Datasheet

HIGH-SPEED ACTIVE FILTERS
511W
20MHz,2nd-OrderButterworthLow-Pass
1/2
OPA2695
+5V
Power-supply
decouplingnotshown.
32.3W 105W
5kW
5kW51W
V
I
0.1 Fm
470W
1/2
OPA2695
4V
I
100pF
158W
0.1 Fm
150pF
12
8
4
0
-4
-8
-12
Frequency(MHz)
20MHz,2ND-ORDERBUTTERWORTH
LOW-PASSFREQUENCYRESPONSE
0.1 100101
Gain(dB)
OPA2695
www.ti.com ...................................................................................................................................................... SBOS354A – APRIL 2008 – REVISED AUGUST 2008
where the 511 Ω feedback resistor is required for
stability. This first stage easily drives the low input
Wideband current-feedback op amps make ideal
resistors required at the input of this high-frequency
elements for implementing high-speed active filters
filter. The second stage is set for a dc gain of +1V/V,
where the amplifier is used as a fixed gain block
carrying the 2.5V operating point through to the
inside a passive RC circuit network. The relatively
output pin and an ac gain of +4V/V. The feedback
constant bandwidth versus gain provides low
resistor has been adjusted to optimize bandwidth for
interaction between the actual filter poles and the
the amplifier itself. As the single-supply frequency
required gain for the amplifier. Figure 73 shows a
response plots show, the OPA2695 in this
typical single-supply buffered filter application. In this
configuration gives greater than 400MHz small-signal
case, one of the OPA2695 channels is used to set up
bandwidth. The capacitor values were chosen to be
the dc operating point and provide impedance
as low as possible, but still large enough to overcome
isolation from the signal source into the 2nd-stage
the effects of the parasitic input capacitance of the
filter. That stage is set up to implement a 20MHz,
amplifier. The resistor values were slightly adjusted to
maximally flat Butterworth frequency response and
give the desired filter frequency response while
provide an ac gain of +4V/V.
accounting for the approximate 1ns propagation delay
through each channel of the OPA2695.
The 51 Ω input matching resistor is optional in this
case. The input signal is ac-coupled to the 2.5V dc
reference voltage developed through the resistor
divider from the +5V power supply. This first stage
acts as a gain of +1V/V voltage buffer for the signal
Figure 73. Buffered Single-Supply Active Filter
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