Datasheet

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SBOS317D − SEPTEMBER 2004 − REVISED AUGUST 2008
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13
APPLICATIONS INFORMATION
WIDEBAND VOLTAGE-FEEDBACK
OPERATION
The combination of speed and dynamic range offered by
the OPA4820 is easily achieved in a wide variety of
application circuits, providing that simple principles of
good design are observed. For example, good
power-supply decoupling, as shown in Figure 1, 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
OPA4820 in all applications, as discussed in the following
sections of this data sheet.
Figure 1 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 1, 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 swings at the output pin
(V
O
in Figure 1). The 100Ω load, combined with the 804Ω
total feedback network load, presents the OPA4820 with
an effective load of approximately 90Ω in Figure 1.
+5V
−5V
−V
S
+V
S
R
S
50Ω
V
O
V
IN
50
Ω
+
2.2µF
+
2.2µF
0.1µF
R
G
402Ω
R
F
402Ω
50Ω Source
50Ω Load
0.1µF
1/4
OPA4820
Figure 1. Gain of +2, High-Frequency Application
and Characterization Circuit
WIDEBAND INVERTING OPERATION
Operating the OPA4820 as an inverting amplifier has
several benefits and is particularly useful when a matched
50Ω source and input impedance is required. Figure 2
shows the inverting gain of −1 circuit used as the basis of
the inverting mode Typical Characteristics.
+5V
−
5V
50
Ω
V
O
V
I
+
0.1µF 2.2µF
+
0.1
µ
F 2.2
µ
F
R
M
57.6
Ω
R
T
205Ω
R
F
402Ω
50Ω Source
50Ω Load
0.01
µ
F
R
G
402Ω
1/4
OPA4820
Figure 2. Inverting G = −1 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
402Ω for a gain of −1) 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
= 57.6Ω in
parallel with the 402Ω 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 2.
The OPA4820 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 2, this requires the
205Ω resistor shown to ground on the noninverting input. The
calculation for this resistor includes a DC-coupled 50Ω
source impedance along with R
G
and R
M
. Although this
resistor will provide cancellation for the bias current, it must
be well decoupled (0.01µF in Figure 2) to filter the noise
contribution of the resistor and the input current noise.
As the required R
G
resistor approaches 50Ω at higher
gains, the bandwidth for the circuit in Figure 2 will far
exceed the bandwidth at that same gain magnitude for the
noninverting circuit of Figure 1. This occurs due to the
lower noise gain for the circuit of Figure 2 when the 50Ω
source impedance is included in the analysis. For
instance, at a signal gain of −10 (R
G
= 50Ω, R
M
= open,
R
F
= 499Ω) the noise gain for the circuit of Figure 2 will
be 1 + 499Ω/(50Ω + 50Ω) = 6 as a result of adding the 50Ω
source in the noise gain equation. This gives considerable
higher bandwidth than the noninverting gain of +10. Using
the 240MHz gain bandwidth product for the OPA4820, an
inverting gain of −10 from a 50Ω source to a 50Ω R
G
gives
42MHz bandwidth, whereas the noninverting gain of +10
gives 27MHz.