Datasheet

OPA843
10
SBOS268C
www.ti.com
FIGURE 1. Gain of +5, High-Frequency Application and
Characterization Circuit.
APPLICATIONS INFORMATION
WIDEBAND NONINVERTING OPERATION
The OPA843’s combination of speed and dynamic range is
useful in a wide variety of application circuits, as long as
simple guidelines common to all high-speed amplifiers 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. Careful
PC board layout and component selection will maximize the
performance of the OPA843 in all applications, as discussed
in the following sections of this data sheet. Figure 1 shows
the gain of +5 configuration used as the basis for most of the
Typical Characteristics. Most of the curves were character-
ized using signal sources with 50Ω driving impedance and
with measurement equipment presenting 50Ω load imped-
ance. In Figure 1, the 50Ω shunt resistor at the input terminal
matches the source impedance of the test generator, while
the 50Ω series resistor at the V
O
terminal provides a match-
ing resistor for the measurement equipment load. Generally,
data sheet specifications refer to the voltage swing at the
output pin (V
O
in Figure 1) while those referring to load power
are at the 50Ω load. The total 100Ω load from the series and
shunt matching resistors, combined with the 502Ω total
feedback network load, presents the OPA843 with an effec-
tive output load of approximately 83Ω.
both the input termination resistor and the gain setting
resistor for the circuit. Although the signal gain for the circuit
of Figure 2 is equal to –8V/V (versus the +5V/V for Figure 1),
their noise gains are equal when the 50Ω source resistor is
included. This has the interesting effect of nearly doubling
the equivalent Gain Bandwidth Product (GBP) for the ampli-
fier. This can be seen in comparing the G = +5 and G = –8
small-signal frequency response curves. Both show approxi-
mately 260MHz bandwidth, but the inverting configuration of
Figure 2 is giving 4dB higher signal gain. If the signal source
is actually the low impedance output of another amplifier, R
G
is increased to the minimum value allowed at the output of
that amplifier and R
F
is adjusted to get the desired gain. It is
critical for stable operation of the OPA843 that this driving
amplifier show a very low output impedance through frequen-
cies exceeding the expected closed-loop bandwidth for the
OPA843.
An optional input termination resistor is also shown in Figure 2.
This R
M
resistor may be used to adjust the input impedance to
lower values when R
G
needs to be adjusted higher. This might
be desirable at lower gains where increasing R
F
will reduce the
output loading improving harmonic distortion performance. For
instance, at a gain of –4 an R
G
set to 50Ω will require a 200Ω
feedback resistor. In this case, adjusting R
F
to 400Ω, setting R
G
to 100Ω, and then adding a 100Ω R
M
resistor will deliver a gain
of –4 with a 50Ω input match.
BUFFERING HIGH-PERFORMANCE ADCs
A single-channel interface using the OPA843 can provide a low
noise/distortion interface to emerging 14-bit Analog-to-Digital
Converters (ADCs) through approximately 5MHz for medium
gain applications. Since the dominant distortion mechanism is
2nd-harmonic distortion, differential circuits using the OPA843
can extend this frequency range and/or power level to much
higher levels. The example on the front page of this data sheet,
for instance, shows better than 93dB SFDR at 5MHz for up to
8V
PP
signals. This is still being limited by the 2nd-harmonic with
WIDEBAND, INVERTING GAIN OPERATION
There can be significant benefits to operating the OPA843 as
an inverting amplifier. This is particularly true when a matched
input impedance is required. Figure 2 shows the inverting
gain circuit used as a starting point for the typical character-
istics showing inverting mode performance.
Driving this circuit from a 50Ω source, and constraining the
gain resistor, R
G
, to equal 50Ω will give both a signal
bandwidth and noise advantage. R
G
in this case is acting as
OPA843
+5V
–5V
–V
S
+V
S
R
S
50Ω
V
O
V
IN
50Ω
+
2.2µF
+
2.2µF
0.1µF
R
G
100Ω
R
F
402Ω
50Ω Source
50Ω Load
0.1µF
FIGURE 2. Inverting G = –8 Specification and Test Circuit.
OPA843
+5V
–5V
R
S
50Ω
V
O
V
I
R
T
+
2.2µF
+
2.2µF
0.1µF
R
M
(optional)
R
F
402Ω
50Ω Source
50Ω Load
0.1µF
R
G
50Ω