OPA2822UG4 Datasheet, PDF (14/33 Pages) Texas Instruments – Dual, Wideband, Low-Noise Operational Amplifier

APPLICATIONS INFORMATION
WIDEBAND NONINVERTING OPERATION
The OPA2822 provides a unique combination of features in
a wideband dual, unity-gain stable, voltage-feedback ampli-
fier to support the extremely high dynamic range require-
ments of emerging communications technologies. Combin-
ing low 2nV/√Hz input voltage noise with harmonic distortion
performance that can exceed 100dBc SFDR through 2MHz,
the OPA2822 provides the highest dynamic range input
interface for emerging high speed 14-bit (and higher) con-
verters. To achieve this level of performance, careful atten-
tion to circuit design and board layout is required.
Figure 1 shows the gain of +2 configuration used as the basis
for the Electrical Characteristics table and most of the Typical
Characteristics at ±6V operation. While the characteristics are
given using split ±6V supplies, most of the electrical and typical
characteristics also apply to a single-supply +12V design where
the input and output operating voltages are centered at the
midpoint of the +12V supply. Operation at ±5V will very nearly
match that shown for the ±6V operating point. Most of the
reference curves were characterized using signal sources with
50Ω driving impedance, and with measurement equipment
presenting a 50Ω load impedance. In Figure 1, the 50Ω shunt
resistor at the V_I terminal matches the source impedance of the
test signal generator, while the 50Ω series resistor at the V_O
terminal provides a matching resistor for the measurement
equipment load. Generally, data sheet voltage swing specifica-
tions are at the output pin (V_O in Figure 1), while output power
(dBm) specifications are at the matched 50Ω load. The total
100Ω load at the output, combined with the total 804Ω total
feedback network load for the noninverting configuration of
Figure 1, presents the OPA2822 with an effective output load of
89Ω. While this is a good load value for frequency response
measurements, distortion will improve rapidly with lighter output
loads. Keeping the same feedback network and increasing the
load to 200Ω will result in a total load of 160Ω for the distortion
performance reported in the Electrical Characteristics table.
+5V
+V_S
0.1µF
6.8µF
50Ω Source
V_I
50Ω
1/2
OPA2822
50Ω Load
V_O ^50Ω
R_F
402Ω
R_G
402Ω
–V_S
–5V
0.1µF
6.8µF
FIGURE 1. Noninverting G = +2 Specification and Test
Circuit.
For higher gains, the feedback resistor (R_F) was held at 402Ω
and the gain resistor (R_G) adjusted to develop the Typical
Characteristics.
Voltage-feedback op amps, unlike current-feedback designs,
can use a wide range of resistor values to set their gains. A low-
noise part like the OPA2822 will deliver low total output noise
only if the resistor values are kept relatively low. For the circuit
of Figure 1, the resistors contribute an input-referred voltage
noise component of 1.8nV/√Hz, which is approaching the value
of the amplifier’s intrinsic 2nV/√Hz. For a more complete
description of the feedback network’s impact on noise, see the
Setting Resistor Values to Minimize Noise section later in this
data sheet. In general, the parallel combination of R_F and R_G
should be < 300Ω to retain the low-noise performance of the
OPA2822. However, setting these values too low can impair
distortion performance due to output loading, as shown in the
distortion versus load data in the Typical Characteristics.
WIDEBAND INVERTING OPERATION
Operating the OPA2822 as an inverting amplifier has several
benefits and is particularly appropriate as part of the hybrid
design in an xDSL receiver application. Figure 2 shows the
inverting gain of –1 circuit used as the basis of the inverting
mode Typical Characteristics.
+5V
+V_S
0.1µF
6.8µF
0.1µF
R_S
309Ω
1/2
OPA2822
50Ω Load
V_O ^50Ω
50Ω Source
V_I
R_G
604Ω
R_M
54.9Ω
R_F
604Ω
–V_S
–5V
0.1µF
6.8µF
FIGURE 2. Inverting G = –1 Specification and Test
Circuit.
In the inverting case, only the R_F element of the feedback
network 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 an effective load of 86Ω in this
inverting configuration. Gain resistor R_G is set to achieve the
desired inverting gain (in this case 604Ω 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 = 54.9Ω in parallel with the 604Ω
gain setting resistor yields a matched input impedance of
50Ω. R_M is needed only when the input must be matched to
a source impedance, as in the characterization testing done
using the circuit of Figure 2.
14
OPA2822
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