OPA2683 Datasheet, PDF (16/33 Pages) Texas Instruments – Very Low-Power, Dual, Current-Feedback Operational Amplifier

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21
18
15
12
9
6
3
1
SINGLE TO DIFFERENTIAL CONVERSION
10
Frequency (MHz)
100 200
FIGURE 8. Small-Signal Bandwidth for Figure 7.
DIFFERENTIAL ACTIVE FILTER
The OPA2683 can provide a very capable gain block for low-
power active filters. The dual 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, 3rd-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
OPA2683 shows minimal bandwidth change with gain, this
feature would not be a constraint in this design. The example
of Figure 9 designs the filter for a differential gain of 5 using
the OPA2683. The resistor values have been adjusted slightly
to account for the amplifier bandwidth effects.
While this circuit is bipolar, using ±5V supplies, it can easily
be adapted to single-supply operation. This is typically done
by providing a supply midpoint reference at the noninverting
inputs, then adding DC blocking caps at each input and in
series with the amplifier gain resistor, R_G. This will add two
real zeroes in the response, transforming the circuit into a
bandpass. Figure 10 shows the frequency response for the
filter of Figure 9.
10MHz, 3RD-ORDER BUTTERWORTH LOW PASS
FREQUENCY RESPONSE
14
11
8
5
2
–1
–4
1
10
20
Frequency (MHz)
FIGURE 10. Frequency Response for 10MHz, 3rd-Order
Butterworth Low-Pass Filter.
47Ω
183Ω
100pF
20Ω
+5V
1/2
OPA2683
953Ω
V_I
75pF
R_G
475Ω
953Ω
47Ω
183Ω
1/2
20Ω
OPA2683
100pF
–5V
357Ω
357Ω
22pF
V_O
FIGURE 9. Low-Power, Differential I/O, 4th-Order Butterworth Active Filter.
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OPA2683
SBOS244H