OPA2832 Datasheet, PDF (25/37 Pages) National Semiconductor (TI) – Dual, Low-Power, High-Speed, Fixed-Gain Operational Amplifier

OPA2832
www.ti.com ............................................................................................................................................. SBOS327C – FEBRUARY 2005 – REVISED AUGUST 2008
DISTORTION PERFORMANCE
The OPA2832 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 62) 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 characteristic curves.
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 OPA2832, 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 71 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
1/2
OPA2832
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 71. 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 1 shows the
general form for the output noise voltage using the
terms shown in Figure 71:
Ǹǒ Ǔ E_O +
ǒ Ǔ _{ENI2 )}
2
I_BNR_S ) 4kTR_S
NG² ) ^ǒI_BIR_F^Ǔ2 ) 4kTR_FNG
(1)
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 Figure 71:
Ǹ ǒ Ǔ E_N +
2
E_NI2 ) ^ǒI_BNR_S^Ǔ2 ) 4kTR_S )
I_BIR_F
NG
)
4kTR_F
NG
(2)
Evaluating these two equations for the circuit and
component values shown in Figure 61 will give a total
output spot noise voltage of 19.3nV/√Hz and a total
equivalent input spot noise voltage of 9.65nV/√Hz.
This 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.
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 OPA2832 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 is done by matching the
DC source resistances appearing at the two inputs.
Evaluating the configuration of Figure 63 (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 × 7.5mV) + (400Ω × 1.5µA)
• = –14.4mV to +15.6mV
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