OPA2695 Datasheet, PDF (23/40 Pages) Texas Instruments – Dual, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2695
www.ti.com..................................................................................................................................................................................................... SBOS354 – APRIL 2008
HIGH-SPEED ACTIVE FILTERS
Wideband current-feedback op amps make ideal
elements for implementing high-speed active filters
where the amplifier is used as a fixed gain block
inside a passive RC circuit network. The relatively
constant bandwidth versus gain provides low
interaction between the actual filter poles and the
required gain for the amplifier. Figure 73 shows a
typical single-supply buffered filter application. In this
case, one of the OPA2695 channels is used to set up
the dc operating point and provide impedance
isolation from the signal source into the 2nd-stage
filter. That stage is set up to implement a 20MHz,
maximally flat Butterworth frequency response and
provide an ac gain of +4V/V.
The 51Ω 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
where the 511Ω feedback resistor is required for
stability. This first stage easily drives the low input
resistors required at the input of this high-frequency
filter. The second stage is set for a dc gain of +1V/V,
carrying the 2.5V operating point through to the
output pin and an ac gain of +4V/V. The feedback
resistor has been adjusted to optimize bandwidth for
the amplifier itself. As the single-supply frequency
response plots show, the OPA2695 in this
configuration gives greater than 400MHz small-signal
bandwidth. The capacitor values were chosen to be
as low as possible, but still large enough to overcome
the effects of the parasitic input capacitance of the
amplifier. The resistor values were slightly adjusted to
give the desired filter frequency response while
accounting for the approximate 1ns propagation delay
through each channel of the OPA2695.
+5V
Power-supply
decoupling not shown.
5kW
0.1mF
100pF
V_I
1/2
51W
5kW _OPA2695
32.3W
105W
150pF
1/2
OPA2695
511W
20MHz, 2nd-Order Butterworth Low-Pass
470W
158W
0.1mF
20MHz, 2ND-ORDER BUTTERWORTH
LOW-PASS FREQUENCY RESPONSE
12
8
4
4V_I
0
-4
-8
-12
0.1
1
10
100
Frequency (MHz)
Figure 73. Buffered Single-Supply Active Filter
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): OPA2695
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