OPA2890 Datasheet, PDF (16/35 Pages) Texas Instruments – Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2890
SBOS364A – DECEMBER 2007 – REVISED DECEMBER 2007
APPLICATIONS INFORMATION
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WIDEBAND VOLTAGE-FEEDBACK
OPERATION
The OPA2890 provides an exceptional combination
of high output power capability in a dual, wideband,
unity-gain stable, voltage-feedback op amp using a
new high slew rate input stage. Typical differential
input stages used for voltage-feedback op amps are
designed to steer a fixed-bias current to the
compensation capacitor, setting a limit to the
achievable slew rate. The OPA2890 uses a new input
stage that places the transconductance element
between two input buffers, using the output currents
as the forward signal. As the error voltage increases
across the two inputs, an increasing current is
delivered to the compensation capacitor. This
configuration provides high slew rate (400V/µs) while
consuming relatively low quiescent current
(1.12mA/ch). This exceptional full-power performance
comes at the price of a slightly higher input noise
voltage than alternative architectures; however, the
8nV/√Hz input voltage noise for the OPA2890 is
exceptionally low for this type of input stage.
Figure 49 shows the DC-coupled, gain of +2V/V, dual
power-supply circuit configuration used as the basis
of the ±5V Electrical Characteristics and Typical
Characteristics. This illustration is for one channel;
the other channel is connected similarly. For test
purposes, the input impedance is set to 50Ω with a
resistor to ground and the output impedance is set to
200Ω. Voltage swings reported in the Electrical
Characteristics are taken directly at the input and
output pins, while output powers (dBm) are at the
matched 50Ω load. For the circuit of Figure 49, the
total effective load is 200Ω || 1.5kΩ. The disable
control line (MSOP-10 package only) is typically left
open for normal amplifier operation. Two optional
components are included in Figure 49. First, an
additional resistor (350Ω) is included in series with
the noninverting input. Combined with the 25Ω dc
source resistance looking back towards the signal
generator, this resistor gives an input bias current
cancelling resistance that matches the 750Ω source
resistance seen at the inverting input (see the DC
Accuracy and Offset Control section). In addition to
the usual power-supply decoupling capacitors to
ground, a 0.1µF capacitor is also included between
the two power-supply pins. In practical printed circuit
board (PCB) layouts, this optional capacitor typically
improves the 2nd-harmonic distortion performance by
3dB to 6dB.
+5V
+V_S
0.1mF
6.8mF
+
50W Source
350W
V_I
50W
V_D
1/2
OPA2890
DIS
100W Load
V_O 100W
0.1mF
R_F
750W
R_G
750W
-V_S
-5V
6.8mF
0.1mF
Figure 49. DC-Coupled, G = +2V/V, Bipolar
Supply, Specification and Test Circuit
Figure 50 illustrates the ac-coupled, gain of +2V/V,
single-supply circuit configuration used as the basis
of the +5V Electrical Characteristics and Typical
Characteristics. Though not a rail-to-rail design, the
OPA2890 requires minimal input and output voltage
headroom compared to other very wideband
voltage-feedback op amps. It delivers a 3V_PP output
swing on a single +5V supply with greater than
80MHz bandwidth. The key requirement of
broadband single-supply operation is to maintain
input and output signal swings within the usable
voltage ranges at both the input and the output. The
circuit of Figure 50 establishes an input midpoint bias
using a simple resistive divider from the +5V supply
(two 698Ω resistors). Separate bias networks would
be required at each input. The input signal is then
ac-coupled into the midpoint voltage bias. The input
voltage can swing to within 1.5V of either supply pin,
giving a 2V_PP input signal range centered between
the supply pins. The input impedance matching
resistor (59Ω) used for testing is adjusted to give a
50Ω input load when the parallel combination of the
biasing divider network is included.
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