OPA631N BURR-BROWN [Burr-Brown Corporation], OPA631N Datasheet - Page 14

• Manufacturer Part Number: OPA631N
• Description: Low Power, Single-Supply OPERATIONAL AMPLIFIERS TM
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

value of 50
direct short to ground on the non-inverting input runs the
risk of a very high frequency instability in the input stage.
OUTPUT CURRENT AND VOLTAGE
The OPA631 and OPA632 provide outstanding output volt-
age capability. Under no-load conditions at +25 C, the
output voltage typically swings closer than 130mV to either
supply rail; the guaranteed swing limit is within 400mV of
either rail (V
The minimum specified output voltage and current specifi-
cations over temperature are set by worst-case simulations at
the cold temperature extreme. Only at cold start-up will the
output current and voltage decrease to the numbers shown in
the guaranteed tables. As the output transistors deliver power,
their junction temperatures will increase, decreasing their
V
increasing their current gains (increasing the available out-
put 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.
To maintain maximum output stage linearity, no output
short-circuit protection is provided. This will not normally
be a problem since most applications include a series match-
ing resistor at the output that will limit the internal power
dissipation if the output side of this resistor is shorted to
ground.
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 OPA631 and OPA632 can be very suscep-
tible 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 sim-
plest 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
sponse at the load. Parasitic capacitive loads greater than
2pF can begin to degrade the performance of the OPA631
and OPA632. Long PC board traces, unmatched cables, and
connections to multiple devices can easily exceed this value.
Always consider this effect carefully, and add the recom-
mended series resistor as close as possible to the output pin
(see Board Layout Guidelines).
BE
S
versus capacitive load and the resulting frequency re-
’s (increasing the available output voltage swing) and
S
®
= +5V).
to damp out parasitic-induced peaking—a
OPA631, OPA632
14
The criterion for setting this R
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
Increasing the noise gain will also reduce the peaking (see
Figure 4).
DISTORTION PERFORMANCE
The OPA631 and OPA632 provide good distortion perfor-
mance into a 150 load. Relative to alternative solutions, it
provides exceptional performance into lighter loads and/or
operating on a single +3V supply. Generally, the 3rd har-
monic will dominate the distortion. Focusing then on the 3rd
harmonic, increasing the load impedance improves distor-
tion directly. Remember that the total load includes
the feedback network; in the non-inverting configuration
(Figure 1) this is sum of R
configuration, it is just R
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 OPA631 and OPA632 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 6 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 6.
FIGURE 6. Noise Analysis Model.
E
RS
R
S
4kTR
4kT
R
G
S
I
BN
E
NI
S
to flatten the response at the load.
F
.
R
F
G
+ R
OPA63x
S
G
resistor is a maximum
, while in the inverting
I
BI
R
F
4kT = 1.6 • 10
at 290 K
4kTR
F
–20
J
E
O