OPA842_15 Datasheet, PDF (11/29 Pages) Texas Instruments – Wideband, Low Distortion, Unity-Gain Stable, Voltage-Feedback OPERATIONAL AMPLIFIER

OPA842
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SBOS267D – NOVEMBER 2002 – REVISED SEPTEMBER 2010
APPLICATION INFORMATION
WIDEBAND CURRENT FEEDBACK
OPERATION
The OPA842 combination of speed and dynamic
range is easily achieved in a wide variety of
application circuits, providing that simple principles of
good design practice are observed. For example,
good power-supply decoupling, as shown in
Figure 37, is essential to achieve the lowest possible
harmonic distortion and smooth frequency response.
Proper printed circuit board (PCB) layout and careful
component selection will maximize the performance
of the OPA842 in all applications, as discussed in the
following sections of this data sheet.
Figure 37 shows the gain of +2 configuration used as
the basis for most of the Typical Characteristics. Most
of the curves were characterized using signal sources
with 50Ω driving impedance and with measurement
equipment presenting 50Ω load impedance. In
Figure 37, the 50Ω shunt resistor at the V_I terminal
matches the source impedance of the test generator
while the 50Ω series resistor at the V_O terminal
provides a matching resistor for the measurement
equipment load. Generally, data sheet specifications
refer to the voltage swing at the output pin (V_O in
Figure 37). The 100Ω load, combined with the 804Ω
total feedback network load, presents the OPA842
with an effective load of approximately 90Ω in
Figure 37.
+5V
+V_S
50W Source
V_IN
50W
0.1mF
2.2mF
OPA842
R_S ^{50W Load}
V_O 50W
R_F
402W
R_G
402W
0.1mF
2.2mF
-V_S
-5V
Figure 37. Gain of +2, High-Frequency
Application and Characterization Circuit
WIDEBAND INVERTING OPERATION
Operating the OPA842 as an inverting amplifier has
several benefits and is particularly useful when a
matched 50Ω source and input impedance is
required. Figure 38 shows the inverting gain of –2
circuit used as the basis of the inverting mode Typical
Characteristics.
+5V
0.1mF
R_T
147W
0.1mF
2.2mF
OPA842
50W Load
V_O ^50W
50W Source
V_I
R_G
200W
R_M
66.5W
R_F
402W
0.1mF
2.2mF
-5V
Figure 38. Inverting G = –2 Specifications and
Test Circuit
In the inverting case, just the feedback resistor
appears as part of the total output load in parallel with
the actual load. For the 100Ω load used in the Typical
Characteristics, this gives a total load of 80Ω in this
inverting configuration. The gain resistor is set to get
the desired gain (in this case, 200Ω for a gain of –2)
while an additional input matching resistor (R_M) can
be used to set the total input impedance equal to the
source if desired. In this case, R_M = 66.5Ω in parallel
with the 200Ω gain setting resistor gives a matched
input impedance of 50Ω. This matching is only
needed when the input needs to be matched to a
source impedance, as in the characterization testing
done using the circuit of Figure 38. The OPA842
offers extremely good dc accuracy as well as low
noise and distortion. To take full advantage of that dc
precision, the total dc impedance looking out of each
of the input nodes must be matched to get bias
current cancellation. For the circuit of Figure 38, this
requires the 147Ω resistor shown to ground on the
noninverting input. The calculation for this resistor
includes a dc-coupled 50Ω source impedance along
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