OPA1652_16 Datasheet, PDF (19/42 Pages) Texas Instruments – Audio Operational Amplifiers

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Application Information (continued)
A) Noise in Noninverting Gain Configuration
R₂
R₁
E_O
R_S
V_S
OPA1652, OPA1654
SBOS477A – DECEMBER 2011 – REVISED AUGUST 2016
Noise at the output:
E_O2 =
1+
R₂
R₁
2
e_n2 +
2
2
R₂
R₁
e₁₂ + e₂₂ +
1+
R₂
R₁
e_s2
Where e_S =
e₁ =
e₂ =
4kTR_S = thermal noise of R_S
4kTR₁ = thermal noise of R₁
4kTR₂ = thermal noise of R₂
B) Noise in Inverting Gain Configuration
R₂
R₁
Noise at the output:
2
E_O2 =
1 + ^R2
R₁ + R_S
e_n2 +
R₂
R₁ + R_S
2
e₁₂ + e₂₂ +
R₂
R₁ + R_S
2
e_s2
R_S
E_O
V_S
Where e_S = 4kTR_S = thermal noise of R_S
e₁ = 4kTR₁ = thermal noise of R₁
e₂ = 4kTR₂ = thermal noise of R₂
Copyright © 2016, Texas Instruments Incorporated
Note: For the OPA165x series of op amps at 1 kHz, e_n = 4.5 nV/√Hz.
Figure 39. Noise Calculation in Gain Configurations
8.1.2 Total Harmonic Distortion Measurements
The OPA165x series op amps have excellent distortion characteristics. THD + noise is below 0.0002% (G = +1,
V_O = 3 V_RMS, BW = 80 kHz) throughout the audio frequency range, 20 Hz to 20 kHz, with a 2-kΩ load (see
Figure 7 for characteristic performance).
The distortion produced by the OPA165x series op amps is below the measurement limit of many commercially
available distortion analyzers. However, a special test circuit (such as Figure 40 shows) can be used to extend
the measurement capabilities.
Op amp distortion can be considered an internal error source that can be referred to the input. Figure 40 shows a
circuit that causes the op amp distortion to be gained up (refer to the table in Figure 40 for the distortion gain
factor for various signal gains). The addition of R₃ to the otherwise standard noninverting amplifier configuration
alters the feedback factor or noise gain of the circuit. The closed-loop gain is unchanged, but the feedback
available for error correction is reduced by the distortion gain factor, thus extending the resolution by the same
amount. Note that the input signal and load applied to the op amp are the same as with conventional feedback
without R₃. The value of R₃ should be kept small to minimize its effect on the distortion measurements.
The validity of this technique can be verified by duplicating measurements at high gain and/or high frequency
where the distortion is within the measurement capability of the test equipment. Measurements for this data
sheet were made with an Audio Precision System Two distortion/noise analyzer, which greatly simplifies such
repetitive measurements. The measurement technique can, however, be performed with manual distortion
measurement instruments.
Copyright © 2011–2016, Texas Instruments Incorporated
Product Folder Links: OPA1652 OPA1654
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