OPA2631 Datasheet, PDF (15/17 Pages) Burr-Brown (TI) – Dual, Low Power, Single-Supply OPERATIONAL AMPLIFIER

DRIVING CAPACITIVE LOADS
One of the most demanding and yet very common load
conditions for an op amp is capacitive loading. Often, the
capacitive load is the input of an A/D converter—including
additional external capacitance which may be recommended
to improve A/D linearity. A high speed, high open-loop gain
amplifier like the OPA2631 can be very susceptible to
decreased stability and closed-loop response peaking when
a capacitive load is placed directly on the output pin. When
the primary considerations are frequency response flatness,
pulse response fidelity and/or distortion, the simplest and
most effective solution is to isolate the capacitive load from
the feedback loop by inserting a series isolation resistor
between the amplifier output and the capacitive load.
The Typical Performance Curves show the recommended
R_S versus capacitive load and the resulting frequency re-
sponse at the load. Parasitic capacitive loads greater than
2pF can begin to degrade the performance of the OPA2631.
Long PC board traces, unmatched cables, and connections to
multiple devices can easily exceed this value. Always con-
sider this effect carefully, and add the recommended series
resistor as close as possible to the output pin (see Board
Layout Guidelines).
The criterion for setting this R_S resistor is a maximum
bandwidth, flat frequency response at the load. For a gain of
+2, the frequency response at the output pin is already
slightly peaked without the capacitive load, requiring rela-
tively high values of R_S to flatten the response at the load.
Increasing the noise gain will also reduce the peaking (see
Figure 6).
DISTORTION PERFORMANCE
The OPA2631 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, the 3rd harmonic will
dominate the distortion. Focusing then on the 3rd harmonic,
increasing the load impedance improves distortion directly.
Remember that the total load includes the feedback network;
in the non-inverting configuration (Figure 1) this is sum of
R_F + R_G, while in the inverting configuration, it is just R_F.
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 6.0nV/√Hz input voltage noise for
the OPA2631 is, however, much lower than comparable
amplifiers. The input-referred voltage noise, and the two
input-referred current noise terms (1.9pA/√Hz), combine to
give low output noise under a wide variety of operating
conditions. Figure 8 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.
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 8.
E_NI
1/2
R_S
I_BN
OPA2631
E_O
E_RS
√4kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6 • 10^–20J
at 290°K
FIGURE 8. Noise Analysis Model.
Equation 1:
( ) ^{( ) ( ) E}O ^{= E}NI^{2 + I}BN^RS ^{2 + 4kTR}S ^{NG2 + I}BI^RF ^{2 + 4kTR}F^NG
Dividing this expression by the noise gain (NG = (1+R_F/R_G))
will give the equivalent input-referred spot noise voltage at
the non-inverting input, as shown in Equation 2.
Equation 2:
( ) E_N =
E_NI² +
I _BN R _S
2
+
4 kTR _S
+


I _BI R _F
NG


2
+
4 kTR _F
NG
Evaluating these two equations for the circuit and compo-
nent values shown in Figure 1 will give a total output spot
noise voltage of 13.1nV/√Hz and a total equivalent input
spot noise voltage of 6.6nV/√Hz. This is including the noise
added by the resistors. This total input-referred spot noise
voltage is not much higher than the 6.0nV/√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
400Ω, and the input attenuation is low.
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 OPA2631
gives even tighter control than comparable products. Al-
though the high-speed input stage does require relatively
high input bias current (typically 11µA out of each input
terminal), the close matching between them may be used to
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OPA2631