RHF330 Datasheet, PDF (15/22 Pages) STMicroelectronics – Rad-hard 1 GHz low noise operational amplifier

RHF330
Noise measurements
Equation 2
eNo²
=
eN² ×
g² + iNn² ×
R2² + iNp² ×
R3² ×
g²
+
R_-----2_--²
R1
×
4kTR1 + 4kTR2 + 1 + ^R-----²--² ×
R1
4kTR3
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
eNo = (Measured)² – (instrumentation)²
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
eNo² = eN² × g² + iNn² × R2² + iNp² × R3² × g² + g × 4kTR2 + 1 + ^R-----²--² × 4kTR3
R1
5.1
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
eNo = eN² × g² + iNn² × R2² + g × 4kTR2
To easily extract the value of eN, the resistance R2 is as low as possible. On the other hand,
the gain must be large enough.
R3=0, gain: g=100
5.2
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 to decrease the thermal noise contribution.
R3=0, gain: g=10
5.3
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 so that its thermal noise contribution is as low as possible
against the iNp contribution.
R3=100 W, gain: g=10
Doc ID 15576 Rev 3
15/22