TSH350 Noise measurements
15/22
Equation 2
The input noise of the instrumentation must be extracted from the measured noise value.
The real output noise value of the driver is:
Equation 3
The input noise is called equivalent input noise because 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:
Equation 4
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:
Equation 5
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=100W, gain: g=10
eNo
2
eN
2
g
2
iNn
2
R2
2
iNp
2
+×+× R3
2
× g
2
×
R2
R1
------- -
2
4kTR1 4kTR2 1
R2
R1
------- -+
2
4kTR3×++×+=
eNo Measured()
2
instrumentation()
2
–=
eNo
2
eN
2
g
2
iNn
2
R2
2
iNp
2
+×+× R3
2
× g
2
× g4kTR21
R2
R1
------- -+
2
4kTR3×+×+=
eNo eN
2
g
2
iNn
2
R2
2
g4kTR2×+×+×=
Obsolete Product(s) - Obsolete Product(s)
