OPA2686 Datasheet, PDF (12/18 Pages) Burr-Brown (TI) – Dual, Wideband, Low Noise, Voltage Feedback OPERATIONAL AMPLIFIER

Figure 6 shows the measured frequency response for the
circuit of Figure 5. This shows the expected gain of
–2 (6dB) with exceptional flatness through 70MHz and a
–3dB bandwidth of 170MHz. Measured distortion into a
100Ω load shows > 5dB improvement through 20MHz over
the performance shown in the Typical Performance Curves.
Into a 500Ω load, the 5MHz, 2Vp-p, 2nd harmonic improves
from –85dBc to –92dBc.
12
9
6
3
0
–3
–6
–9
–12
–15
–18
1
170MHz
10
100
500
Frequency (MHz)
FIGURE 6. Low Noise Figure IF Amplifier.
DC-COUPLED, SINGLE-TO-DIFFERENTIAL ADC
DRIVER
Many very high performance CMOS ADCs are intended
to operate with a differential input signal. Translating a
single-ended source to this differential input while con-
trolling the common-mode operating voltage can present
a considerable challenge where high SFDR is required.
Figure 7 shows one way to do this where very low
harmonic distortion is required and good common-mode
control is desired.
This particular example is set for a signal gain of 4 from
the single-ended input to the differential output voltage.
Since the common-mode control signal (from the output
of the OPA680) is fed into the midpoint of the two gain
resistors (124Ω), this DC control path requires a very low
source impedance through high frequencies to maintain
the desired signal path gain. A wideband, unity gain
stable, voltage-feedback op amp like the OPA680 makes
an ideal choice to provide this low output impedance DC
control signal. This op amp also compares the output
common-mode voltage to the desired V_CM, and servos the
OPA2686 common-mode output voltage to that value
using an integrator loop. This holds the output common-
mode voltage precisely at V_CM while giving the low
output impedance required of the circuit.
V_I
100Ω Input
Impedance
130Ω
78.7Ω
24pF
124Ω
124Ω
191Ω
+5V
1/2
OPA2686
Power Supply De-Coupling
Not Shown
357Ω
+2.5V ±2V_I
20Ω
357Ω
2.2pF
25.5Ω
+5V
357Ω
2.2pF
357Ω
+5V
100Ω
OPA680
10kΩ
0.1µF
5kΩ
–5V
0.1µF
10kΩ
+5V
V+
80pF
14-Bit
10MSPS
V_CM
1µF
24pF
1/2
OPA2686
49.9Ω
–5V
+2.5V ±2V
20Ω
V–
80pF
FIGURE 7. DC-Coupled, Single-to-Differential High SFDR ADC Driver.
®
OPA2686
12