OPA4820ID Datasheet, PDF (20/34 Pages) Texas Instruments – Quad, Unity-Gain Stable, Low-Noise, Voltage-Feedback Operational Amplifier

OPA4820
SBOS317D − SEPTEMBER 2004 − REVISED AUGUST 2008
Usually, it is best to set the absolute values of R₂ and R₁
equal to R_F and R_G, respectively; this equalizes the divider
resistances and cancels the effect of input bias currents.
However, it is sometimes useful to scale the values of R₂
and R₁ in order to adjust the loading on the driving source,
V₁. In most cases, the achievable low-frequency CMRR
will be limited by the accuracy of the resistor values. The
85dB CMRR of the OPA4820 itself will not determine the
overall circuit CMRR unless the resistor ratios are
matched to better than 0.003%. If it is necessary to trim the
CMRR, then R₂ is the suggested adjustment point.
4-CHANNEL DAC TRANSIMPEDANCE
AMPLIFIER
High-frequency Digital-to-Analog Converters (DACs)
require a low-distortion output amplifier to retain their
SFDR performance into real-world loads. See Figure 14
for a single-ended output drive implementation. In this
circuit, only one side of the complementary output drive
signal is used. The diagram shows the signal output
current connected into the virtual ground-summing
junction of the OPA4820, which is set up as a
transimpedance stage or I-V converter. The unused
current output of the DAC is connected to ground. If the
DAC requires its outputs to be terminated to a compliance
voltage other than ground for operation, then the
appropriate voltage level may be applied to the
noninverting input of the OPA4820.
High−Speed
DAC
I_D
1/4
OPA4820
V_O = I_D R_F
R_F
C_F
C_D
GBP →Gain Bandwidth
Product (Hz) for the OPA4820.
I_D
Figure 14. 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 OPA4820 that may
cause peaking in the closed-loop frequency response. C_F
is added across R_F to compensate for this noise-gain
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peaking. To achieve a flat transimpedance frequency
response, this pole in the feedback network should be set
to:
Ǹ ¹
2pR_FC_F
+
GBP
4pR_FC_D
(3)
which will give a corner frequency f_−3dB of approximately:
Ǹ f_*3dB +
GBP
2pR_FC_D
(4)
ACTIVE FILTERS
Most active filter topologies will have exceptional
performance using the broad bandwidth and unity-gain
stability of the OPA4820. 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 15 shows an example Sallen-Key low-pass filter, in
which the OPA4820 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 OPA4820 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).
C₁
150pF
+5V
R₁
124Ω
V₁
R₂
505Ω
C₂
1/4
100pF OPA4820
V_O
Power−supply
decoupling not shown.
R_F
402Ω
−5V
R_G
402Ω
Figure 15. 5MHz Butterworth Low-Pass Active
Filter
20