AD8421ARMZ-R7 datasheet(25/29 Pages) AD

25 / 29 page

AD8421

Data Sheet

Rev. 0 | Page 24 of 28

Voltage Noise of the Instrumentation Amplifier

The voltage noise of the instrumentation amplifier is calculated

using three parameters: the device output noise, the input noise,

and the RG resistor noise. It is calculated as follows:

Total Voltage Noise

(

) (

Resistor

Noise

Input

Noise

Output

For example, for a gain of 100, the gain resistor is 100 Ω. Therefore,

the voltage noise of the in-amp is

()

(

100

= 3.5 nV/√Hz

Current Noise of the Instrumentation Amplifier

Current noise is converted to a voltage by the source resistance.

The effect of current noise can be calculated by multiplying the

specified current noise of the in-amp by the value of the source

resistance.

For example, if the R1 source resistance in Figure 69 is 4 kΩ,

and the R2 source resistance is 1 kΩ, the total effect from the

current noise is calculated as follows:

() (

= 0.8 nV/√Hz

Total Noise Density Calculation

To determine the total noise of the in-amp, referred to input,

combine the source resistance noise, voltage noise, and current

noise contribution by the sum of squares method.

For example, if the R1 source resistance in Figure 69 is 4 kΩ, the

R2 source resistance is 1 kΩ, and the gain of the in-amp is 100,

the total noise, referred to input, is

= 9.6 nV/√Hz

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