OPA642N BURR-BROWN [Burr-Brown Corporation], OPA642N Datasheet - Page 10

• Manufacturer Part Number: OPA642N
• Description: Wideband, Low Distortion, Low Gain OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

DAC TRANSIMPEDANCE AMPLIFIER
High frequency DDC DACs require a low distortion output
amplifier to retain their SFDR performance into real-world
loads. A single-ended output drive implementation is shown
in Figure 4. In this circuit, only one side of the complemen-
tary output drive signal is used. The diagram shows the
signal output current connected into the virtual ground
summing junction of the OPA642, which is set up as a
transimpedance stage or “I-V converter”. The unused cur-
rent output of the DAC is connected to ground. If the DAC
requires its outputs terminated to a compliance voltage other
than ground for operation, then the appropriate voltage level
may be applied to the non-inverting input of the OPA642.
The DC gain for this circuit is equal to R
cies, the DAC output capacitance will produce a zero in the
noise gain for the OPA642 that may cause peaking in the
closed-loop frequency response. C
compensate for this noise gain peaking. To achieve a flat
transimpedance frequency response, this pole in the feed-
back network should be set to:
which will give a corner frequency ƒ
FIGURE 4. Wideband, Low Distortion DAC Transimpedance
ACTIVE FILTERS
Most active filter topologies will deliver exceptional perfor-
mance using the broad bandwidth and unity gain stability of
the OPA642. Topologies employing capacitive feedback
require a unity gain stable voltage feedback op amp. Sallen-
Key filters simply use the op amp as a non-inverting gain
stage inside an RC network. Either current or voltage feed-
back op amps may be used in Sallen-Key implementations.
High Speed
DAC
Amplifier.
I
I
O
O
1 / 2 R
®
OPA642
–3dB
F
C
C
D
F
GBP / 2 R
GBP
Product for the OPA642
GBP / 4 R
OPA642
Gain Bandwidth
C
R
F
F
F
is added across R
-3dB
F
C
F
F
D
of approximately:
. At high frequen-
C
D
V
O
= I
O
R
F
F
to
10
Figure 5 shows an example Sallen-Key low pass filter, in
which the OPA642 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 band-
width provided by the OPA642 in this configuration. This
filter may be combined with the ADC driver suggestions to
provide moderate (2-pole) Nyquist filtering, limiting noise
and out of band components into the input of an ADC. This
filter will deliver the exceptionally low harmonic distortion
required by high SFDR A/D converters such as the ADS804
(12-bit, 10MSPS, 80dB SFDR).
OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES
Since the OPA642 is a unity gain stable voltage feedback op
amp, a wide range of resistor values may be used for the
feedback and gain setting resistors. The primary limits on
these values are set by dynamic range (noise and distortion)
and parasitic capacitance considerations. For a non-inverting
unity gain follower application, the feedback connection
should be made with a 25 resistor—not a direct short. This
will isolate the inverting input capacitance from the output
pin and improve the frequency response flatness. Usually,
the feedback resistor value should be between 200
1k . Below 200 , the feedback network will present
additional output loading which can degrade the harmonic
distortion performance of the OPA642. Above 1k , the
typical parasitic capacitance (approximately 0.2pF) across
the feedback resistor may cause unintentional band-limiting
in the amplifier response.
A good rule of thumb is to target the parallel combination of
R
combined impedance R
input capacitance, placing an additional pole in the feedback
network and thus a zero in the forward response. Assuming
a 2pF total parasitic on the inverting node, holding R
< 200
constraint implies that the feedback resistor R
to several k at high gains. This is acceptable as long as the
pole formed by R
in parallel is kept out of the frequency range of interest.
FIGURE 5. 5MHz Butterworth Low Pass Active Filter.
F
V
I
and R
will keep this pole above 400MHz. By itself, this
124
G
(Figure 1) to be less than about 200 . The
F
and any parasitic capacitance appearing
505
100pF
F
|| R
G
150pF
interacts with the inverting
402
OPA642
402
F
can increase
F
V
|| R
O
and
G