OPA3690_14 Datasheet, PDF (24/39 Pages) Texas Instruments – Triple, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER

OPA3690
SBOS237G – MARCH 2002 – REVISED MARCH 2010
NOISE PERFORMANCE
High slew rate, unity-gain stable, voltage-feedback op
amps usually achieve their slew rate at the expense
of a higher input noise voltage. The 5.5nV/√Hz input
voltage noise for the OPA3690 is, however, much
lower than comparable amplifiers. The input-referred
voltage noise, and the two input-referred current
noise terms, combine to give low output noise under
a wide variety of operating conditions. Figure 50
shows the op amp 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
1/3
R_S
OPA3690
I_BN
E_O
E_RS
Ö4kTR_S
4kT
RG
R_F
Ö4kTRF
R_G
I_BI
4kT = 1.6E - 20J
at 290°K
Figure 50. Op Amp Noise Analysis Model
www.ti.com
The total output spot noise voltage can be computed
as the square root of the sum of all squared output
noise voltage contributors. Equation 4 shows the
general form for the output noise voltage using the
terms shown in Figure 50.
E_O = E_NI2 + (I_BNR_S)² + 4kTR_S NG² + (I_BIR_F)² + 4kTR_FNG
(4)
Dividing this expression by the noise gain [NG = (1 +
R_F/R_G)] will give the equivalent input-referred spot
noise voltage at the noninverting input, as shown in
Equation 5.
E_N =
E_NI2 + (I_BNR_S)² + 4kTR_S +
I_BIR_F
2
+
NG
4kTR_F
NG
(5)
Evaluating these two equations for the OPA3690
circuit and component values (see Figure 36) gives a
total output spot noise voltage of 12.3nV/√Hz and a
total equivalent input spot noise voltage of 6.1nV/√Hz.
This is including the noise added by the bias current
cancellation resistor (100Ω) on the noninverting input.
This total input-referred spot noise voltage is only
slightly higher than the 5.5nV/√Hz specification for the
op amp voltage noise alone. This will be the case as
long as the impedances appearing at each op amp
input are limited to the previously recommend
maximum value of 300Ω. Keeping both (R_F || R_G) and
the noninverting input source impedance less than
300Ω will satisfy both noise and frequency response
flatness considerations. As the resistor-induced noise
is relatively negligible, additional capacitive
decoupling across the bias current cancellation
resistor (R_B) for the inverting op amp configuration of
Figure 47 is not required.
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