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OPA2690 Datasheet(PDF) 14 Page - Texas Instruments

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Part No. OPA2690
Description  Triple, Wideband, Fixed Gain Video BUFFER AMPLIFIER With Disable
Download  27 Pages
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Manufacturer  TI [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI - Texas Instruments

OPA2690 Datasheet(HTML) 14 Page - Texas Instruments

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OPA3692
SBOS228E
14
www.ti.com
OPERATING SUGGESTIONS
GAIN SETTING
Setting the gain with the OPA3692 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 ratios) change significantly over tempera-
ture and process, external resistors should not be used to
modify the gain.
OUTPUT CURRENT AND VOLTAGE
The OPA3692 provides output voltage and current capabili-
ties 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 15
Ω load (the minimum
tested load), it is tested to deliver more than
±160mA.
The specifications described previously, 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 OPA3692 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 OPA3692 can drive
±2.5V into 25Ω or ±3.5V into 50Ω without exceeding the
output capabilities or the 1W dissipation limit. A 100
Ω load
line (the standard test circuit load) shows the full
±3.9V
output swing capability, as shown in the Electrical Character-
istics.
The minimum specified output voltage and current over-
temperature are set by worst-case simulations at the cold
temperature extreme. Only at cold start-up does the output
current and voltage decrease to the numbers shown in the
Electrical Characteristic tables. As the output transistors
deliver power, their junction temperatures increase, decreas-
ing their VBEs (increasing the available output voltage swing)
and increasing their current gains (increasing the available
output current). In steady-state operation, the available out-
put voltage and current is always greater than that shown in
the over-temperature specifications because the output stage
junction temperatures are higher than the minimum specified
operating ambient.
To protect the output stage from accidental shorts to ground
and the power supplies, output short-circuit protection is
included in the OPA3692. This circuit acts to limit the maxi-
mum source or sink current to approximately 250mA.
DRIVING CAPACITIVE LOADS
One of the most demanding, but 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 that may be recommended to improve
ADC linearity. A high-speed amplifier like the OPA3692 can be
very susceptible to decreased stability and closed-loop re-
sponse peaking when a capacitive load is placed directly on
the output pin. When the amplifier 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 solutions to this problem have been sug-
gested. When the primary considerations are frequency re-
sponse 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.
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 capaci-
tive load pole, thus increasing the phase margin and improv-
ing stability.
The Typical Characteristics show the recommended RS ver-
sus capacitive load and the resulting frequency response at
the load. Parasitic capacitive loads greater than 2pF can begin
to degrade the performance of the OPA3692. Long PCB
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 OPA3692 output pin
(see the Board Layout Guidelines section).
DISTORTION PERFORMANCE
The OPA3692 provides good distortion performance into a
100
Ω 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 dominates 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
RF + RG, whereas in the inverting configuration, it is just RF.
Also, providing an additional supply decoupling capacitor (0.1
µF)
between the supply pins (for bipolar operation) improves the
2nd-order distortion slightly (3dB to 6dB).
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 little less than the expected
3X rate. Where the test power doubles, the 2nd-harmonic
increases only by less than the expected 6dB, whereas the 3rd-


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