [+ or -]10-V output range, 53 mV is about 173 LSBs. Table 1 shows the IR error induced in a number of different combinations of wire size, output current, and cable length.

There are three options for reducing this IR drop error. First, you can minimize the distance between the D/A output and the device it is driving. Second, you can increase the size of your wire to reduce the series resistance. However, it is not always possible to do either of these, which leads to option three: use a board with sense leads or connections. The sense capability is designed to automatically compensate for IR losses in the system.

Basically, the sense leads are connected in parallel with the main D/A output leads but do not conduct any current. This allows the D/A converter to adjust its output so the voltage at the load is the desired level and not the output at the D/A converter itself. Many D/A output devices, particularly those designed to drive higher currents, will have sense leads that may be used.

Monotonicity

Presumably, if you command your output to go to a higher voltage, it will do so regardless of the overall accuracy. However, this is not necessarily the case. D/A converters exhibit an error called differential nonlinearity (DNL).

In essence, DNL error represents the variation in output step size between adjacent codes. Ideally, commanding the output to increase by 1 LSB would cause the output to change by an amount equal to the overall output resolution. However, D/A converters are not perfect, and increasing the digital code written to a D/A by one may cause the output to change 0.5 LSB, 1.3 LSB, or any other arbitrary number.

A D/A channel is said to be monotonic if every time you increase the digital code written to the D/A converter the output voltage does indeed increase. If the D/A converter DNL is less than [+ or -]1 bit, the converter will be monotonic.

If not, commanding a higher output voltage could cause the output to drop. In control applications, this can be very problematic because it theoretically becomes possible for the system to lock onto a false set point distant from the one desired.

Output Type

Unlike a myriad of sensor-specific input configurations for A/Ds, D/A outputs typically come in two flavors: voltage and current. Be sure to specify the right type for your system. Some devices offer a mixture of voltage and current outputs although most have only a single type.

If your system requires both, you may want to consider a current output since the current outputs often can be converted to a suitable voltage output with the simple installation of a shunt resistor. The accuracy of the shunt resistor-created voltage output is very dependent on the accuracy of the resistor used.

Also, the shunt resistor will be in parallel with any load or device connected to it. Be sure the input impedance of the device driven is high enough to not impact the shunt function.

Output Drive

Be sure to investigate how much current is required by whatever device you are attempting to drive with the D/A output channel. Most D/A channels are limited to less than [+ or -]5 or [+ or -]10 mA maximum.

Some vendors offer higher output currents as standard. For higher output drive, it often is possible to add an external buffer amplifier. If you are driving more than 10 mA, you will likely need to specify a system with sense leads if you need to maintain high system accuracy.

Output Range

The output range must be matched to your application requirement. Like its A/D sibling, it is possible for a D/A channel to drive a smaller range than its maximum although there is a reduction of effective resolution.

Most D/A output modules are designed to drive [+ or -]10 V; however, some will directly drive outputs up to [+ or -]40 V. Still higher voltages may be accommodated with external buffer devices. At voltages greater than [+ or -]40 V, safety becomes an important factor.

A final note regarding increasing the output range of a D/A channel: If the device being driven is either isolated from the D/A output system or if it utilizes differential inputs, it may be possible to double the effective output range by using two channels that drive their outputs in opposite directions.

Output Update Rate