OPA2690 Datasheet, PDF (15/30 Pages) Burr-Brown (TI) – Dual, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2690
www.ti.com
HIGH-PERFORMANCE DAC
TRANSIMPEDANCE AMPLIFIER
High-frequency DDS Digital-to-Analog Converters
(DACs) require a low distortion output amplifier to retain
their SFDR performance into real-world loads. Figure 5
shows a single-ended output drive implementation. The
diagram shows the signal output current(s) connected into
the virtual ground summing junction(s) of the OPA2690,
which is set up as a transimpedance stage or I-V converter.
If the DAC requires that its outputs terminate to a
compliance voltage other than ground for operation, the
appropriate voltage level may be applied to the
noninverting input of the OPA2690. The DC gain for this
circuit is equal to R_F. At high frequencies, the DAC output
capacitance (C_D in Figure 5) will produce a zero in the
noise gain for the OPA2690 that may cause peaking in the
closed-loop frequency response. C_F is added across R_F to
compensate for this noise gain peaking. To achieve a flat
transimpedance frequency response, the pole in each
feedback network should be set to:
Ǹ ¹
2pR_FC_F
+
GBP
4pR_FC_D
(1)
which will give a cutoff frequency f_−3dB of approximately:
Ǹ f_*3dB +
GBP
2pR_FC_D
(2)
SBOS238D − JUNE 2002 − REVISED DECEMBER 2004
High−Speed
DAC
I_O
50Ω
1/2
OPA2690
R_F1
C_F1
C_D1
R_F2
V_O = I_O R_F
C_F2
− I_O
C_D2
1/2
OPA2690
−V_O = −I_O R_F
50Ω
GBP →Gain Bandwidth
Product (Hz) for the OPA2690
Figure 5. DAC Transimpedance Amplifier
WIDEBAND VIDEO MULTIPLEXING
One common application for video speed amplifiers that
include a disable pin is to wire multiple amplifier outputs
together, then select which one of several possible video
inputs to source onto a single line. This simple wired-OR
video multiplexer can be easily implemented using the
OP2690I-14D (SO-14 package only), as shown in
Figure 6.
V_DIS
+5V
Video 1
Video 2
2kΩ
146Ω
75Ω
340Ω
340Ω
146Ω
75Ω
2kΩ
+5V
DISA
1/2
O PA 2 690
−5V
402Ω
402Ω
+5V
1/2
O PA 2 690
DISB
−5V
82.5Ω
75Ω Cable
RG−59
82.5Ω
75Ω Load
Figure 6. 2-Channel Video Multiplexer (SO-14 package only)
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