ADA4665-2

Rev. 0 | Page 15 of

20

APPLICATIONS INFORMATION

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 common-

mode 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 (see Figure 5 and Figure 8).

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.

Figure 55 shows a low-side current sensing circuit, and Figure 56

shows a high-side current sensing circuit using the ADA4665-2.

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.

1/2

ADA4665-2

16V

RL

R2

1MΩ

R1

10kΩ

RS

0.1Ω

R4

1MΩ

R3

10kΩ

VOUT*

*VOUT = AMPLIFIER GAIN × VOLTAGE ACROSS RS

= 100 × RS × I

= 10 × I

I

16V

SUPPLY

I

Figure 55. Low-Side Current Sensing Circuit

1/2

ADA4665-2

16V

RL

R4

1MΩ

R3

10kΩ

RS

0.1Ω

R2

1MΩ

R1

10kΩ

I

16V

SUPPLY

I

VOUT*

*VOUT = AMPLIFIER GAIN × VOLTAGE ACROSS RS

= 100 × RS × I

= 10 × I

Figure 56. High-Side Current Sensing Circuit

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.

Figure 57 shows a two-pole low-pass filter. It is configured as a

unity-gain filter with cutoff frequency at 10 kHz. Resistor and

capacitor values are chosen to give a quality factor, Q, of 1/√2

for a Butterworth filter, which has maximally flat pass-band

frequency response. Figure 58 shows the frequency response of

the low-pass Sallen-Key filter. The response falls off at a rate of

40 dB per decade after the cutoff frequency of 10 kHz.