OPA4830 Datasheet, PDF (34/45 Pages) National Semiconductor (TI) – Quad, Low-Power, Single-Supply, Wideband Operational Amplifier

OPA4830
SBOS350A – DECEMBER 2006 – REVISED MAY 2008.................................................................................................................................................... www.ti.com
DISTORTION PERFORMANCE
The OPA4830 provides good distortion performance
into a 150Ω load. Relative to alternative solutions, it
provides exceptional performance into lighter loads
and/or operating on a single +3V supply. Generally,
until the fundamental signal reaches very high
frequency or power levels, the 2nd-harmonic
dominates the distortion with a negligible
3rd-harmonic component. Focusing then on the
2nd-harmonic, increasing the load impedance
improves distortion directly. Remember that the total
load includes the feedback network; in the
noninverting configuration (see Figure 74) this is sum
of R_F + R_G, while in the inverting configuration, only
R_F needs to be included in parallel with the actual
load. Running differential suppresses the
2nd-harmonic, as shown in the Differential Typical
Characteristics.
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 9.2nV/√Hz input
voltage noise for the OPA4830 however, is much
lower than comparable amplifiers. The input-referred
voltage noise and the two input-referred current noise
terms (2.8pA/√Hz) combine to give low output noise
under a wide variety of operating conditions.
Figure 89 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
1/4
OPA4830
E_O
R_S
I_BN
E_RS
4kTR_S
4kT
R_G
R_F
4kTR_F
R_G
I_BI
4kT = 1.6E - 20J
at 290°K
Figure 89. 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 8 shows the
general form for the output noise voltage using the
terms shown in Figure 89:
E_O = _(ENI² _{+ (IBNRS)}² _{+ 4kTRS)NG}² _{+ (IBIRF)}² _{+ 4kTRFNG}
(8)
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; this result is shown
in Equation 9:
E_N =
E_NI² + (I_BNR_S)² + 4kTR_S +
I_BIR_F
NG
2
+
4kTR_F
NG
(9)
Evaluating these two equations for the circuit and
component values shown in Figure 72 gives a total
output spot noise voltage of 19.3nV/√Hz and a total
equivalent input spot noise voltage of 9.65nV/√Hz.
This value is including the noise added by the
resistors. This total input-referred spot noise voltage
is not much higher than the 9.2nV/√Hz specification
for the op amp voltage noise alone.
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