OPA2830 Datasheet, PDF (30/43 Pages) National Semiconductor (TI) – Dual, Low-Power, Single-Supply, Wideband OPERATIONAL AMPLIFIER

OPA2830
SBOS309D – AUGUST 2004 – REVISED AUGUST 2008.................................................................................................................................................. www.ti.com
OUTPUT CURRENT AND VOLTAGES
The OPA2830 provides outstanding output voltage
capability. For the +5V supply, under no-load
conditions at +25°C, the output voltage typically
swings closer than 90mV to either supply rail.
The minimum specified output voltage and current
specifications over temperature are set by worst-case
simulations at the cold temperature extreme. Only at
cold startup will the output current and voltage
decrease to the numbers shown in the ensured
tables. As the output transistors deliver power, their
junction temperatures will increase, decreasing their
V_BEs (increasing the available output voltage swing)
and increasing their current gains (increasing the
available output current). In steady-state operation,
the available output voltage and current will always
be greater than that shown in the over-temperature
specifications, since the output stage junction
temperatures will be higher than the minimum
specified operating ambient.
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
ADC—including additional external capacitance which
may be recommended to improve ADC linearity. A
high-speed, high open-loop gain amplifier like the
OPA2830 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 Characteristic curves show the
recommended R_S versus capacitive load and the
resulting frequency response at the load. Parasitic
capacitive loads greater than 2pF can begin to
degrade the performance of the OPA2830. Long PC
board traces, unmatched cables, and connections to
multiple devices can easily exceed this value. Always
consider this effect carefully, and add the
recommended series resistor as close as possible to
the output pin (see the Board Layout Guidelines
section).
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 relatively high values of R_S to flatten the
response at the load. Increasing the noise gain will
also reduce the peaking (see Figure 77).
DISTORTION PERFORMANCE
The OPA2830 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 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 72) 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 differentially
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 OPA2830 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 85 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
OPA2830
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 85. 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 85:
Ǹǒ Ǔ E_O +
ǒ Ǔ _{ENI2 )}
2
I_BNR_S ) 4kTR_S
NG² ) ^ǒI_BIR_F^Ǔ2 ) 4kTR_FNG
(1)
30
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