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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

In Part 1, I covered digital inputs, outputs, and I/O points, as well as bridging the gap between relatively fragile high-speed CMOS devices and the real world. In this article, I discuss the basics of analog interfacing.

Embedded systems are used in so many different applications, itıs impossible to cover all the analog I/O requirements. Here, I simply offer a few circuits and components that have proven adequate for many applications in the past.

ADCs

Acquiring analog signals requires an analog to digital converter (ADC). There are more ADCs on the market than can be counted in a week. When Iım in the market for an ADC, I usually look at Analog Devices, Burr Brown, Linear Technologies, TI, and National Semiconductor, though not necessarily in that order.

I currently have a few favorite devices. For inexpensive 12-bit conversion, Iım partial to the Texas Instruments TLC2543. This device has an internal 11-channel analog multiplexer, comes in several 20-pin packages, and costs around $4.80.

For applications requiring a fast 12-bit converter in a tiny package, Iım fond of the Analog Devices AD7887. This device is available in an 8-pin Micro-SOIC or Narrow-SOIC package. The Micro-SOIC is particularly impressive with its 0.025" pitch leads and 0.196" ı 0.244" footprint.

The AD7887 has one or two analog inputs depending on how you configure the device. It will sample at 125 kilosamples per second and operate on supply rails between 2.7 and 5.25 V.

The one shortcoming of the AD7887 is the lack of an internal voltage reference. Analog Devices designed this part to be low power enough that the AD7887ıs V_DD pin can be connected directly to the output of a reference, which means the external reference can not only be used as a voltage reference but also as a voltage regulator. This scheme provides excellent decoupling from noise that may be on the raw power supply rails.

The ChipCenter Supercatalog gives a quote from Avnet of $4.90 for the AD7887AR, which seems a bit expensive for a single-channel 12-bit converter. But given the other features the AD7887 offers, five bucks is a great deal.

There is currently a whole slew of 24-bit delta-sigma converters on the market. The AD77xx family is possibly the most well-known family of 24-bit converters. The family has parts that are intended to interface directly to several common classes of sensors.

Burr Brown has the 24-bit ADS1211, which is a full-featured yet inexpensive part. I used this part in a project and was quite happy with the price/performance ratio. The project only required 16 noise-free bits, which was easily achieved with the ADS1211.

Burr Brown tried to roll so many features into the ı1211 that the serial communications protocol is a bit overwhelming at first glance. There are several options for configuring data I/O lines and the self-clocked versus externally clocked protocol.

The ı1211 also has many options for configuring the converterıs internal sampling, filtering, calibration, and gain characteristics. All of these are controlled through the serial port. The ı1211 has an internal 4-1 analog mux and comes in a 24-pin package. The mux actually has eight inputs and two outputs. Internally the ADS1211 has a true differential input, with the mux, this gives the user four channels of true differential analog input.

Although itıs a sophisticated device, the ı1211 is low in cost. To assist developers, Burr-Brown has published several meaty application notes focusing on various aspects of the ADS1211. There are other members in the ADS121x family. The app notes cover all variants equally well.

If physical space is a concern, I recommend looking at the LTC2400. This is a micro-power (200 ıA) delta-sigma 24-bit ADC in an 8-pin SOIC. The LTC2400 doesnıt require an external oscillator, although one may be used. This ADC has one single-ended analog input. A bigger brother, the LTC2408, has eight inputs and is in a 28-pin SSOP.

The LTC2400 claims to be insensitive to PCB layout. The cute little demo board available for the LTC2400 is a two-layer board with only a single ground plane. As a further demonstration of the LTC2400ıs ability to deliver high-performance in a relatively noisy environment, the demo board derives its power from the RS-232 connection with its host PC. All things considered, the LTC2400 does an excellent job, in a small space, at a low cost.

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