AN-940 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AN-940 Datasheet PDF : 12 Pages
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AN-940
OP AMP
NOISE MODEL
en
RESISTOR
NOISE
RS
–+ +
in
–
in
COMBINED RTI NOISE
(Vni, TOTAL)
–+ +
–
Application Note
RESISTOR
NOISE
R2
R1
RESISTOR
NOISE
R2
R1
Figure 4. Simplifying the Amplifier Noise Circuit
NOISE GAIN
The noises previously discussed can be grouped into referred-
to-input (RTI) noise of the amplifier circuit. To calculate the
total output noise of the amplifier circuit, the total combined
noise on the input must be multiplied by the amplifier circuit’s
noise gain. Noise gain is the gain of the amplifier’s circuit for
referred-to-input noise and it is typically used to determine the
stability of the amplifier circuit.
To simplify the noise gain calculation, the noise sources in the
simple amplifier circuit in Figure 1 can be reduced to a single
total RTI noise source (Vni, TOTAL), as shown in Figure 4. It is a
common practice to lump the total combined RTI noise to the
noninverting input of the amplifier.
Vno, TOTAL GN Vni, TOTAL
where:
Vno, TOTAL is the total referred-to-output (RTO) noise.
Vni, TOTAL is the total referred-to-input (RTI) noise
GN
1
R1
R2
where:
GN is the noise gain.
R1 is the feedback equivalent impedance.
R2 is the gain setting equivalent impedance.
In some cases, the noise gain and the signal gain are not equiv-
alent (see Figure 5). Note that the closed-loop bandwidth is
determined by dividing the gain bandwidth product (or unity
gain frequency) by the noise gain of the amplifier circuit.
NOISE
SOURCE
+
CASE 1:
SIGNAL
SOURCE
–
R2
R1
CASE 2:
SIGNAL
SOURCE
Figure 5. Signal Gain vs. Noise Gain
Case 1: In a noninverting configuration, both the signal gain
and the noise gain are equal to 1 + R1/R2.
Case 2: In an inverting configuration, signal gain is equal to
−(R1/R2), but the noise gain is still equal to 1 + R1/R2.
Rev. D | Page 6 of 12
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