board delivered with descriptive documentation. The sum-
mary information for these boards is shown in the table
below.
over-temperature specifications because the output stage
junction temperatures are higher than the minimum specified
operating ambient.
DRIVING CAPACITIVE LOADS
BOARD
PART
NUMBER
LITERATURE
REQUEST
NUMBER
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 amplifier like the OPA692 can be
very susceptible to decreased stability and frequency re-
sponse peaking when a capacitive load is placed directly on
the output pin. When the amplifier’s open-loop output resis-
tance is considered, this capacitive load introduces an addi-
tional pole in the signal path that can decrease the phase
margin. Several external solutions to this problem have been
suggested. 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 capaci-
tive load. This does not eliminate the pole from the loop
response, but rather shifts it and adds a zero at a higher
frequency. The additional zero acts to cancel the phase lag
from the capacitive load pole, thus increasing the phase
margin and improving stability.
PRODUCT
PACKAGE
OPA692ID
OPA692IDBV
SO-8
SOT23-6
DEM-OPA68xU
DEM-OPA6xxN
SBOU009
SBOU010
To request any of these boards, check the Texas Instruments
web site at www.ti.com.
OPERATING SUGGESTIONS
GAIN SETTING
Setting the gain with the OPA692 is very easy. For a gain of
+2, ground the –IN pin and drive the +IN pin with the signal.
For a gain of +1, leave the –IN pin open and drive the +IN pin
with the signal. For a gain of –1, ground the +IN pin and drive
the –IN pin with the signal. As the internal resistor values (not
their ratio) change over temperature and process, external
resistors should not be used to modify the gain.
OUTPUT CURRENT AND VOLTAGE
The OPA692 provides output voltage and current capabilities
that are unsurpassed in a low-cost monolithic op amp. Under
no-load conditions at +25°C, the output voltage typically
swings closer than 1V to either supply rail; the tested swing
limit is within 1.2V of either rail. Into a 15Ω load (the minimum
tested load), it is specified to deliver more than ±160mA.
The Typical Characteristics show the recommended “RS vs
Capacitive Load” and the resulting frequency response at the
load. Parasitic capacitive loads greater than 2pF can begin to
degrade the performance of the OPA692. Long PC board
traces, unmatched cables, and connections to multiple de-
vices can easily cause this value to be exceeded. Always
consider this effect carefully, and add the recommended
series resistor as close as possible to the OPA692 output pin
(see the Board Layout Guidelines section).
The specifications described previously, though familiar in
the industry, consider voltage and current limits separately. In
many applications, it is the voltage times current, or V-I
product, which is more relevant to circuit operation. Refer to
the “Output Voltage and Current Limitations” plot in the
Typical Characteristics. The X- and Y-axes of this graph show
the zero-voltage output current limit and the zero-current
output voltage limit, respectively. The four quadrants give a
more detailed view of the OPA692 output drive capabilities,
noting that the graph is bounded by a safe operating area of
1W maximum internal power dissipation. Superimposing
resistor load lines onto the plot shows that the OPA692 can
drive ±2.5V into 25Ω, or ±3.5V into 50Ω without exceeding
the output capabilities or the 1W dissipation limit. A 100Ω
load line (the standard test circuit load) shows the full ±3.9V
output swing capability (see the Electrical Characteristics).
DISTORTION PERFORMANCE
The OPA692 provides good distortion performance into a
100Ω load on ±5V supplies. Relative to alternative solutions, it
provides exceptional performance into lighter loads and/or
operating on a single +5V supply. Generally, until the funda-
mental 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 1) this is the sum
of RF + RG, while in the inverting configuration, it is just RF.
Also, providing an additional supply decoupling capacitor
(0.1µF) between the supply pins (for bipolar operation) im-
proves the 2nd-order distortion slightly (3dB to 6dB).
The minimum specified output voltage and current 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
Electrical Characteristics. As the output transistors deliver
power, their junction temperatures increase, decreasing their
In most op amps, increasing the output voltage swing increases
harmonic distortion directly. The Typical Characteristics show the
2nd-harmonic increasing at a little less than the expected 2x rate
while the 3rd-harmonic increases at a much lower rate than the
expected 3x. Where the test power doubles, the difference
between it and the 2nd-harmonic decreases less than the
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 volt-
age and current is always greater than that shown in the
OPA692
SBOS236C
17
www.ti.com