Datasheet

OPA890
+5V
+
DIS
-5V
50 LoadW
50W
50WV
O
V
I
50 SourceW
R
G
750W
R
F
750W
+
6.8 Fm
0.1 Fm 6.8 Fm
0.1 Fm
0.1 Fm
324W
OPA890
www.ti.com SBOS369B –MAY 2007–REVISED DECEMBER 2009
APPLICATION INFORMATION
WIDEBAND VOLTAGE-FEEDBACK
OPERATION
The OPA890 provides an exceptional combination of
low quiescent current with a 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 OPA890 uses an input stage that places the
transconductance element between two input buffers,
using the combined 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 increasing current
provides very high slew rate (500V/μs) while
consuming relatively low quiescent current (1.1mA).
This exceptional full-power performance comes at the
price of a slightly higher input noise voltage than
alternative architectures. The 8nV/√Hz input voltage
noise for the OPA890 is low for this combination of
input stage and low quiescent current.
Figure 46. DC-Coupled, G = +2, Bipolar Supply,
Specification and Test Circuit
Figure 46 shows the dc-coupled, gain of +2, dual
power-supply circuit configuration used as the basis
Figure 47 shows the ac-coupled, gain of +2,
of the ±5V Electrical Characteristics and Typical
single-supply circuit configuration used as the basis
Characteristics. For test purposes, the input
of the +5V Electrical Characteristics and Typical
impedance is set to 50Ω with a resistor to ground and
Characteristics. Though not a rail-to-rail design, the
the output impedance is set to 50Ω with a series
OPA890 requires minimal input and output voltage
output resistor. Voltage swings reported in the Typical
headroom compared to other very wideband
Characteristics are taken directly at the input and
voltage-feedback op amps. It delivers a 2V
PP
output
output pins, while output powers (dBm) are at the
swing on a single +5V supply with > 100MHz
matched 50Ω load. For the circuit of Figure 46, the
bandwidth. The key requirement of broadband
total effective load will be 100Ω 1.5kΩ. The disable
single-supply operation is to maintain input and
control line is typically left open to ensure normal
output signal swings within the usable voltage ranges
amplifier operation. Two optional components are
at both the input and the output. The circuit of
included in Figure 46. An additional resistor (324Ω) is
Figure 47 establishes an input midpoint bias using a
included in series with the noninverting input.
simple resistive divider from the +5V supply (two
Combined with the 25Ω dc source resistance looking
698Ω resistors). The input signal is then ac-coupled
back towards the signal generator, this configuration
into the midpoint voltage bias. The input voltage can
gives an input bias current cancelling resistance that
swing to within 1.5V of either supply pin, giving a
matches the 375Ω source resistance seen at the
2V
PP
input signal range centered between the supply
inverting input (see the DC Accuracy and Offset
pins. The input impedance matching resistor (59Ω)
Control section). In addition to the usual power-supply
used for testing is adjusted to give a 50Ω input load
decoupling capacitors to ground, a 0.1μF capacitor is
when the parallel combination of the biasing divider
included between the two power-supply pins. In
network is included.
practical printed circuit board (PCB) layouts, this
optional-added capacitor typically improves the
2nd-harmonic distortion performance by 3dB to 6dB.
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Product Folder Link(s): OPA890