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I/O FOR EMBEDDED CONTROLLERS

Part 2: Analog I/O
by Bob Perrin

Start ı Single Conditioning ı DACs ı Thatıs All Folks ı Sources and PDF

DACs

Like ADCs, DACs are abundant. You can get DACs with current or voltage outputs, internal or external references, and parallel or serial digital interfaces.

DACs designed for audio applications often do not have the stability and low offset required for control systems. These devices are generally available in 16-bit resolutions at rock-bottom prices. If your application can tolerate, null, or otherwise compensate for these offsets, an audio DAC may be something to consider. The saving grace of audio DACs is the fact that they exhibit monotonic performance.

Cost, space, and power consumption are often the primary considerations for selecting a DAC. Analog Devices has the AD53xx family of DACs. These devices come in SOT-23 six-pin packages and can be had for a song. These parts typically consume less than 150 ıA, and operate over a 2.7ı5.5-V supply range.

Table 1 compares the three devices that I have used in past projects. The data in Table 1 was extracted from the Analog Devices web site. The full AD53xx family currently has nine parts with an additional 15 parts planned. The full table can be found at www.analog.com/support/standard_linear/selection _guides/AD53xx.html

Like any device, the AD5300 parts have pros and cons. The biggest con of these devices is initial offset. The second biggest con is the "relative accuracy" much like integral nonlinearly (INL) for ADCs. In the pros column are size, cost, scalability, 10-ıs settling time, and a 1-V/ıs slew rate. Overall, this family of converters offers a solution for many common applications that require a digitally controllable voltage.

Signal conditioning for analog outputs is a similar problem to single conditioning for analog inputs. The circuit shown in Figure 1 is excellent for mapping a DAC output voltage to a channel output voltage.

Often an analog output is required to deliver more than the few milliamps a jellybean op-amp can deliver. When this happens, you have several options. You can use a discrete BJT or MOSFET follower in the output stage of the your circuit. This option works fine if you only need to deliver a positive or a negative voltage. But if the output is to be bipolar, a simple transistor wonıt work.

Class B amplifiers made from discrete parts have certain difficulties. The plethora of components is expensive. The biggest problem Iıve had is crossover distortion. The easiest way around all this discrete design is to buy an op-amp that has the drive capacity you need.

The Burr-Brown OPA548 is great for creating high-current analog outputs. The device comes in a 7-pin TO-220 or a 7-pin D²PAK. The part can deliver continuous 3 A of current and a peak of 5 A. Burr-Brownıs "budgetary 1000-piece pricing" is $5.45. One drawback of the OPA548 is that it requires a minimum supply rail of 8 V (or ı4 V in a split-rail system).

The OPA548 gives the designer the ability to limit the maximum current with a single resistor. The current limit can also be digitally programmed using an external DAC.

Another important detail to consider when designing analog output stages is power-up state. For example, say you have a DAC that delivers 0ı5 V feeding a circuit (similar to that shown in Figure 1) that maps the 0ı5 V to ı10 V. The power-up state of the DAC may be 0 V. That will map to a +10-V channel output.

If that isnıt acceptable, one solution is to add a tristate to the output with a relay. Figure 4 shows how this scheme works. The pull-down resistor holds the channelıs output at ground when the relay is open.

Figure 4ıThis is a sure way to assure a benign power-up state.

Another solution is to use a flip-flop and a small MOSFET connected to the system RESET to pull a critical node low or high. If this type of scheme is used, you must take into account the various offsets in the output stage. I have found that getting this scheme to initialize a channelıs output to exactly zero can be difficult. If you can live with the small errors associated with this type of scheme, it is a low-cost solution.

The last solution is to select an output amplifier that has the ability to tristate or shutdown its output. The OPA548 has this capability.

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