ADA4528-2ARMZ-R7 datasheet(20/24 Pages) AD

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ADA4528-1/ADA4528-2

Data Sheet

Rev. F | Page 20 of 24

Source Resistance

With 5.6 nV/√Hz of broadband noise at 1 kHz (VSY = 2.5 V

and AV = +100), the ADA4528-1/ADA4528-2 are among the

lowest noise zero-drift amplifiers currently available in the

industry. Therefore, it is important to carefully select the input

source resistance to maintain a total low noise.

The total input referred broadband noise (en total) from any

amplifier is primarily a function of three types of noise: input

voltage noise, input current noise, and thermal (Johnson) noise

from the external resistors.

These uncorrelated noise sources can be summed up in a root

sum squared (rss) manner using the following equation:

en total = [en2 + 4 kTRS + (in × RS)2]1/2

where:

en is the input voltage noise of the amplifier (V/√Hz).

k is the Boltzmann’s constant (1.38 × 10−23 J/K).

T is the temperature in Kelvin (K).

RS is the total input source resistance (Ω).

in is the input current noise of the amplifier (A/√Hz).

The total equivalent rms noise over a specific bandwidth is

expressed as

en,rms = en total × √BW

where BW is the bandwidth in hertz.

This analysis is valid for broadband noise calculation. If the

bandwidth of concern includes the chopping frequency, more

complicated calculations must be made to include the effect of

the noise energy spectrum at the chopping frequency (see the

Residual Voltage Ripple section).

With a low source resistance of RS < 1 kΩ, the voltage noise

of the amplifier dominates. As source resistance increases, the

thermal noise of RS dominates. As the source resistance increases

further, where RS > 100 kΩ, the current noise becomes the main

contributor to the total input noise. A good selection table for low

noise op amps can be found in theAN-940Application Note, Low

Noise Amplifier Selection Guide for Optimal Noise Performance.

Voltage Noise Density with Different Gain Configurations

Figure 65 shows the voltage noise density vs. closed-loop gain of a

zero-drift amplifier from a leading competitor. The voltage noise

density of the amplifier increases from 11 nV/√Hz to 21 nV/√Hz

as the closed-loop gain decreases from 1000 to 1.

100

1000

CLOSED-LOOP GAIN (V/V)

VSY = 5V

f = 100Hz

COMPETITOR A

Figure 65. Competitor A: Voltage Noise Density vs. Closed-Loop Gain

Figure 66 shows the voltage noise density vs. frequency of the

ADA4528-1/ADA4528-2 for three different gain configurations.

The ADA4528-1/ADA4528-2 offer a constant input voltage

noise density of 6 nV/√Hz to 7 nV/√Hz, regardless of the gain

configuration.

100

10k

FREQUENCY (Hz)

AV = 10

AV = 100

AV = 1

VSY = 5V

VCM = VSY/2

Figure 66. Voltage Noise Density vs. Frequency with Different Gain

Configurations

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