OPA2680 Datasheet, PDF (16/21 Pages) Burr-Brown (TI) – Dual Wideband, Voltage Feedback OPERATIONAL AMPLIFIER With Disable

several performance benefits. Figure 9 shows a typical
inverting configuration where the I/O impedances and
signal gain from Figure 1 are retained in an inverting
circuit configuration.
+5V
0.1µF
R_B
146Ω
+
0.1µF
6.8µF
1/2
OPA2680
R_O
V_O 50Ω
50Ω Load
50Ω
Source
V_I
R_G
200Ω
R_M
67Ω
R_F
402Ω
V_O
V_I
= –2
0.1µF
6.8µF
+
ance is added in series with R_G for calculating the noise gain
(NG). The resultant NG is 2.8 for Figure 9, as opposed to only
2 if R_M could be eliminated as discussed above. The band-
width will therefore be slightly lower for the gain of –2 circuit
of Figure 9 than for the gain of +2 circuit of Figure 1.
The third important consideration in inverting amplifier
design is setting the bias current cancellation resistor on the
non-inverting input (R_B). If this resistor is set equal to the
total DC resistance looking out of the inverting node, the
output DC error, due to the input bias currents, will be
reduced to (Input Offset Current) • R_F. If the 50Ω source
impedance is DC-coupled in Figure 9, the total resistance to
ground on the inverting input will be 228Ω. Combining this
in parallel with the feedback resistor gives the R_B = 146Ω
used in this example. To reduce the additional high fre-
quency noise introduced by this resistor, it is sometimes
bypassed with a capacitor. As long as R_B <350Ω, the
capacitor is not required since the total noise contribution of
all other terms will be less than that of the op amp’s input
noise voltage. As a minimum, the OPA2680 requires an R_B
value of 50Ω to damp out parasitic-induced peaking—a
direct short to ground on the non-inverting input runs the
risk of a very high frequency instability in the input stage.
–5V
OUTPUT CURRENT AND VOLTAGE
FIGURE 9. Gain of –2 Example Circuit.
In the inverting configuration, three key design consider-
ation 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), R_G
may be set equal to the required termination value and R_F
adjusted to give the desired gain. This is the simplest
approach and results in optimum bandwidth and noise per-
formance. However, at low inverting gains, the resultant
feedback resistor value can present a significant load to the
amplifier output. For an inverting gain of –2, setting R_G to
50Ω for input matching eliminates the need for R_M but
requires a 100Ω feedback resistor. This has the interesting
advantage that the noise gain becomes equal to 2 for a 50Ω
source impedance—the same as the non-inverting circuits
considered above. However, the amplifier output will now
see the 100Ω feedback resistor in parallel with the external
load. In general, the feedback resistor should be limited to
the 200Ω to 1.5kΩ range. In this case, it is preferable to
increase both the R_F and R_G values as shown in Figure 8, and
then achieve the input matching impedance with a third
resistor (R_M) to ground. The total input impedance 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 influences the bandwidth. For
the example in Figure 9, the R_M value combines in parallel
with the external 50Ω source impedance, yielding an effec-
tive driving impedance of 50Ω || 67Ω = 28.6Ω. This imped-
The OPA2680 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
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 Typical
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 quadrants give a
more detailed view of the OPA2680’s output drive capabili-
ties, noting that the graph is bounded by a “Safe Operating
Area” of 1W maximum internal power dissipation for each
channel separately. Superimposing resistor load lines onto
the plot shows that the OPA2680 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 capabil-
ity, as shown in the typical specifications.
The minimum specified output voltage and current specifi-
cations 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
®
OPA2680
16