Datasheet

OPA693
18
SBOS285A
www.ti.com
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Two printed circuit (PC) boards are available to assist in the
initial evaluation of the circuit performance using the OPA693 in
its two package styles. Both are available free as unpopulated
PC boards delivered with descriptive documentation. The sum-
mary information for these boards is shown in Table I.
either the output capabilities or the 1W dissipation limit. A
100Ω load line (the standard test-circuit load) shows full
±3.8V output swing capability, as shown in the Typical
Characteristics.
The minimum specified output voltage and current specifica-
tions 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 over-temperature min/max specifications. As the out-
put transistors deliver power, their junction temperatures
increase, which decreases their V
BE
’s (increasing the avail-
able output voltage swing) and increases their current gains
(increasing the available output current). In steady state
operation, the available output voltage and current will al-
ways be greater than that shown in the over-temperature
characteristics 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 matching
resistor at the output that limits the internal power dissipation
if the output side of this resistor is shorted to ground.
However, shorting the output pin directly to an adjacent
positive power supply pin will, in most cases, destroy the
amplifier. If additional protection to a power-supply short is
required, consider a small series resistor in the power supply
leads. Under heavy output loads, this will reduce the avail-
able output voltage swing. A 5Ω series resistor in each
supply lead will limit the internal power dissipation to < 1W for
an output short while decreasing the available output voltage
swing only 0.5V, for up to 100mA desired load currents.
Always place the 0.1µF power supply decoupling capacitors
after these supply current limiting resistors directly on the
device supply pins.
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 OPA693 can be very susceptible to decreased stability
and may give closed-loop response peaking when a capaci-
tive load is placed directly on the output pin. When the
amplifier’s open loop output resistance is considered, this
capacitive load introduces an additional pole in the signal path
that can decrease the phase margin. Several external solu-
tions to this problem have been suggested. When the primary
considerations are frequency response flatness, pulse re-
sponse fidelity and/or distortion, the simplest and most effec-
tive solution is to isolate the capacitive load from the feedback
loop by inserting a series isolation resistor between the ampli-
fier output and the capacitive 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.
To request either of these boards, check the Texas Instru-
ments web site at www.ti.com.
OPERATING SUGGESTIONS
GAIN SETTING
Setting the gain for the OPA693 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, either leave the –IN pin open and drive the +IN
pin or drive both the +IN and –IN pins as shown in Figure 7.
For a gain of –1, ground the +IN pin and drive the –IN pin with
the input signal. An external resistor may be used in series
with the –IN pin to reduce the gain. However, since the internal
resistors (R
F
and R
G
) have a tolerance and temperature drift
different than the external resistor, the absolute gain accuracy
and gain drift over temperature will be relatively poor com-
pared to the previously described standard gain connections
using no external resistor.
OUTPUT CURRENT AND VOLTAGE
The OPA693 provides output voltage and current capabilities
that can easily support multiple video loads and/or 100Ω
loads with very low distortion. Under no-load conditions at
25°C, the output voltage typically swings to 1V of either
supply rail; the tested swing limit is within 1.2V of either rail.
Into a 15Ω load (the minimum tested load), it is tested to
deliver more than ±90mA.
The specifications described above, though familiar in the
industry, consider voltage and current limits separately. In
many applications, it is the voltage × 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 OPA693’s 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 OPA693 can
drive ±3.4V into 20Ω or ±3.7V into 50Ω without exceeding
DEMO BOARD LITERATURE
PART REQUEST
PRODUCT PACKAGE NUMBER NUMBER
OPA693ID SO-8 DEM-OPA68xU SBOU009
OPA693IDBV SOT23-6 DEM-OPA6xxN SBOU010
TABLE I. Demo Board Ordering Information.