OPA2846_14 Datasheet, PDF (12/30 Pages) Texas Instruments – Dual, Wideband, Low-Noise, Voltage-Feedback Operational Amplifier

OPA2846
SBOS274C −JUNE 2003 − REVISED AUGUST 2008
APPLICATIONS INFORMATION
WIDEBAND, NONINVERTING OPERATION
The OPA2846 provides a unique combination of
features—low input voltage noise along with a very low
distortion output stage—to give one of the highest dynamic
range dual op amps available. Its very high Gain
Bandwidth Product (GBP) can be used either to deliver
high signal bandwidths at high gains, or to deliver very low
distortion signals at moderate frequencies and lower
gains. To achieve the full performance of the OPA2846,
careful attention to printed circuit board (PCB) layout and
component selection is required, as discussed in the
remaining sections of this data sheet.
Figure 1 shows the noninverting gain of +10 circuit used as
the basis of the Electrical Characteristics and most of the
Typical Characteristics. Most of the curves were charac-
terized 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
generator, while the 50Ω series resistor at the V_O terminal
provides a matching resistor for the measurement equip-
ment 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 503Ω
total feedback network load, presents the OPA2846 with
an effective output load of 83Ω for the circuit of Figure 1.
0.1µF
+5V
+V_S
6.8µF
50Ω Source
V_I
50Ω
1/2
O PA 2 846
50Ω Load
V_O ^50Ω
R_F
453Ω
R_G
50Ω
6.8µF
0.1µF
− V_S
−5V
Figure 1. Noninverting, G = +10 Specification and
Test Circuit
Voltage-feedback op amps, unlike current-feedback
designs, can use a wide range of resistor values to set their
gains. The circuit of Figure 1, and the specifications at
other gains, uses the constraint that R_G should always be
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set to 50Ω and R_F adjusted to get the desired gain.
Observing this guideline will ensure that the thermal noise
contribution of the feedback network is insignificant
compared to the 1.2nV/√Hz input voltage noise for the op
amp itself.
WIDEBAND, INVERTING GAIN OPERATION
Operating the OPA2846 as an inverting amplifier has
several benefits and is particularly appropriate when a
matched input impedance is required. Figure 2 shows the
inverting gain circuit used as the basis of the inverting
mode Typical Characteristics.
+5V
+V_S
0.1µF 6.8µF
0.1µF
91Ω
1 /2
OPA2846
50Ω Load
V_O 50Ω
50Ω Source ^RG
R_F
50Ω
1kΩ
V_I
0.1µF 6.8µF
−V_S
−5V
Figure 2. Inverting, G = −20 Characterization
Circuit
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 acts as both the input
termination resistor and the gain setting resistor for the
circuit. Although the signal gain (V_O/V_I) for the circuit of
Figure 2 is double that for Figure 1, the noise gains are in
fact equal when the 50Ω source resistor is included. This
has the interesting effect of doubling the equivalent GBP
of the amplifier. This can be seen in comparing the G = +10
and G = −20 small-signal frequency response curves. Both
show approximately 250MHz bandwidth, but the inverting
configuration of Figure 2 gives 6dB higher signal gain. If
the signal source is actually the low impedance output of
another amplifier, R_G should be increased to the minimum
load resistance value allowed for that amplifier and R_F
should be adjusted to achieve the desired gain. For stable
operation of the OPA2846, it is critical that this driving
amplifier show a very low output impedance at frequencies
beyond the expected closed-loop bandwidth for the
OPA2846.