OPA2684 Datasheet, PDF (17/33 Pages) Texas Instruments – MINIMAL BANDWIDTH CHANGE VERSUS GAIN, 170MHz BANDWIDTH AT G = +2

24
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 OPA2684 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
OPA2684 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 5 using the
OPA2684. 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.
50Ω
232Ω
100pF
20Ω
+5V
1/2
OPA2684
800Ω
V_I
75pF
400Ω
800Ω
50Ω
232Ω
1/2
20Ω
OPA2684
100pF ^–5V
357Ω
357Ω
22pF
V_O
FIGURE 9. Low-Power, Differential I/O, 4th-Order Butterworth Active Filter.
OPA2684
17
SBOS239D
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