OPA4684 Datasheet, PDF (15/26 Pages) Burr-Brown (TI) – Quad, Low-Power, Current-Feedback OPERATIONAL AMPLIFIER

common-mode level to be set up. The desired output com-
mon-mode voltage, V_CM, is cut in half and applied to the
noninverting input of the 2nd stage. The signal path in this
stage sees a gain of –1 while this (1/2 • V_CM) voltage sees a
gain of +2. The output of this 2nd stage is then the original
common-mode voltage plus the inverted signal from the
output of the first stage. The output of this 2nd stage then
appears directly at the output of the noninverting final stage.
The inverting node of the inverting output stage is also
biased to the common-mode voltage, equal to the common-
mode voltage appearing at the output of the 2nd stage,
creating no current flow and placing the desired V_CM at the
output of this stage as well.
LOW-POWER, DIFFERENTIAL I/O, 4th-ORDER ACTIVE FILTER
The OPA4684 can give a very capable gain block for low-
power active filters. The quad design lends itself very well to
differential active filters. Where the filter topology is looking
for a simple gain function to implement the filter, the
noninverting configuration is preferred to isolate the filter
elements from the gain elements in the design. Figure 9
shows an example of a very low-power, 10MHz, 4th-order
Butterworth, low-pass Sallen-Key filter. Often, these filters
are designed at an amplifier gain of 1 to minimize amplifier
bandwidth interaction with the desired filter shape. Since the
OPA4684 shows minimal bandwidth change with gain, this
would not be a constraint in this design. The example of
Figure 9 designs the filter for a differential gain of 4 in each
differential stage. This DC-coupled design gives a signal gain
of 16V/V in the passband with a f_–3dB at 10MHz. The design
places the higher Q stage first to allow the lower Q 2nd stage
to roll off the peaked noise of the first stage. The resistor
values have been adjusted slightly to account for the ampli-
fier group delay.
1kΩ _VCM
2
V_I
1/4
R_{I OPA4684}
0.1µF
1kΩ
1/4
OPA4684
+5V
V_CM
1/4
OPA4684
800Ω
800Ω
800Ω
R_G
800Ω
800Ω
50Ω
1/4
OPA4684
800Ω
+V_I
(1
+
800Ω
R_G
)
V_CM
–V_I
(1
+
800Ω
R_G
)
–5V
FIGURE 8. High Gain, DC-Coupled, Single to Differential Conversion.
66.5Ω
249Ω
100pF
1/4
OPA4684
800Ω
V_I
50pF
534Ω
800Ω
49.9Ω
392Ω
100pF
+5V
1/4
OPA4684
800Ω
50pF
534Ω
800Ω
V_O/V_I = 16V/V
f_–3dB = 10MHz
P_D = 68mW
V_O
66.5Ω
249Ω
1/4
OPA4684
100pF
49.9Ω
392Ω
1/4
OPA4684
–5V
100pF
G_D = 4, W_O = 2π 10MHz, Q = 1.31
G_D = 4, W_O = 2π 10MHz, Q = 0.54
FIGURE 9. Low-Power, Differential I/O, 4th-Order Butterworth Active Filter.
OPA4684
15
SBOS240B
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