OPA2846_14 Datasheet, PDF (17/30 Pages) Texas Instruments – Dual, Wideband, Low-Noise, Voltage-Feedback Operational Amplifier

OPA2846
www.ti.com
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 OPA2846. Figure 10 shows a
specific example of this application, where the input
single-to-differential conversion is provided by an input
transformer. Once the signal source is purely differential,
the circuit of Figure 10 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 10, the secondary of the
transformer 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
SBOS274C −JUNE 2003 − REVISED AUGUST 2008
circuit (10 in this case), each amplifier is operating at a
relatively low noise gain (3.5V/V). This low-noise gain at
low frequencies gives high loop gain for distortion
suppression in the baseband. External compensation
capacitors (18pF and 2.1pF) are included to hold the
frequency response flat, as described in the Low-Gain
Compensation For Improved SFDR section of this data
sheet. The common-mode operating voltage is fed into
each amplifier’s noninverting input. Since these are equal,
and will appear at each inverting input as well, no DC
current is produced through the transformer secondary
due to this common-mode operating voltage. Since no
current flows due to V_CM, the output will operate at V_CM as
well. This is one of the few common-mode operating point
control techniques that requires no current to flow. This
makes the common-mode control aspect of this circuit
essentially non-interactive with the signal path. To provide
a 2V_PP differential signal operating at a 2.5V output
common-mode requires a 2.0V to 3.0V output swing on
each output. Tested performance over frequency for the
circuit of Figure 10 is shown in Figure 11.
Power−supply decoupling
not shown.
V_CM
+5V
1000pF
1/2
R₁
OPA2846
18pF
2.1pF
V_I
50Ω
1:2
100Ω
100Ω
500Ω
500Ω
V_CM +10V_I
2.1pF
Single−to−Differential
Gain of 10
18pF
1/2
V_CM ^OPA2846
R₁
1000pF
−5V
L
500Ω
V+
C
R₂
2.5V
1µF
14−Bit
10MSPS
ADS850
V_CM
500Ω
L
V−
C
R₂
Figure 10. AC-Coupled, Single-to-Differential High SFDR ADC Driver
17