OPA3832_07 Datasheet, PDF (24/32 Pages) Burr-Brown (TI) – Triple, Low-Power, High-Speed, Fixed-Gain Operational Amplifier

OPA3832
SBOS370 – DECEMBER 2006
DISTORTION PERFORMANCE
The OPA3832 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 +3.3V supply.
Generally, until the fundamental signal reaches very
high frequency or power levels, the 2nd-harmonic will
dominate 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 47) this is the
sum of R_F + R_G, while in the inverting configuration,
only R_F needs to be included in parallel with the
actual load.
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 OPA3832,
however, is much lower than comparable amplifiers.
The input-referred voltage noise and the two
input-referred current noise terms (2.2pA/√Hz)
combine to give low output noise under a wide
variety
of
operating
conditions.
Figure 55 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
OPA3832
E_O
R_S
I_BN
E_RS
Ö4kTR_S
4kT
R_G
R_F
Ö4kTR_F
R_G
I_BI
4kT = 1.6E - 20J
at 290°K
Figure 55. 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 1 shows the
general form for the output noise voltage using the
terms shown in Figure 55:
E_O
E_NI ²
2
I_BN R_S
4kTR_S NG²
2
I_BI R_F
4kTR_F NG
(1)
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 Figure 55:
2
E_N
E_NI2
2
I_BNR_S
4kTR_S
I_BIR_F
NG
4kTR_F
NG
(2)
Evaluating these two equations for the circuit and
component values shown in Figure 46 gives a total
output spot noise voltage of 18.8nV/√Hz and a total
equivalent input spot noise voltage of 9.42nV/√Hz.
This total includes 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.
DC ACCURACY AND OFFSET CONTROL
The balanced input stage of a wideband
voltage-feedback op amp allows good output dc
accuracy in a wide variety of applications. The
power-supply current trim for the OPA3832 gives
even tighter control than comparable products.
Although the high-speed input stage does require
relatively high input bias current (typically 5µA out of
each input terminal), the close matching between
them may be used to reduce the output dc error
caused by this current. This configuration matches
the dc source resistances appearing at the two
inputs. Evaluating the configuration of Figure 48
(which has matched dc input resistances), using
worst-case +25°C input offset voltage and current
specifications, gives a worst-case output offset
voltage equal to:
• (NG = noninverting signal gain at dc)
• ±(NG × V_OS(MAX)) + R_F × I_OS(MAX))
• = ±(2 × 80mV) + (400Ω × 1.5µA)
• = –15.4mV to +16.6mV
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