OPA3684 Datasheet, PDF (16/32 Pages) Burr-Brown (TI) – Low-Power, Triple Current-Feedback OPERATIONAL AMPLIFIER With Disable

The first stage buffers the video DAC output to the first
3rd-order filter section. This stage also provides group delay
equalization while the 2nd and 3rd stages each give a 3rd-
order low-pass response with sin(x)/x equalization. Figure 10
shows the frequency response for the filter of Figure 9.
20
10
0
–10
–20
–30
–40
–50
0
f_–3dB
1
10
100
Frequency (MHz)
FIGURE 10. Video Filter Frequency Response.
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to assist in
the initial evaluation of circuit performance using the OPA3684
in its two package options. Both of these are offered free of
charge as unpopulated PCBs, delivered with a user's guide.
The summary information for these fixtures is shown in
Table I.
PRODUCT
OPA3684ID
OPA3684IDBQ
PACKAGE
SO-14
SSOP-16
ORDERING
NUMBER
DEM-OPA-SO-3B
DEM-OPA-SSOP-3B
LITERATURE
NUMBER
SBOU018
SBOU019
TABLE I. Demonstration Fixtures by Package.
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA3684
product folder.
MACROMODELS
Computer simulation of circuit performance using SPICE is
often useful in predicting the performance of analog circuits
and systems. This is particularly true for Video and RF
amplifier circuits where parasitic capacitance and inductance
can have a major effect on circuit performance. Check the TI
web site (www.ti.com) for SPICE macromodels within the
OPA3684 product folder. These models do a good job of
predicting small-signal AC and transient performance under
a wide variety of operating conditions. They do not do as well
in predicting distortion or dG/dP characteristics. Most of
these models do not attempt to distinguish between the
package types in their small-signal AC performance.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO OPTIMIZE BANDWIDTH
Any current-feedback op amp like the OPA3684 can hold
high bandwidth over signal-gain settings with the proper
adjustment of the external resistor values. A low-power part
like the OPA3684 typically shows a larger change in band-
width due to the significant contribution of the inverting input
impedance to loop-gain changes as the signal gain is changed.
Figure 11 shows a simplified analysis circuit for any current-
feedback amplifier.
V_I
i_ERR
α
R_I
R_G
V_O
Z_(S) i_ERR
R_F
FIGURE 11. Current-Feedback Transfer Function Analysis
Circuit.
The key elements of this current-feedback op amp model
are:
α ⇒ Buffer gain from the noninverting input to the inverting input
R_I ⇒ Buffer output impedance
i_ERR ⇒ Feedback error current signal
Z_(S) ⇒ Frequency-dependent open-loop transimpedance gain
from i_ERR to V_O
The buffer gain is typically very close to 1.00 and is normally
neglected from signal gain considerations. It will, however,
set the CMRR for a single op amp differential
amplifier configuration. For the buffer gain α < 1.0 and
CMRR = –20 • log(1 – α). The closed-loop input stage buffer
used in the OPA3684 gives a buffer gain more closely
approaching 1.00 and this shows up in a slightly higher
CMRR than previous current-feedback op amps.
R_I, the buffer output impedance, is a critical portion of the
bandwidth control equation. The OPA3684 reduces this
element to approximately 4.0Ω using the local loop gain of
the input buffer stage. This significant reduction in output
impedance, on very low power, contributes significantly to
extending the bandwidth at higher gains.
A current-feedback op amp senses an error current in the
inverting node (as opposed to a differential input error volt-
age for a voltage-feedback op amp) and passes this on to
the output through an internal frequency-dependent
16
OPA3684
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