OPA2694ID Datasheet, PDF (17/28 Pages) Texas Instruments – Dual, Wideband, Low-Power, Current Feedback Operational Amplifier

OPA2694
www.ti.com
As the desired signal gain increases, this equation will
eventually predict a negative R_F. A somewhat subjective
limit to this adjustment can also be set by holding R_G to a
minimum value of 20Ω. Lower values will load both the
buffer stage at the input and the output stage, if R_F gets too
low, actually decreasing the bandwidth. Figure 11 shows
the recommended R_F versus NG for ±5V operation. The
values for R_F versus gain shown here are approximately
equal to the values used to generate the Typical
Characteristics. They differ in that the optimized values
used in the Typical Characteristics are also correcting for
board parasitics not considered in the simplified analysis
leading to Equation (3). The values shown in Figure 11
give a good starting point for design where bandwidth
optimization is desired.
450
400
350
300
250
200
150
0
5
10
15
20
Noise Gain
Figure 11. Feedback Resistor vs Noise Gain
The total impedance going into the inverting input may be
used to adjust the closed-loop signal bandwidth. Inserting
a series resistor between the inverting input and the
summing junction will increase the feedback impedance
(denominator of Equation (2)), decreasing the bandwidth.
This approach to bandwidth control is used for the
inverting summing circuit of Figure 4. The internal buffer
output impedance for the OPA2694 is slightly influenced
by the source impedance looking out of the noninverting
input terminal. High source resistors will have the effect of
increasing R_I, decreasing the bandwidth.
OUTPUT CURRENT AND VOLTAGE
The OPA2694 provides output voltage and current
capabilities that are not usually found in wideband
amplifiers. Under no-load conditions at 25°C, the output
voltage typically swings closer than 1.2V to either supply
rail; the +25°C swing limit is within 1.2V of either rail. Into
a 15Ω load (the minimum tested load), it is tested to deliver
more than ±55mA.
SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013
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 OPA2694
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 OPA2694 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 ±3.4V output swing
capability, as shown in the Electrical Charateristics.
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 Characteristic tables. As the output
transistors deliver power, the junction temperatures will
increase, decreasing both V_BE (increasing the available
output voltage swing) and increasing the 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 that may be
recommended to improve ADC linearity. A high-speed,
high open-loop gain amplifier like the OPA2694 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 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 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 capacitive load. This does not eliminate the
pole from the loop response, but rather shifts it and adds
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