OPA2889 Datasheet, PDF (16/35 Pages) Burr-Brown (TI) – Dual, Low-Power, Wideband, Voltage Feedback OPERATIONAL AMPLIFIER with Disable

OPA2889
SBOS373 – JUNE 2007
APPLICATIONS INFORMATION
www.ti.com
WIDEBAND VOLTAGE-FEEDBACK
OPERATION
The OPA2889 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 OPA2889 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 (250V/μs)
while consuming very low quiescent current
(460μA/ch). This exceptional full-power performance
comes at the price of a slightly higher input noise
voltage than alternative architectures. The 8.4nV/√Hz
input voltage noise for the OPA2889 is exceptionally
low for this type of input stage.
Figure 50 shows the dc-coupled, gain of +2V/V, dual
power-supply circuit configuration used as the basis
of the ±5V Electrical Characteristics and ±5V Typical
Chararacteristics. 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
100Ω. 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 50, the
total effective load will be 100Ω || 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 50. 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 375Ω 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 included between the two
power-supply pins. In practical printed circuit board
(PCB) layouts, this optional-added 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
OPA2889
DIS
50W Load
V_O 50W
0.1mF
R_F
750W
R_G
750W
-V_S
-5V
6.8mF
0.1mF
Figure 50. DC-Coupled, G = +2, Bipolar Supply,
Specification and Test Circuit
Figure 51 illustrates the ac-coupled, gain of +2V/V,
single-supply circuit configuration used as the basis
of the +5V Electrical Characteristics and +5V Typical
Chararacteristics. Though not a rail-to-rail design, the
OPA2889 requires minimal input and output voltage
headroom compared to other very wideband
voltage-feedback op amps. It delivers a 2.8V_PP
output swing on a single +5V supply with > 50MHz
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 51 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 1.1V 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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