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| OPA1612-Q1 Datasheet, PDF (20/31 Pages) Texas Instruments – SoundPlus High-Performance, Bipolar-Input Audio Operational Amplifier | |||
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OPA1612-Q1
SLOS931A â NOVEMBER 2015 â REVISED NOVEMBER 2015
Typical Application (continued)
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Noise in Noninverting Gain Configuration
R2
R1
EO
RS
VS
Noise at the output:
EO2
=
êëé1+
R2
R1
ù2
ú
û
en2
+ e12
+
e22
+ (in R2 )2
+ eS2
+
(in RS )2
êëé1+
R2
R1
ù2
ú
û
where
·
eS =
4kTRS
´
éê1+
ë
R2
R1
ù
ú
û
=
therma l
noise
of
RS
·
e1 =
4kTR1
´
é
êë
R2
R1
ù
úû
=
therma l
noise
of
R1
· e2 = 4kTR2 = thermal noise of R2
Noise in Inverting Gain Configuration
R2
R1
Noise at the output:
EO2
=
éê1+
ë
R2
R1 + RS
ù2
ú
û
en2
+
e12
+
e22
+ (in R2 )2
+
eS2
RS
VS
EO
where
·
eS =
4kTRS
´
é
ê
ë
R2
R1 + RS
ù
ú
û
=
therma l
noise
of
RS
·
e1 =
4kTR1
´
é
ê
ë
R2
R1 + RS
ù
ú
û
=
therma l
noise
of
R1
· e2 = 4kTR2 = thermal noise of R2
At 1 kHz, en = 1.1 nV/âHz and in = 1.7 pA/âHz.
Figure 38. Noise Calculation in Gain Configurations
9.2.2.2 Total Harmonic Distortion Measurements
The OPA1612-Q1 op amp has excellent distortion characteristics. THD + noise is below 0.00008% (G = +1, VO =
3 VRMS, BW = 80 kHz) throughout the audio frequency range, 20 Hz to 20 kHz, with a 2-k⦠load (see Figure 7 for
characteristic performance).
The distortion produced by OPA1612-Q1 op amp is below the measurement limit of many commercially available
distortion analyzers. However, a special test circuit (such as Figure 39 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 39 shows a
circuit that causes the op amp distortion to be 101 times (or approximately 40 dB) greater than that normally
produced by the op amp. 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 a factor of 101, thus extending the resolution by 101. Note that the input signal and
load applied to the op amp are the same as with conventional feedback without R3. Keep the value of R3 small to
minimize its effect on the distortion measurements.
20
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Copyright © 2015, Texas Instruments Incorporated
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