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.
Small size and high performance are also attributes of Linear Technology's LTC6915A PGA. This instrumentation amplifier comes in a 16-lead SSOP or 12-lead DFN, which allows it to be placed very close to the sensor/transducer. The company claims that it would require about six times the size of a circuit board area if occupied by an equivalent discrete solution. The zero-drift unit features a wide programmable gain range up to 4096 and 0.1% gain accuracy. It also features 50-µV/°C drift and 125 dB of CMRR independent of gain.
For applications in which low distortion is critical, a solution may be found in the HFA11XX family of PGA buffers from Intersil. They feature low distortion levels down to −73 dBc and noise levels down to 7 to 9 nV/√Hz. Texas Instruments chooses to have separate low-noise preamplifier and gain-amplification stages using its PowerPAD package for the THS7001/7002 single/dual PGAs. As a result, the device achieves very low noise levels down to 1.7 nV/√Hz.
Other notable PGAs include Analog Devices' AD628, the first common-mode difference amplifier with programmable gain, and Linear Technology's LTC1564 PGA. Besides the amplifier, the LTC1564 features an eighth-order software-programmable anti-aliasing filter.
PGAs are also integrated with other front-end circuitry, as is the case with Maxim Integrated Products' MAX14XX family of signal-conditioning ASICs that include a PGA. They fit directly between a sensor/transducer and the ADC. Maxim also offers an ADC driver IC, the MAX2055. This low-distortion (−76 dBc 2nd harmonic and −69 dBc 3rd harmonic) driver features a differential output that's specifically designed to drive high-speed ADCs.
Some PGA vendors also offer digital potentiometer and voltage-divider ICs to drive a PGA's front end. Maxim comes in with the MAX5420/21 digitally programmable voltage dividers, and Analog Devices has its AD5321, a digital potentiometer with nonvolatile memory for programmable-gain and attenuation applications.
In addition, Xicor produces a number of PGA devices that are optimized to perform as digitally controlled potentiometers (DCPs). This manufacturer's DCPs come in single, dual, and quad versions.
THE INTEGRATED SOLUTION
Many companies now integrate the PGA and other signal-conditioning circuits directly on the same chip housing the ADC. This approach can be beneficial where space is at a premium, and it can provide higher performance than a separate PGA and ADC. However, it can also cost much more. Besides being less flexible than if the PGAs and ADCs were separate, integrating the PGA on the ADC generally means having to deal with higher clock noise levels.
One of the devices in the MAX14XX family mentioned earlier, the MAX1457 sensor-linear IC with a linearized front end, includes a 12-bit ADC (Fig. 2). Analog Devices offers the ADC7707 high-accuracy signal-conditioning 16-bit sigma-delta ADC, which includes a multiplexer, a buffer, a PGA, a charge-balancing circuit, a serial interface, and a clock generator. A similar unit is available without a multiplexer (the ADC-7715). Analog Devices' ADC7708/18 sigma-delta 6-bit/24-bit ADCs also have the same circuits as the AD7707. But they target low-voltage and low-power applications, as does the ADC7714 24-bit sigma-delta ADC.
An interesting highly integrated ADC is the ADC7731 24-bit sigma-delta unit. It offers all the circuitry of the ADC7707, plus a calibration microcontroller (Fig. 3). This IC targets low-noise high-throughput applications. Maxim also makes available a dual 6-bit ADC with an integrated PGA. This unit, which features a 90-sample/s throughput rate, consumes just 550 mW.
Will we see more ADCs containing PGAs and other signal-conditioning circuits? There's no escaping the fact that high levels of integration are the norm for the IC industry, and PGAs and other signal-conditioning support circuits are no exception. So expect to see the PGA moving onto the same die housing as the ADC as on-chip performance parameters like clock-induced noise are solved and unit ADC prices drop for high-resolution devices.
In fact, designers have the ultimate goal of putting the sensor/transducer circuits on such a highly integrated ADC. That will be the gateway to the final challenge: marrying real-world analog signals with the digital world of computers. It also will bring into sharper focus the two different philosophies of analog and digital circuit design.
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