Datasheet

OPA683
15
SBOS221E
www.ti.com
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to assist in
the initial evaluation of circuit performance using the OPA683
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.
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, the CMRR =
–20 • log(1 – α). The closed loop input stage buffer used in
the OPA683 gives a buffer gain more closely approaching
1.00 and this shows up in a slightly higher CMRR than any
previous current feedback op amp. The 60dB typical CMRR
shown in the Electrical Characteristics implies a buffer gain
of 0.9990.
R
I
, the buffer output impedance, is a critical portion of the
bandwidth control equation. The OPA683 reduces this ele-
ment to approximately 4.5Ω using the loop-gain of the input
buffer stage. This significant reduction in buffer output im-
pedance, on very low power, contributes significantly to
extending the bandwidth at higher gains.
ORDERING LITERATURE
PRODUCT PACKAGE NUMBER NUMBER
OPA683ID SO-8 DEM-OPA-SO-1A SBOU009
OPA683IDBQ SOT23-6 DEM-OPA-SOT-1A SBOU010
TABLE I. Demonstration Fixtures by Package.
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA683
product folder.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO OPTIMIZE
BANDWIDTH
Any current-feedback op amp like the OPA683 can hold high
bandwidth over signal gain settings with the proper adjust-
ment of the external resistor values. A low-power part like the
OPA683 typically shows a larger change in bandwidth due to
the significant contribution of the inverting input impedance
to loop-gain changes as the signal gain is changed. Figure
10 shows a simplified analysis circuit for any current feed-
back amplifier.
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
OPA683
+5V
DIS
60Ω
R
G
R
G
R
I
1.4kΩ
50Ω
22pF
V
I
V
O
1.4kΩ
0.1µF
V
O
= – V
I
IN
IN
CM
ADS820
10-Bit
20MSPS
2.5V
DC
+2.5V
2V
PP
Max
FIGURE 9. Low Power, Single-Supply, ADC Driver.
R
F
V
O
R
G
R
I
Z
(S)
i
ERR
i
ERR
α
V
I
FIGURE 10. Current Feedback Transfer Function Analysis
Circuit.