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.