ADA4505-2

Rev. 0 | Page 14 of 20

THEORY OF OPERATION

The ADA4505-2 is a unity-gain stable CMOS rail-to-rail input/

output operational amplifier designed to optimize performance

in current consumption, PSRR, CMRR, and zero crossover distor-

tion, all imbedded in a small package. The typical offset voltage

is 500 μV, with a low peak-to-peak voltage noise of 2.95 μV p-p

from 0.1 Hz to 10 Hz and a voltage noise density of 55 nV/√Hz

at 1 kHz.

The ADA4505-2 was designed to solve two key problems in low

voltage battery-powered applications: battery voltage decrease

over time and rail-to-rail input stage distortion.

In battery-powered applications, the supply voltage available to

the IC is the voltage of the battery. Unfortunately, the voltage of

a battery decreases as it discharges itself through the load. This

voltage drop over the lifetime of the battery causes an error in

the output of the op amps. Some applications requiring precision

measurements during the entire lifetime of the battery use

voltage regulators to power up the op amps as a solution. If a

design uses standard battery cells, the op amps experience a

supply voltage change from roughly 3.2 V to 1.8 V during the

lifetime of the battery. This means that for a PSRR of 70 dB

minimum in a typical op amp, the input-referred offset error is

approximately 440 μV. If the same application uses the ADA4505-2

with a 100 dB minimum PSRR, the error is only 14 μV. It is

possible to calibrate out this error or to use an external voltage

regulator to power the op amp, but these solutions can increase

system cost and complexity. The ADA4505-2 solves the impasse

with no additional cost or error-nullifying circuitry.

The second problem with battery-powered applications is the

distortion caused by the standard rail-to-rail input stage. Using

a CMOS non-rail-to-rail input stage (that is, a single differential

for normal operation is commonly over 1 V, a single differential

pair input stage op amp greatly restricts the allowable input

voltage range when using a low supply voltage. This limitation

restricts the number of applications where the non-rail-to-rail

input op amp was originally intended to be used. To solve this

problem, a dual differential pair input stage is usually implemented

(see Figure 48); however, this technique has its own drawbacks.

One differential pair amplifies the input signal when the common-

mode voltage is on the high end, whereas the other pair amplifies

the input signal when the common-mode voltage is on the low

end. This method also requires a control circuitry to operate the

two differential pairs appropriately. Unfortunately, this topology

leads to a very noticeable and undesirable problem: if the signal

level moves through the range where one input stage turns off

and the other one turns on, noticeable distortion occurs (see

Figure 49).

Q2

Q3

Q1

Q4

Figure 48. A Typical Dual Differential Pair Input Stage Op Amp

(Dual PMOS Q1 and Q2 Transistors Form the Lower End of the Input Voltage

Range Whereas Dual NMOS Q3 and Q4 Compose the Upper End)

0

–300

–100

100

300

1.5

3.5

5.0

1.0

0.5

2.5

4.5

4.0

3.0

2.0

–200

–150

–250

–50

0

50

150

200

250

Figure 49. Typical Input Offset Voltage vs. Common-Mode Voltage

Response in a Dual Differential Pair Input Stage Op Amp (Powered by 5 V

Supply; Results of Approximately 100 Units per Graph Are Displayed)

This distortion forces the designer to come up with impractical

ways to avoid the crossover distortion areas, therefore narrowing

the common-mode dynamic range of the operational amplifier.

The ADA4505-2 solves this crossover distortion problem by

using an on-chip charge pump to power the input differential

pair. The charge pump creates a supply voltage higher than the

voltage of the battery, allowing the input stage to handle a wide

range of input signal voltages without using a second differential

pair. With this solution, the input voltage can vary from one

supply extreme to the other with no distortion, thereby restoring

the op amp full common-mode dynamic range.