Datasheet

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SBOS303C − JUNE 2004 − REVISED AUGUST 2008
www.ti.com
17
OPA820
High−Speed
DAC
V
O
=I
D
R
F
R
F
C
F
GBP
→
Gain Bandwidth
Product (Hz) for the OPA820.
C
D
I
D
I
D
Figure 8. Wideband, Low-Distortion DAC
Transimpedance Amplifier
The DC gain for this circuit is equal to R
F
. At high frequencies,
the DAC output capacitance (C
D
) will produce a zero in the
noise gain for the OPA820 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, this pole in the
feedback network should be set to:
1
2pR
F
C
F
+
GBP
4pR
F
C
D
Ǹ
which will give a corner frequency f
−3dB
of approximately:
f
*3dB
+
GBP
2pR
F
C
D
Ǹ
ACTIVE FILTERS
Most active filter topologies will have exceptional performance
using the broad bandwidth and unity-gain stability of the
OPA820. Topologies employing capacitive feedback require
a unity-gain stable, voltage-feedback op amp. Sallen-Key
filters simply use the op amp as a noninverting gain stage
inside an RC network. Either current- or voltage-feedback op
amps may be used in Sallen-Key implementations.
Figure 9 shows an example Sallen-Key low-pass filter, in
which the OPA820 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 240MHz
bandwidth provided by the OPA820 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 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).
OPA820
+5V
−
5V
R
2
505
Ω
C
1
150pF
R
1
124
Ω
V
O
V
1
R
G
402
Ω
R
F
402
Ω
C
2
100pF
Power−supply
decoupling not shown.
Figure 9. 5MHz Butterworth Low-Pass Active
Filter
Another type of filter, a high-Q bandpass filter, is shown in
Figure 10. The transfer function for this filter is:
V
OUT
V
IN
+
s
R
3
)R
4
R
1
R
4
C
1
s
2
) s
1
R
1
C
1
)
R
3
R
2
R
4
R
5
C
1
C
2
with w
O
2
+
R
3
R
2
R
4
R
5
C
1
C
2
and
w
O
Q
+
1
R
1
C
1
For the values chosen in Figure 10:
f
O
+
w
O
2p
] 1MHz
and Q = 100
See Figure 11 for the frequency response of the filter
shown in Figure 10.
OPA820
OPA820
V
OUT
R
3
500
Ω
R
4
500
Ω
R
5
158
Ω
C
2
1000pF
R
1
15.8k
Ω
R
2
158
Ω
V
IN
C
1
1000pF
Figure 10. High-Q 1MHz Bandpass Filter
(1)
(2)
(3)
(4)
(5)
(6)