TSH350 Datasheet, PDF (14/21 Pages) STMicroelectronics – 550 MHz, Low Noise Current Feedback Amplifier

TSH350
Noise Measurements
The input noise of the instrumentation must be extracted from the measured noise value. The real output
noise value of the driver is:
eNo = (Measured)² – (instrumentation)²
Equation 3
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:
eNo2 = eN2 × g2 + iNn2 × R22 + iNp2 × R32 × g2 + g × 4kTR2 + 1 + ^R-----²--² × 4kTR3
R1
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:
eNo = eN2 × g2 + iNn2 × R22 + g × 4kTR2
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
14/21