GENERAL DESCRIPTION

This family of amplifiers has ultralow offset, drift, and bia scurrent. The AD8571/AD8572/AD8574 are single, dual, and quad amplifiers, respectively, featuring rail-to-rail input and output swings. All are guaranteed to operate from 2.7 V to 5 V single supply. The AD8571/AD8572/AD8574 provide benefits previously found only in expensive auto-zeroing or chopper-stabilized amplifiers. Using Analog Devices, Inc., topology, these zero drift amplifiers combine low cost with high accuracy. (No external capacitors are required.) Using a spread-spectrum, auto-zero technique, the AD8571/AD8572/AD8574 eliminate the intermodulation effects from interaction of the chopping function with the signal frequency in ac applications. With an offset voltage of only 1 μV and drift of 0.005 μV/°C, the AD8571/AD8572/AD8574 are perfectly suited for applications where error sources cannot be tolerated. Position and pressure sensors, medical equipment, and strain gage amplifiers benefit greatly from nearly zero drift over their operating temperature range. Many more systems require the rail-to-rail input and output swings provided by the AD8571/AD8572/AD8574.The AD8571/AD8572/AD8574 are specified for the extended industrial/ automotive temperature range (−40°C to +125°C).  The AD8572 dual amplifier is available in 8-lead narrow SOIC and surface-mount TSSOP packages.

 

FEATURES

Low offset voltage: 1 μV

Input offset drift: 0.005 μV/°C

Rail-to-rail input and output swing

5 V/2.7 V single-supply operation

High gain: 145 dB typical

CMRR: 140 dB typical

PSRR: 130 dB typical

Ultralow input bias current: 10 pA typical

Low supply current: 750 μA per op amp

Overload recovery time: 50 μs

No external capacitors required

 

APPLICATIONS

Temperature sensors

Pressure sensors

Precision current sensing

Strain gage amplifiers

Medical instrumentation

Thermocouple amplifiers

 

AMPLIFIER ARCHITECTURE

Each AD8571/AD8572/AD8574 op amp consists of two amplifiers: a main amplifier and a secondary amplifier that is used to correct the offset voltage of the main amplifier. Both consist of a rail-to-rail input stage, allowing the input common-mode voltage range to reach both supply rails. The input stage consists of an NMOS differential pair operating concurrently with a parallel PMOS differential pair. The outputs from the differential input stages are combined in another gain stage whose output is used to drive a rail-to-rail output stage.The wide voltage swing of the amplifier is achieved by using two output transistors in a common-source configuration. The output voltage range is limited by the drain-to-source resistance of these transistors. As the amplifier is required to source or sink more output current, the voltage drop across these transistors increases due to their on resistance (RDS). Simply put, the output voltage does not swing as close to the rail under heavy output current conditions as it does with light output current. This is a characteristic of all rail-to-rail output amplifiers. Figure 12 and Figure 13 show how close the output voltage can get to the rails with a given output current. The output of the AD8571/ AD8572/AD8574 is short-circuit protected to approximately 50 mA of current.The AD8571/AD8572/AD8574 amplifiers have exceptional gain, yielding greater than 120 dB of open-loop gain with a load of 2 kΩ. Because the output transistors are configured in a common-source configuration, the gain of the output stage, and thus the open-loop gain of the amplifier, is dependent on the load resistance. Open-loop gain decreases with smaller load resistances, which is another characteristic of rail-to-rail output amplifiers.

1/f NOISE CHARACTERISTICS

Another advantage of auto-zero amplifiers is their ability to cancel flicker noise. Flicker noise, also known as 1/f noise, is noise inherent in the physics of semiconductor devices and increases 3 dB for every octave decrease in frequency. The 1/f corner frequency of an amplifier is the frequency at which the flicker noise is equal to the broadband noise of the amplifier. At lower frequencies, flicker noise dominates, causing higher degrees of error for sub-Hertz frequencies or dc precision applications.Because the AD8571/AD8572/AD8574 amplifiers are selfcorrecting op amps, they do not have increasing flicker noise at lower frequencies. In essence, low frequency noise is treated as a slowly varying offset error and is greatly reduced with autocorrection. The correction becomes more effective as the noise frequency approaches dc, offsetting the tendency of the noise to increase exponentially as frequency decreases, which allows the AD8571/AD8572/AD8574 to have lower noise near dc than standard low noise amplifiers that are susceptible to 1/f noise.