Datasheet

GBP
2 R Cp
F D
=
f
-3dB
V = I R
O D F
R
F
C
F
GBP Gain Bandwidth
Product (Hz) for the OPA842
?
C
D
I
D
I
D
High-Speed
DAC
OPA842
+5V
-5V
R
2
505W
C
1
150pF
R
1
124W
V
O
V
1
R
G
402W
RF
402W
C
2
100pF
Power-supply
decoupling not shown.
OPA842
GBP
4 R Cp
F D
=
1
2 R Cp
F F
OPA842
SBOS267D –NOVEMBER 2002–REVISED SEPTEMBER 2010 www.ti.com
DAC TRANSIMPEDANCE AMPLIFIER which will give a corner frequency (f
–3dB
) of
approximately:
High-frequency digital-to-analog converters (DACs)
require a low-distortion output amplifier to retain the
SFDR performance into real-world loads. A
(2)
single-ended output drive implementation is shown in
Figure 41. In this circuit, only one side of the
ACTIVE FILTERS
complementary output drive signal is used. The
diagram shows the signal output current connected
Most active filter topologies will have exceptional
into the virtual ground-summing junction of the
performance using the broad bandwidth and
OPA842, which is set up as a transimpedance stage
unity-gain stability of the OPA842. Topologies
or I-V converter. The unused current output of the
employing capacitive feedback require a unity-gain
DAC is connected to ground. If the DAC requires its
stable, voltage-feedback op amp. Sallen-Key filters
outputs terminated to a compliance voltage other than
simply use the op amp as a noninverting gain stage
ground for operation, then the appropriate voltage
inside an RC network. Either current- or
level may be applied to the noninverting input of the
voltage-feedback op amps may be used in
OPA842.
Sallen-Key implementations.
See Figure 42 for an example Sallen-Key low-pass
filter, in which the OPA842 is set up to deliver a
low-frequency gain of +2. The filter component values
have been selected to achieve a maximally flat
Butterworth response with a 5MHz, –3dB bandwidth.
The resistor values have been slightly adjusted to
compensate for the effects of the 150MHz bandwidth
provided by the OPA842 in this configuration. This
filter may be combined with the ADC driver
suggestions to provide moderate (two-pole) Nyquist
filtering, limiting noise, and out-of-band harmonics
into the input of an ADC. This filter will deliver the
exceptionally low harmonic distortion required by high
SFDR ADCs such as the ADS850 (14-bit, 10MSPS,
82dB SFDR).
Figure 41. Wideband, Low-Distortion DAC
Transimpedance Amplifier
The dc gain for this circuit is equal to R
F
. At high
frequencies, the DAC output capacitance will produce
a zero in the noise gain for the OPA842 that may
cause peaking in the closed-loop frequency
response. CF is added across RF to compensate for
this noise-gain peaking. To achieve a flat
transimpedance frequency response, this pole in the
feedback network should be set to:
(1)
Figure 42. 5MHz Butterworth Low-Pass Active
Filter
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Product Folder Link(s): OPA842