OPA2686 Datasheet, PDF (13/18 Pages) Burr-Brown (TI) – Dual, Wideband, Low Noise, Voltage Feedback OPERATIONAL AMPLIFIER

Each side of the OPA2686 in this circuit is operating at a
relatively low noise gain. To hold excellent frequency re-
sponse flatness, the inverting gain compensation capacitors
are included at the inverting nodes and across the feedback
resistors, as described in the “Low Gain Compensation for
Improved SFDR” section in this data sheet. Operating at
+2.5V common-mode requires a DC level shifting current
through the feedback resistors. Since this current is to the
supply midpoint, pull-up resistors equal to the feedback
resistors are connected to the positive supply to keep the
output stage signal currents equal and bipolar. This signifi-
cantly improves 2nd harmonic distortion.
To deliver a 2Vp-p differential input signal on a 2.5V
common-mode voltage, each output must swing between
2.0V and 3.0V. Tested harmonic distortion performance for
this condition from 1MHz to 10MHz is shown in Figure 8.
–70
–75
2nd Harmonic
–80
–85
3rd Harmonic
–90
–95
1
10
Frequency (MHz)
FIGURE 8. Harmonic Distortion vs Frequency for the
Circuit of Figure 7.
In this case, the 2nd harmonic distortion is still dominant
due to slight signal path imbalances—even though this
circuit does provide matched noise gain. The distortion
levels, however, are very low. Thus, narrowband applica-
tions which are impacted by only 3rd-order terms will see
very low single- and two-tone distortion levels.
AC-COUPLED, SINGLE-TO-DIFFERENTIAL ADC
DRIVER
Where the signal path may be AC-coupled, a very balanced,
high SFDR dual op amp interface circuit can easily be
provided by the OPA2686. Figure 9 shows a specific ex-
ample of this application where the input single-to-differen-
tial conversion is provided by an input transformer. Once
the signal source is purely differential, the circuit of Figure
9 provides low harmonic distortion with a common-mode
control path that does not interact with the signal path gain.
If the source is already differential, such as at the output of
a balanced mixer, the input transformer could be replaced by
blocking capacitors.
In the example of Figure 9, the secondary of the trans-
former is connected into the two inverting path gain
resistors (100Ω). These resistors provide both an input
impedance match (assuming a 50Ω source on the primary
of this 1:2 step-up transformer) and set the signal gain for
each amplifier along with the 500Ω feedback resistors.
Although relatively high signal gain is provided by this
circuit (10 in this case), each amplifier is operating at a
relatively low noise gain (3.5 at DC). This low noise gain
at low frequencies gives high loop gain for distortion
suppression in the baseband. External compensation ca-
pacitors (18pF and 2.1pF) are included to hold the fre-
quency response flat, as described in the “Low Gain
Power Supply De-Coupling
Not Shown
V_CM
+5V
1000pF
1/2
OPA2686
18pF
2.1pF
V_I
50Ω
1:2
100Ω
100Ω
500Ω
500Ω
V_CM +10V_I
Single-to-Differential
Gain of 10
2.1pF
18pF
1/2
V_CM ^OPA2686
1000pF
–5V
20Ω
V+
80pF
500Ω
14-Bit
10MSPS
2.5V
V_CM
1µF
500Ω
20Ω
V–
80pF
FIGURE 9. AC-Coupled, Single-to-Differential High SFDR ADC Driver.
13
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OPA2686