Datasheet

Data Sheet AD8428

Rev. A | Page 17 of 20

Voltage Noise of the Instrumentation Amplifier Total Noise Density Calculation

Unlike other instrumentation amplifiers in which an external

resistor is used to set the gain, the voltage noise specification

of the AD8428 already includes the input noise, output noise,

and the R

resistor noise.

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 45 is 4 k

and the R2 source resistance is 1 k, the total noise, referred

to input, is

Current Noise of the Instrumentation Amplifier

The contribution of current noise to the input stage in nV/√Hz

is calculated by multiplying the source resistance in k by the

specified current noise of the instrumentation amplifier in

pA/√Hz.

HznV/0.112.65.19.8

222

=++

For example, if the R1 source resistance in Figure 45 is 4 k

and the R2 source resistance is 1 k, the total effect from the

current noise is calculated as follows:

()()

HznV/2.625.2365.115.14

=+=×+×