OPA3690ID Datasheet, PDF (16/39 Pages) Texas Instruments – Triple, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3690
SBOS237G – MARCH 2002 – REVISED MARCH 2010
ANALOG DELAY LINE
The circuit on the front page of this data sheet shows
an analog delay line using the OPA3690. The first op
amp buffers the delay line from the source, and can
be used to establish the dc operating point if single
+5V supply operation is desired. The last two
sections provide an analog delay function given by
Equation 1:
Delay
=
1
+
2t
(2pft)²
,
for
each
section.
(1)
Where:
t = RC.
f represents the frequency components of interest
in the input signal.
For input frequencies below 0.39/2pt = 2.5MHz, the
delay will be within 15% of the desired value (2t).
The circuit on the front page gives a delay of 50ns
per stage for a total delay of 100ns. Excellent pulse
fidelity will be retained as long as the first five
harmonics are delayed equally. For the circuit on the
front page, the 5th-harmonic should be = 2.5MHz/5,
which will support a square wave up to 500kHz, with
good pulse response. The input rise-and-fall times
also need to be = 0.30/2.5MHz = 120ns in order to
keep the spectral energy within this 2.5MHz limit.
Quicker rise or fall times will cause propagation delay
errors and excessive pre-shoot.
Shorter delays may be implemented at higher
frequencies by adjusting R and C. To maintain bias
current cancellation, it is best to simply reduce C
without changing R. The analog delay line pulse
response is shown in Figure 40.
800
600
400
200
0
-200
-400
-600
-800
Input
Output
106ns
Time (200ns/div)
Figure 40. Analog Delay Line Pulse Response
The 1pF capacitors limit the noise, while maintaining
good pulse response. If desired, these two capacitors
may be removed for circuits that produce less delay.
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INSTRUMENTATION DIFFERENTIAL
AMPLIFIER
Figure 41 shows an instrumentation differential
amplifier based on the OPA3690. This application
benefits from the OPA3690 dc precision,
common-mode rejection, high impedance input, and
low-current noise. The resistors on the last
(difference) amplifier were selected to keep the loads
equal on the input stage op amps. The matched
loads and a careful PCB layout can improve
2nd-harmonic distortion at higher frequencies.
Figure 42 shows the frequency response of the
instrumentation differential amplifier.
V₁
1/3
OPA3690
402W
806W
402W
1/3
OPA3690
V₂
200W
200W
1/3
OPA3690
402W
402W
V_OUT
V_OUT = 2 (V₁ - V₂)
Figure 41. Instrumentation Amplifier
10
5
0
-5
-10
-15
-20
10
100
400
Frequency (MHz)
Figure 42. Frequency Response for the OPA3690
as an Instrumentation Amplifier
In applications where current-feedback amplifiers or
fixed-gain amplifiers can be used, the OPA3691 or
OPA3692 will provide wider bandwidth
instrumentation amplifiers.
16
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