OPA2889 Datasheet, PDF (18/35 Pages) Burr-Brown (TI) – Dual, Low-Power, Wideband, Voltage Feedback OPERATIONAL AMPLIFIER with Disable

OPA2889
SBOS373 – JUNE 2007
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 54 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 OPA2889, 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 OPA2889. The dc gain for
this circuit is equal to R_F. At high frequencies, the
DAC output capacitance (C_D in Figure 54) produces
a zero in the noise gain for the OPA2889 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:
1 ₌ GBP
2pR_FC_F
4pR_FC_D
(1)
which gives a cutoff frequency f_–3dB of approximately:
f = ^GBP
-3dB
2pR_FC_D
(2)
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WIDEBAND VIDEO MULTIPLEXING
One common application for video speed amplifiers
that include a disable pin is to wire multiple amplifier
outputs together, then select one of several possible
video inputs to source onto a single line. This simple
wired-OR video multiplexer can be easily
implemented using the OP2889IDGS (MSOP-10
package only), as shown in Figure 55.
High-Speed
DAC
I_O
50W
1/2
OPA2889
R_F1
C_F1
C_D1
R_F2
V_O = I_O R_F
C_F2
-I_O
C_D2
1/2
OPA2889
-V_O = -I_O R_F
50W
GBP ® Gain Bandwidth
Product (Hz) for the OPA2889
Figure 54. DAC Transimpedance Amplifier
+5V
V_DIS
2kW
+5V
Video 1
146W
75W
DISA
1/2
OPA2889
340W
340W
-5V
402W
402W
+5V
Video 2
146W
75W
2kW
1/2
OPA2889
DISB
-5V
82.5W
75W Cable
RG-59
82.5W
75W Load
Figure 55. 2-Channel Video Multiplexer (SO-14 package only)
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