OPA3680 Burr-Brown, OPA3680 Datasheet - Page 12

• Manufacturer Part Number: OPA3680
• Description: Triple / Wideband / Voltage-Feedback OPERATIONAL AMPLIFIER With Disable
• Manufacturer: Burr-Brown

output voltage can swing to within 1V of either supply pin
while delivering >100mA output current. A demanding 100
load to a midpoint bias is used in this characterization circuit.
The new output stage circuit used in the OPA3680 can
deliver large bipolar output currents into this midpoint load
with minimal crossover distortion, as shown in the
supply, Harmonic Distortion vs Supply Voltage plot.
ANALOG DELAY LINE
The diagram on the front page of this data sheet shows an
analog delay line using the OPA3680. 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 opera-
tion is desired. The last two sections provide an analog delay
function given by Equation 1:
where, f represents the frequency components of interest in
the input signal. For input frequencies below 0.39/2
2.5MHz the delay will be within 15% of the desired value
(2 ). 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 5 harmonics are delayed
equally. For the circuit shown 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 1pF capacitors limit the noise, while maintaining good
pulse response. If desired, these two capacitors may be
removed for circuits that produce less delay.
FIGURE 3. Analog Delay Line’s Pulse Response.
–200
–400
–600
–800
800
600
400
200
0
Delay
®
RC
OPA3680
Input
1
2
2
f
Time (200ns/div)
Output
2
,
for each section
106ns
5V
(1)
=
12
INSTRUMENTATION DIFFERENTIAL AMPLIFIER
Figure 4 shows an instrumentation differential amplifier
based on the OPA3680. This application benefits from the
OPA3680’s 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 PC board layout can improve 2nd harmonic distor-
tion at higher frequencies.
BUFFERED 2 x 1 MULTIPLEXER
Using two of the three channels in an OPA3680 to select one
of two possible input signals, then using the 3rd to isolate the
summing point and drive the load, will give a very flexible,
wideband, multiplexing capability. Figure 5 shows one ex-
ample of this where the two input stages have been set up for
a gain of +2.
Summing the two output signals together at the output stage
buffer’s non-inverting input through 400
excellent isolation between the two channels to be main-
tained. When one channel is operating, the other will see an
attenuated version of the active channel’s signal on its
inverting node. In this circuit, that signal is attenuated by
20dB at this inactive inverting input—this will keep the
swing low enough on the off channel to avoid parasitic turn
on at that input stage. The desired signal is attenuated by
0.6V/V due to this resistor divider, then recovered by the
gain set in the output stage.
One modification to this circuit would give a high speed
switched gain. The same signal would be fed into both
inputs and each amplifier would be set to a different gain.
FIGURE 4. Instrumentation Amplifier.
V
V
1
2
499
OPA3680
OPA3680
1/3
1/3
249
249
124
249
OPA3680
124
249
1/3
resistors allows
= 2 (V
V
1
OUT
– V
2
)