Noise Measurements TSH300
15/18
The input noise of the instrumentation must be extracted from the measured noise value. The
real output noise value of the driver is:
The input noise is called the Equivalent Input Noise as it is not directly measured but is
evaluated from the measurement of the output divided by the closed loop gain (eNo/g).
After simplification of the fourth and the fifth term of Equation 2 we obtain:
Measurement of the input voltage noise
eN
If we assume a short-circuit on the non-inverting input (R3=0), from Equation 4 we can derive:
In order to easily extract the value of eN, the resistance R2 will be chosen to be as low as
possible. In the other hand, the gain must be large enough:
R3=0, gain: g=100
Measurement of the negative input current noise
iNn
To measure the negative input current noise iNn, we set R3=0 and use Equation 5. This time
the gain must be lower in order to decrease the thermal noise contribution:
R3=0, gain: g=10
Measurement of the positive input current noise
iNp
To extract iNp from Equation 3, a resistance R3 is connected to the non-inverting input. The
value of R3 must be chosen in order to keep its thermal noise contribution as low as possible
against the iNp contribution:
R3=100Ω, gain: g=10
eNo Measured()
2
instrumentation
()
2
– (Equation 3)=
eNo
2
eN
2
g
2
iNn
2
R2
2
iNp
2
+×+× R3
2
× g
2
× g4kTR2g
2
4kTR3×+× (Equation 4)+=
eNo eN
2
g
2
iNn
2
R2
2
g4kTR2×+×+× (Equation 5)=
