OPA3681 Datasheet, PDF (17/21 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

50Ω
Source
V_I
R_G
226Ω
R_M
64.9Ω
+5V
Power supply
de-coupling
not shown
1/3
OPA3681
DIS
50Ω Load
V_O ^50Ω
R_F
464Ω
–5V
FIGURE 11. Inverting Gain of –2 with Impedance Matching.
In the inverting configuration, two key design consider-
ations must be noted. The first is that the gain resistor (R_G)
becomes part of the signal channel input impedance. If input
impedance matching is desired (which is beneficial when-
ever the signal is coupled through a cable, twisted pair, long
PC board trace or other transmission line conductor), it is
normally necessary to add an additional matching resistor to
ground. R_G by itself is normally not set to the required input
impedance since its value, along with the desired gain, will
determine a R_F which may be non-optimal from a frequency
response standpoint. The total input impedance for the
source becomes the parallel combination of R_G and R_M.
The second major consideration, touched on in the previous
paragraph, is that the signal source impedance becomes part
of the noise gain equation and will have slight effect on the
bandwidth through Equation 1. The values shown in Figure
11 have accounted for this by slightly decreasing R_F (from
Figure 1) to re-optimize the bandwidth for the noise gain of
Figure 11 (NG = 2.82) In the example of Figure 11, the R_M
value combines in parallel with the external 50Ω source
impedance, yielding an effective driving impedance of
50Ω || 64Ω = 28.1Ω. This impedance is added in series with
R_G for calculating the noise gain—which gives NG = 2.82.
This value, along with the R_F of Figure 10 and the inverting
input impedance of 45Ω, are inserted into Equation 3 to get
a feedback transimpedance nearly equal to the 589Ω opti-
mum value.
Note that the non-inverting input in this bipolar supply
inverting application is connected directly to ground. It is
often suggested that an additional resistor be connected to
ground on the non-inverting input to achieve bias current
error cancellation at the output. The input bias currents for
a current feedback op amp are not generally matched in
either magnitude or polarity. Connecting a resistor to ground
on the non-inverting input of the OPA3681 in the circuit of
Figure 11 will actually provide additional gain for that
input’s bias and noise currents, but will not decrease the
output DC error since the input bias currents are not matched.
OUTPUT CURRENT AND VOLTAGE
The OPA3681 provides output voltage and current capabili-
ties that are unsurpassed in a low cost dual monolithic op
amp. Under no-load conditions at 25°C, the output voltage
typically swings closer than 1V to either supply rail; the
guaranteed swing limit is within 1.2V of either rail. Into a
15Ω load (the minimum tested load), it is guaranteed to
deliver more than ±135mA.
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 Typi-
cal Performance Curves. 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 quad-
rants give a more detailed view of the OPA3681’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 OPA3681 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,
as shown in the Specifications Table.
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
guaranteed tables. As the output transistors deliver power,
their junction temperatures will increase, decreasing their
V_BE’s (increasing the available output voltage swing) and
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. However, shorting the output pin directly to the
adjacent positive power supply pins will, in most cases,
destroy the amplifier. If additional short-circuit protection
is required, consider a small series resistor in the power
supply leads. Under heavy output loads, this will reduce the
available output voltage swing. A 5Ω series resistor in each
power supply lead will limit the internal power dissipation to
less than 1W for an output short circuit 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 supply pins.
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OPA3681