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OPA2690 Datasheet(PDF) 20 Page - Burr-Brown (TI)

[Old version datasheet] Texas Instruments acquired Burr-Brown Corporation. Click here to check the latest version.
Part No. OPA2690
Description  Dual, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable
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Maker  BURR-BROWN [Burr-Brown (TI)]
Homepage  http://www.burr-brown.com
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OPA2690 Datasheet(HTML) 20 Page - Burr-Brown (TI)

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OPA2690
SBOS238D − JUNE 2002 − REVISED DECEMBER 2004
www.ti.com
20
feedback resistor. This has the interesting advantage that
the noise gain becomes equal to 2 for a 50
Ω source
impedance—the same as the noninverting circuits
considered in the previous section. The amplifier output,
however, will now see the 100
Ω feedback resistor in
parallel with the external load. In general, the feedback
resistor should be limited to the 200
Ω to 1.5kΩ range. In
this case, it is preferable to increase both the RF and RG
values (see Figure 8), and then achieve the input matching
impedance with a third resistor (RM) to ground. The total
input impedance becomes the parallel combination of RG
and RM.
1/2
O P A 269 0
50
R
F
402
R
G
200
R
B
146
R
M
67
Source
+5V
−5V
R
O
50
0.1
µF
6.8
µF
+
0.1
µF
0.1
µF
6.8
µF
+
50
Ω Load
V
O
V
I
=
−2
V
O
V
I
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 RM value
combines in parallel with the external 50
Ω source
impedance, yielding an effective driving impedance of
50
Ω 67Ω = 28.6Ω. This impedance is added in series
with RG for calculating the noise gain (NG). The resultant
NG is 2.8 for Figure 12, as opposed to only 2 if RM 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 (RB). 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)
× RF. If the 50Ω source
impedance is DC-coupled in Figure 10, the total
resistance to ground on the inverting input will be 228
Ω.
Combining this in parallel with the feedback resistor gives
the RB = 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
RB < 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 RB 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 15
Ω 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 100
Ω 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 VBEs (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.


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