OPA3681 Datasheet, PDF (15/21 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

+5V
74HC238
D₁
D₂
Y₀
Y₁
Y₂
+5V
Power Supply
De-Coupling Not Shown
20Ω
G = +2
1/3
10Ω
OPA3681
499Ω
499Ω
V_IN
20Ω
G = +4
1/3
10Ω
50Ω Load
50Ω
50Ω
OPA3681
140Ω
422Ω
20Ω
G = +8
1/3
10Ω
OPA3681
35.7Ω
249Ω
–5V
FIGURE 8. Wideband Programmable Gain.
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO
OPTIMIZE BANDWIDTH
A current-feedback op amp like the OPA3681 can hold an
almost constant bandwidth over signal gain settings with the
proper adjustment of the external resistor values. This is shown
in the Typical Performance Curves; the small signal bandwidth
decreases only slightly with increasing gain. These curves also
show that the feedback resistor has been changed for each gain
setting. The resistor “values” on the inverting side of the circuit
for a current feedback op amp can be treated as frequency
response compensation elements while their “ratios” set the
signal gain. Figure 9 shows the small-signal frequency response
analysis circuit for the OPA3681.
V_I
i_ERR
α
R_I
R_G
V_O
Z_(S) i_ERR
R_F
The key elements of this current-feedback op amp model are:
α → Buffer gain from the non-inverting 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
FIGURE 9. Current Feedback Transfer Function Analysis
Circuit.
amplifier configuration. For a buffer gain α < 1.0, the
CMRR = –20 • log (1– α) dB.
R_I, the buffer output impedance, is a critical portion of the
bandwidth control equation. The OPA3681 is typically 42Ω.
®
15
OPA3681