OPA2690 Datasheet, PDF (20/30 Pages) Burr-Brown (TI) – Dual, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2690
SBOS238D − JUNE 2002 − REVISED DECEMBER 2004
feedback resistor. This has the interesting advantage that
the noise gain becomes equal to 2 for a 50Ω source
impedance—the same as the noninverting circuits
considered in the previous section. The amplifier output,
however, 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 (see 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.
+5V
0.1µF
R_B
146Ω
+
0.1µ F
6.8µF
1/2
O PA 269 0
R_O
V_O 50Ω
50Ω Load
50Ω
Source
V_I
R_G
200Ω
R_M
67Ω
R_F
402Ω
V_{O = −2}
V_I
0.1µ F
6.8µ F
+
− 5V
Figure 12. Gain of −2 Example Circuit
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 12, the R_M value
combines in parallel with the external 50Ω source
impedance, yielding an effective driving impedance of
50Ω 67Ω = 28.6Ω. This impedance is added in series
with R_G for calculating the noise gain (NG). The resultant
NG is 2.8 for Figure 12, as opposed to only 2 if R_M could
be eliminated as discussed above. The bandwidth will
therefore be slightly lower for the gain of −2 circuit of
Figure 12 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 noninverting 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 10, the total
resistance to ground on the inverting input will be 228Ω.
20
www.ti.com
Combining this in parallel with the feedback resistor gives
the R_B = 146Ω used in this example. To reduce the
additional high-frequency noise introduced by this resistor,
it is sometimes bypassed with a capacitor. As long as
R_B < 350Ω, the capacitor is not required because the total
noise contribution of all other terms will be less than that
of the op amp input noise voltage. As a minimum, the
OPA2690 requires an R_B value of 50Ω to damp out
parasitic-induced peaking—a direct short to ground on the
noninverting input runs the risk of a very high-frequency
instability in the input stage.
OUTPUT CURRENT AND VOLTAGE
The OPA2690 provides exceptional output voltage and
current capabilities 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 specified
swing limit is within 1.2V of either rail. Into a 15Ω load (the
minimum tested load), it will 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 OPA2690
output drive capabilities, 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 OPA2690
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 (see the
Electrical Characteristics).
The minimum specified output voltage and current
specifications 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, their junction
temperatures increase, decreasing their V_BEs (increasing
the available output voltage swing) and increasing their
current gains (increasing the available output current). In
steady-state operation, the available output voltage and
current is always greater than that shown in the
over-temperature specifications because the output stage
junction temperatures will be 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 OPA2690. The circuit acts to
limit the maximum source or sink current to approximately
250mA.