National Semiconductor Corporation introduced the industry’s highest-precision programmable gain amplifier (PGA), enabling a new level of signal-conditioning performance in sensor interface applications and data acquisition systems targeting the industrial and instrumentation markets.

 

National’s LMP8100A delivers guaranteed 0.03 percent gain accuracy using a software adjustable gain (in 1V/V increments) from 1V/V to 16V/V over the full industrial temperature range of -40 degrees C to 125 degrees C. National also offers the LMP8100 semi-precision grade PGA, which provides guaranteed 0.075 percent gain accuracy over the -40 degrees C to 85 degrees C temperature rage. The amplifiers closed-loop gain is set by an array of 16 precision thin-film resistors. At the core of the PGAs is a precision 33 MHz CMOS input, rail-to-rail, input/output (RRIO) operational amplifier with a typical open-loop gain of 110 dB.

 

Many instrumentation products require the designer to scale the output of a sensor to the full-scale input of an analog-to-digital converter (ADC) to ensure optimal sensitivity. National offers this analog signal-path solution for instrumentation products by combining the LMP8100A with a 1 mega-samples per second (MSPS), 12-bit, one-channel ADC -- such as the ADC121S021, ADC121S051 or ADC121S101.

 

 

 

You cannot imagine -- unless you are an applications engineer at an analog semiconductor company -- how many requests I receive asking for directions to a source that can describe how to span the input range of an ADC. With more and more digital design engineers being directed to do just "that little bit" of analog design themselves -- instead of hiring a consultant -- the usual end result is a design that uses a lot more bits in the ADC resolution than were actually needed. A precision monolithic PGA that can help set that spanning range is a much needed device.

 

One of the great benefits of the new parts is that they don't compromise with the "fit all" attitude of gaining from an attenuated signal stage -- the norm for PGAs, especially those combined with an ADC.

 

The parts -- the difference between them being that the LMP8100A offers 0.03% gain accuracy while the spec on the LMP8100 is slackened to 0.075% -- are 33 MHz (unity gain) op amps (12 V/µs slew rate) with an open-loop gain of about 110 dB. The feedback loop consists of 16 cascaded thin-film resistors with switching to change the closed-loop gain from +1 V/V to +16 V/V in 1 V/V steps. The CMOS input resistance appears to unchanged at over 10 GΩ (at 10 Hz). The bandwidth does, of course, fall as the gain increases, reducing to 9.5 MHz at full gain.

 

The part is characterized at both 5 V and 3.3 V with a little over 5 mA quiescent with either rail. Power down current at 5 V is 3.5 µA (typical) and 1.8 µA at 3.3 V.

 

Control of gain, 4 different frequency compensation settings (to maximize frequency response at the higher gains), power down, and an input zeroing switch (allowing the output offset voltage to be measured for system calibration) are on a three-wire serial interface to the double-buffered register. The same data stream can be used to daisy-chain multiple LMP8100s. That situation might arise using two parts as a sensor bridge amplifier or, potentially, in some video application.

 

Apart from spanning an ADC correctly, these parts, the LMP8100A particularly, make for a new era in sensor accuracy -- with the gain error maintained at its low rate over the full temperature range of -40°C to +125°C -- with very good power numbers. Fabricated in National's VIP50 BiCMOS these parts are properly priced and will do extremely well in the market in high accuracy sensor and other data acquisition applications.

 

The LMP8100A and LMP8100 are shipping in SOIC-14 prices at $7 and $2.50, respectively, in 1000-piece lots.