OPA2680 Datasheet, PDF (11/21 Pages) Burr-Brown (TI) – Dual Wideband, Voltage Feedback OPERATIONAL AMPLIFIER With Disable

APPLICATIONS INFORMATION
WIDEBAND VOLTAGE FEEDBACK OPERATION
The OPA2680 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 achiev-
able slew rate. The OPA2680 uses a new input stage which
places the transconductance element between two input
buffers, using their output currents as the forward signal. As
the error voltage increases across the two inputs, an increas-
ing current is delivered to the compensation capacitor. This
provides very high slew rate (1800V/µs) while consuming
relatively low quiescent current (6.4mA/ch.). This excep-
tional full power performance comes at the price of a slightly
higher input noise voltage than alternative architectures. The
4.8nV/√Hz input voltage noise for the OPA2680 is excep-
tionally low for this type of input stage.
Figure 1 shows the DC-coupled, gain of +2, dual power
supply circuit configuration used as the basis of the ±5V
Specifications and Typical Performance Curves. This 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 50Ω
with a series output resistor. Voltage swings reported in the
specifications are taken directly at the input and output pins,
while output powers (dBm) are at the matched 50Ω load. For
the circuit of Figure 1, the total effective load will be 100Ω
|| 804Ω. The disable control line (SO-14 package only) is
typically left open to guarantee normal amplifier operation.
Two optional components are included in Figure 1. An
additional resistor (175Ω) is included in series with the non-
inverting input. Combined with the 25Ω DC source resis-
tance looking back towards the signal generator, this gives an
input bias current cancelling resistance that matches the
200Ω 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 PC board layouts, this optional-added capacitor will
typically improve the 2nd harmonic distortion performance
by 3dB to 6dB.
Figure 2 shows the AC-coupled, gain of +2, single supply
circuit configuration which is the basis of the +5V Specifi-
cations and Typical Performance Curves. Though not a “rail-
to-rail” design, the OPA2680 requires minimal input and
output voltage headroom compared to other very wideband
voltage feedback op amps. It will deliver a 3Vp-p output
swing on a single +5V supply with >150MHz bandwidth.
The key requirement of broadband single-supply operation is
to maintain input and output signal swings within the useable
voltage ranges at both the input and the output. The circuit
of Figure 2 establishes an input midpoint bias using a simple
resistive divider from the +5V supply (two 698Ω resistors).
Separate 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 2Vp-p 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. Again, an additional resistor (50Ω in
this case) is included directly in series with the non-inverting
input. This minimum recommended value provides part of
the DC source resistance matching for the non-inverting
input bias current. It is also used to form a simple parasitic
pole to roll off the frequency response at very high frequen-
cies (>500MHz) using the input parasitic capacitance. The
gain resistor (R_G) is AC-coupled, giving the circuit a DC
gain of +1, which puts the input DC bias voltage (2.5V) on
the output as well. The output voltage can swing to within
1V of either supply pin while delivering >100mA output
+5V
0.1µF
6.8µF
+
+5V
+V_S
50Ω Source
175Ω
V_I
50Ω
V_D
1/2
OPA2680
DIS
V_O 50Ω
50Ω Load
0.1µF
R_F
402Ω
698Ω
0.1µF
6.8µF
0.1µF
50Ω
DIS
V_I
698Ω
V_D
1/2
V_O 100Ω
59Ω
OPA2680
V_S/2
R_F
402Ω
R_G
402Ω
6.8µF
0.1µF
R_G
402Ω
0.1µF
–5V
FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifi- FIGURE 2. AC-Coupled, G = +2, Single Supply Specifica-
cation and Test Circuit.
tion and Test Circuit.
®
11
OPA2680