RHF310 Ver la hoja de datos (PDF) - STMicroelectronics
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RHF310 Datasheet PDF : 22 Pages
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Noise measurements
RHF310
Equation 2
eNo2
=
eN2
×
g2
+
i
N
n2
×
R
22
+
i
N
p2
×
R
32
×
g2
+
R-----2--
R1
2
×
4kT
R1
+
4
kT
R
2
+
1
+
R-----2--
R1
2
×
4
kT
R
3
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 fifth terms of Equation 2, you obtain:
Equation 4
eNo2 = eN2 × g2 + iNn2 × R22 + iNp2 × R32 × g2 + g × 4kTR2 + 1 + R-----2--2 × 4kTR3
R1
4.1
Measurement of the input voltage noise eN
Assuming a short-circuit on the non-inverting input (R3=0), from Equation 4 you can derive:
Equation 5
eNo = eN2 × g2 + iNn2 × R22 + g × 4kTR2
To easily extract the value of eN, the resistance R2 must be as low as possible. On the other
hand, the gain must be high enough.
R3=0, gain: g=100
4.2
Measurement of the negative input current noise iNn
To measure the negative input current noise iNn, R3 is set to zero and Equation 5 is used.
This time, the gain must be lower in order to decrease the thermal noise contribution.
R3=0, gain: g=10
4.3
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 selected so as to keep its thermal noise contribution as low as possible
against the iNp contribution.
R3=100 Ω, gain: g=10
14/22
Doc ID 15577 Rev 3
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