Datasheet

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 R
G
resistor noise. It is calculated as follows:
Total Voltage Noise =
(
)
(
)
(
)
222
/ ResistorRofNoiseNoiseInputGNoiseOutput
G
++
For example, for a gain of 100, the gain resistor is 100 . Therefore,
the voltage noise of the in-amp is
()
()
2
2
2
1.042.3100/60 ×+ +
= 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 k,
and the R2 source resistance is 1 k, the total effect from the
current noise is calculated as follows:
()()
22
2.012.04 ×+×
= 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 k, the
R2 source resistance is 1 k, and the gain of the in-amp is 100,
the total noise, referred to input, is
222
8.05.39.8 ++ = 9.6 nV/√Hz