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.
SINGLE TO DIFFERENTIAL CONVERSION
24
21
18
15
12
9
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, RG. 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.
6
3
1
10
100
200
10MHz, 3RD-ORDER BUTTERWORTH LOW PASS
Frequency (MHz)
FREQUENCY RESPONSE
14
FIGURE 8. Small-Signal Bandwidth for Figure 7.
11
8
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
5
2
–1
–4
1
10
20
Frequency (MHz)
FIGURE 10. Frequency Response for 10MHz, 3rd-Order
Butterworth Low-Pass Filter.
100pF
+5V
20Ω
47Ω
183Ω
1/2
OPA2683
953Ω
357Ω
357Ω
75pF
RG
475Ω
VI
VO
22pF
953Ω
1/2
OPA2683
20Ω
47Ω
183Ω
–5V
100pF
FIGURE 9. Low-Power, Differential I/O, 4th-Order Butterworth Active Filter.
OPA2683
SBOS244H
16
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