THS4031-EP Datasheet, PDF (21/37 Pages) Texas Instruments – 100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

THS4031-EP
THS4032-EP
www.ti.com........................................................................................................................................................................................... SLOS610 – NOVEMBER 2008
NOISE CALCULATIONS AND NOISE FIGURE
Noise can cause errors on very small signals. This is especially true when amplifying small signals. The noise
model for the THS403x, shown in Figure 52, includes all of the noise sources as follows:
• e_n = Amplifier internal voltage noise (nV/√Hz)
• IN+ = Noninverting current noise (pA/√Hz)
• IN– = Inverting current noise (pA/√Hz)
• e_Rx = Thermal voltage noise associated with each resistor (e_Rx = 4 kTR_x)
R_S
e_Rs
e_n
Noiseless
e_ni
+
_
e_no
IN+
e_Rf
R_F
IN−
e_Rg
R_G
Figure 52. Noise Model
Ǹ _{ǒ Ǔ ǒ ǒ ǓǓ ǒ Ǔ} ^{The total equivalent input noise density (eni) is calculated by using the following equation:}
eni +
ǒ_enǓ² _{) IN )}
2
RS ) IN–
2
RF ø RG ) 4 kTRs ) 4 kT RF ø RG
Where:
k = Boltzmann’s constant = 1.380658 × 10^−23
T = Temperature in degrees Kelvin (273 +°C)
R_F || R_G = Parallel resistance of R_F and R_G
(1)
To get the equivalent output noise of the amplifier, just multiply the equivalent input noise density (e_ni) by the
overall amplifier gain (A_V).
ǒ Ǔ eno + eni AV
+
e_ni
1
)
R_F
R_G
(Noninverting Case)
(2)
As the previous equations show, to keep noise at a minimum, small-value resistors should be used. As the
closed-loop gain is increased (by reducing R_G), the input noise is reduced considerably because of the parallel
resistance term. This leads to the general conclusion that the most dominant noise sources are the source
resistor (R_S) and the internal amplifier noise voltage (e_n). Because noise is summed in a root-mean-squares
method, noise sources smaller than 25% of the largest noise source can be effectively ignored. This advantage
can greatly simplify the formula and make noise calculations much easier to calculate.
For more information on noise analysis, refer to the application note, Noise Analysis for High-Speed Op Amps
(SBOA066).
Copyright © 2008, Texas Instruments Incorporated
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