OPA2890 Datasheet, PDF (18/38 Pages) Texas Instruments – Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2890
SBOS364C – DECEMBER 2007 – REVISED DECEMBER 2009
HIGH-PERFORMANCE DAC
TRANSIMPEDANCE AMPLIFIER
High-frequency DDS digital-to-analog converters
(DACs) require a low distortion output amplifier to
retain SFDR performance into real-world loads.
Figure 53 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 OPA2890, 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 OPA2890. The dc gain for
this circuit is equal to R_F. At high frequencies, the
DAC output capacitance (C_D in Figure 53) produces a
zero in the noise gain for the OPA2890 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 OP2890IDGS (MSOP-10
package only), as shown in Figure 54.
High-Speed
DAC
I_O
-I_O
50W
1/2
OPA2890
R_F1
C_F1
C_D1
R_F2
V_O = I_O R_F
C_F2
C_D2
1/2
OPA2890
-V_O = -I_O R_F
50W
GBP ® Gain Bandwidth
Product (Hz) for the OPA2890
Figure 53. DAC Transimpedance Amplifier
+5V
V_DIS
2kW
+5V
Video 1
305W
75W
DISA
1/2
OPA2890
634W
634W
-5V
750W
750W
+5V
Video 2
305W
75W
2kW
1/2
OPA2890
DISB
-5V
82.5W
75W Cable
RG-59
82.5W
75W Load
Figure 54. 2-Channel Video Multiplexer (MSOP-10 package only)
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