[Technology Report]
Take The PGA Tour Through Analog-Digital Interfacing
As real-world analog signals close in on the ubiquitous digital world of computing, a host of PGA and supporting products is being honed to meet data-acquisition needs.
Lying at designers' fingertips is an ever-widening range of programmable-gain amplifiers (PGAs) to handle the interface between real-world analog outputs of sensors/transducers in data-acquisition systems and the digital world of signal processing. Monolithic and highly integrated PGAs are now replacing modular and hybrid solutions, providing more programmable steps, higher accuracy and throughput, and smaller package sizes (see the online table of representative devices at Drill Deeper 7697).
Due to their very nature, analog signals emanating from sensors and transducers must work with fairly high dynamic ranges. This in turn requires the use of continuous gain stages to boost these signals before any actual digital processing takes place—something that a PGA provides.
PGAs are a subset of variable-gain amplifiers (VGAs). But while VGAs offer variable and continuous gain control, PGAs do so under software control in fixed steps (generally in 6-dB steps). Researchers are working on finer-resolution steps of as much as 0.5 dB, which they think is possible. Impressive gains also are being made in the VGA arena, although this report deals mostly with PGAs (see "A Look At The Larger Picture," p. 54).
Multichannel data-acquisition systems typically use many different types of sensors/transducers, including thermocouples, Wheatstone bridges, thermistors, strain gauges, and ultrasound systems. Though these sensors/transducers are based on a variety of physical principles, most produce a voltage as an output. Even those that produce an intermediate value, such as capacitance or resistance, eventually transform that value into a voltage for further processing in a data-acquisition system (Fig. 1).
Outputs from these sensors/transducers can span a very wide range, requiring a PGA to handle their interface to an analog-to-digital converter (ADC). In industrial process-control systems, for example, low-frequency signals may vary from a few millivolts to several volts. The PGA is needed to match this wide sensor/transducer output range to a particular ADC's input range. Typically, it's needed where the ratio of the lowest signal levels to the highest signal levels on input data-acquisition channels is on the order of 2 or higher. Otherwise, the resolution of the following ADC won't be fully utilized.
A 12-bit ADC accepting a signal that's less than one-tenth of the ADC's full-scale input may provide only 8 bits of resolution, unless it's amplified by the PGA before it reaches the ADC. "A PGA allows the gain of an acquired signal to be under software control with a wide gain-bandwidth product," explains Eric Soule, Linear Technology's product marketing manager, Signal Conditioning Products Group. "This prevents clipping and allows the use of a less expensive ADC, say, a 12-bit unit instead of a 16-bit unit."
But PGAs can do much more. They buffer the ADC's input from the previous stage (usually a multiplexer) to prevent loading caused by the multiplexer's on-resistance. PGAs also provide differential to single-ended conversion, needed for most track-and-hold type ADCs. On top of that, they supply common-mode rejection when connected to the output of differential multiplexers.
Many types of PGAs and support components are available on the market. They include standalone op amps specifically designed as PGAs, ASICs, PGAs integrated with programmable filters, instrumentation-amplifier PGAs, digital potentiometer front ends for op amps, digitally programmable voltage dividers for PGAs, and ADC drivers. In some cases, PGAs are integrated on the same chip as the ADC.
For applications that don't involve a wide dynamic range of signals, the PGA may not be necessary. Amplifier products are available to interface sensors/transducers directly to the ADC. Just one example is the MAX-1494 instrumentation amplifier from Maxim Integrated Products, which is suited for applications involving gain ranges of 250 V/V or less.
RUNNING THE PERFORMANCE GAMUT As can be seen from the online table, PGAs are available with specifications that run the performance gamut. Many of these PGAs are optimized for specific performance parameters, such as high gain stability and accuracy, low drift, low distortion, high output drive current, high slew rates, fast settling times, high levels of common-mode rejection ratio (CMRR), low power drain, and small size.
Some versions, such as the MCP6S2X family of PGAs from Microchip Technology with two-, six-, and eight-channel inputs, include a multiplexer and allow gain control and input channel selection over the serial peripheral interface (SPI) bus. Others, such as the LMH6718 IC from National Semiconductor, are dual PGA chips with high-output (200-mA) drive signals.
If you're looking for high performance in a small package, one option is Analog Devices' AD8555 digitally programmable signal-conditioning auto-zero amplifier housed in a tiny eight-lead SOIC. This package includes the amplifier, a comparator, a resistor trim-pot that uses the company's DigiTrim technology, and a buffer. Its total input offset drift of 50 nV/°C is about one-twentieth that of other competitive products.
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