GENERAL DESCRIPTION

The ADA4665-2 is a rail-to-rail input/output dual amplifier optimized for lower power budget designs. The ADA4665-2 offers a low supply current of 400 μA maximum per amplifier at 25°C and 600 μA maximum per amplifier over the extended industrial temperature range. This feature makes the ADA4665-2 well suited for low power applications. In addition, the ADA4665-2 has a very low bias current of 1 pA maximum, low offset voltage drift of 3 μV/°C, and bandwidth of 1.2 MHz. The combination of these features, together with a wide supply voltage range from 5 V to 16 V, allows the device to be used in a wide variety of other applications, including process control, instrumentation equipment, buffering, and sensor front ends. Furthermore, its rail-to-rail input and output swing adds to its versatility. The ADA4665-2 is specified from −40°C to +125°C and is available in standard SOIC and MSOP packages.

 

RAIL-TO-RAIL INPUT OPERATION

The ADA4665-2 is a unity-gain stable CMOS operational amplifier designed with rail-to-rail input/output swing capability to optimize performance. The rail-to-rail input feature is vital to maintain the wide dynamic input voltage range and to maximize signal swing to both supply rails. For example, the rail-to-rail input feature is extremely useful in buffer applications where the input voltage must cover both the supply rails.

The input stage has two input differential pairs, nMOS and pMOS. When the input common-mode voltage is at the low end of the input voltage range, the pMOS input differential pair is active and amplifies the input signal. As the input commonmode voltage is slowly increased, the pMOS differential pair gradually turns off while the nMOS input differential pair turns on. This transition is inherent to all rail-to-rail input amplifiers that use the dual differential pairs topology. For the ADA4665-2, this transition occurs approximately 1 V away from the positive rail and results in a change in offset voltage due to the different offset voltages of the differential pairs.

 

CURRENT SHUNT SENSOR

Many applications require the sensing of signals near the positive or the negative rails. Current shunt sensors are one such application and are mostly used for feedback control systems. They are also used in a variety of other applications, including power metering, battery fuel gauging, and feedback controls in electrical power steering. In such applications, it is desirable to use a shunt with very low resistance to minimize the series voltage drop. This not only minimizes wasted power, but also allows the measurement of high currents while saving power. The ADA4665-2 provides a low cost solution for implementing current shunt sensors.

A typical shunt resistor of 0.1 Ω is used. In these circuits, the difference amplifier amplifies the voltage drop across the shunt resistor by a factor of 100. For true difference amplification, matching of the resistor ratio is very important, where R1/R2 = R3/R4. The rail-to-rail feature of the ADA4665-2 allows the output of the op amp to almost reach 16 V (the power supply of the op amp). This allows the current shunt sensor to sense up to approximately 1.6 A of current.

 

ACTIVE FILTERS

The ADA4665-2 is well suited for active filter designs. An active filter requires an op amp with a unity-gain bandwidth at least 100 times greater than the product of the corner frequency, fc, and the quality factor, Q. An example of an active filter is the Sallen-Key, one of the most widely used filter topologies. This topology gives the user the flexibility of implementing either a low-pass or a high-pass filter by simply interchanging the resistors and capacitors. To achieve the desired performance, 1% or better component tolerances are usually required.