Datasheet

OPA2684
17
SBOS239D
www.ti.com
FIGURE 8. Small-Signal Bandwidth for Figure 7.
FIGURE 9. Low-Power, Differential I/O, 4th-Order Butterworth Active Filter.
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
24
21
18
15
12
9
6
3
Frequency (MHz)
1 20010 100
SINGLE TO DIFFERENTIAL CONVERSION
Gain (dB)
V
I
V
O
1/2
OPA2684
1/2
OPA2684
232Ω50Ω
75pF 22pF
100pF
232Ω50Ω
100pF
800Ω
20Ω
20Ω
800Ω
–5V
+5V
357Ω
357Ω
400Ω
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.
14
11
8
5
2
–1
–4
Frequency (MHz)
12010
10MHz, 3RD-ORDER BUTTERWORTH, LOW PASS,
FREQUENCY RESPONSE
Differential Gain (dB)
FIGURE 10. Frequency Response for 10MHz, 3rd-Order
Butterworth Low-Pass Filter.