OPA4872-EP Datasheet, PDF (17/24 Pages) Texas Instruments – 4:1 HIGH-SPEED MULTIPLEXER

OPA4872-EP
www.ti.com........................................................................................................................................................................................... SBOS444 – DECEMBER 2008
NOISE PERFORMANCE
The OPA4872 offers an excellent balance between
voltage and current noise terms to achieve low output
noise. The inverting current noise (19 pA/√Hz) is
significantly lower than earlier solutions, while the
input voltage noise (4.5 nV/√Hz) is lower than most
unity-gain stable, wideband, voltage-feedback op
amps. As long as the ac source impedance looking
out of the noninverting node is less than 100 Ω, this
current noise will not contribute significantly to the
total output noise. The op amp input voltage noise
and the two input current noise terms combine to give
low output noise under a wide variety of operating
conditions. Figure 34 shows the OPA4872 noise
analysis model with all the noise terms included. In
this model, all noise terms are taken to be noise
voltage or current density terms in either nV/√Hz or
pA/√Hz.
E_NI
OPA4872
E_O
R_S
I_BN
E_RS
Ö4kTR_S
R_F
Ö^{( ) EO =}
E_NI2
+
(I_BNR_S)²
+
4kTR_S
NG²
+
2
(I_BIR_F)
+
4kTR_FNG
(9)
Dividing this expression by the noise gain (NG = (1 +
R_F/R_G)) gives the equivalent input-referred spot noise
voltage at the noninverting input, as shown in
Equation 10.
Ö ^{( ) EO =}
E_NI2 + (I_BNR_S)² + 4kTR_S +
I_BIR_F
2
+
4kTR_F
NG NG
(10)
Evaluating these two equations for the OPA4872
circuit and component values (see Figure 27) gives a
total output spot noise voltage of 14.2 nV/√Hz and a
total equivalent input spot noise voltage of 7.1
nV/√Hz. This total input-referred spot noise voltage is
higher than the 4.5-nV/√Hz specification for the
OPA4872 voltage noise alone. This voltage reflects
the noise added to the output by the inverting current
noise times the feedback resistor. If the feedback
resistor is reduced in high-gain configurations, the
total input-referred voltage noise given by
Equation 10 approaches only the 4.5 nV/√Hz of the
op amp itself. For example, going to a gain of +10
using R_F = 178 Ω gives a total input-referred noise of
4.7 nV/√Hz.
Ö4kTR_F
R_G
I_BI
4kT
- 20
4kT = 1.6 x 10 J
R_G
at 290K
Figure 34. Op Amp Noise Analysis Model
The total output spot noise voltage can be computed
as the square root of the sum of all squared output
noise voltage contributors. Equation 9 shows the
general form for the output noise voltage using the
terms shown in Figure 35.
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