OPA2846_14 Datasheet, PDF (16/30 Pages) Texas Instruments – Dual, Wideband, Low-Noise, Voltage-Feedback Operational Amplifier

OPA2846
SBOS274C −JUNE 2003 − REVISED AUGUST 2008
www.ti.com
V_I
50Ω Input
Impedance
12Ω
95Ω
+5V
1/2
OPA2846
750Ω
Power−supply decoupling
not shown.
750Ω _200Ω
1.3µH
93.8Ω
+5V
100Ω
O PA8 20
93.8Ω
93.8Ω
+5V
750Ω
750Ω
−5V
0.1µF
3.8kΩ
0.1µF
2.01kΩ
3.8kΩ
+5V
V+
30pF
14−Bit
10MSPS
ADS850
2.5V
V_CM
1µF
1/2
OPA2846
49.9Ω
−5V
200Ω
1.3µH
V−
Voltage at V+ to V− = 2V_PP
2nd−order Butterworth post filter f_−3dB = 18MHz.
Figure 8. DC-Coupled, Single-to-Differential High SFDR ADC Driver
Operating at +2.5V output 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 significantly improves 2nd-har-
monic distortion.
One side of the OPA2846 is operating at a gain of +9 with
some attenuation of the input signal to have an equivalent
+8 gain. The other side of the OPA2846 is operating at a
gain of −8.
To deliver a 2V_PP 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 9.
In this case, the 2nd-harmonic distortion is still dominant
due to slight signal path imbalances. The distortion levels,
however, are very low. Thus, narrowband applications
which are impacted by only 3rd-order terms will see very
low single- and two-tone distortion levels.
−70
−75
−80
2nd−Harmonic
−85
3rd−Harmonic
−90
−95
1
10
Frequency (MHz)
Figure 9. Harmonic Distortion vs Frequency for
the Circuit of Figure 8
For more information on the 2nd-order post filter, refer to
RLC Filter Design for ADC Interface Applications
(SBAA108), available for download at www.ti.com.
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