Datasheet

OPA695
21
SBOS293G
www.ti.com
FIGURE 12. High Power, Wideband AC-Coupled Arbitrary Waveform Driver.
OPA695
+5V
–5V
+5V
–5V
464Ω66.5Ω
0.01µF
200Ω
20Ω
50Ω
OPA695
464Ω
66.5Ω
0.01µF
200Ω
50Ω
DAC
20mA Peak Output
Differential
Filter
I
O
50Ω Source
1.4:1
±3.5V
±3.5V
Power supply decoupling not shown.
20Ω
DIS
DIS
I
O
noninverting input), then inverts the complementary output
current to produce a signal that ranges from this 1.4V at zero
output current down to 0V at maximum output current level
(assuming a 20mA maximum output current). This will give a
very wideband (> 800MHz) video signal capability.
ARBITRARY WAVEFORM DRIVER
The OPA695 may be used as the output stage for moderate
output power Arbitrary Waveform Driver applications. Driving
out through a series 50Ω matching resistor into a 50Ω
matched load will allow up to a 4.0V
PP
swing at the matched
load (15dBm) when operating the OPA695 on a ±5V power
supply. This level of power is available for gains of either ±8
with a flat response through 100MHz. When interfacing
directly from a complementary current output DAC, consider
the circuit of Figure 11, modified for the peak output currents
of the particular DAC being considered. Where purely
AC-coupled output signals are required from a complemen-
tary current output DAC, consider a push-pull output stage
using the circuit of Figure 12. The resistor values here have
been calculated for a 20mA peak output current DAC which
produces up to a 5V
PP
swing at the matched load (18dBm).
This approach will give higher power at the load with much
lower 2nd-harmonic distortion.
For a 20mA peak output current DAC, the mid-scale current
of 10mA will give a 2V DC output common-mode operating
voltage due to the 200Ω resistor to ground at the outputs.
The total AC impedance at each output is 50Ω, giving a
±0.5V swing around this 2V common-mode voltage for the
DAC. These resistors also act as a current divider, sending
75% of the DAC output current through the feedback resistor
(464Ω). The blocking capacitor references the OPA695 out-
put voltage to ground, and turns the unipolar DAC output
current into a bipolar swing of 0.75 • 20mA • 464Ω = 7V
PP
at
each amplifier output. Each output is exactly 180° out-of-
phase from the other, producing double 7V
PP
into the match-
ing resistors. To limit the peak output current and improve
distortion, the circuit of Figure 12 is set up with a 1.4:1 step-
down transformer. This reflects the 50Ω load to be 100Ω at
the primary side of the transformer. For the maximum 14V
PP
swing across the outputs of the two amplifiers, the matching
resistors will drop this to 7V
PP
at the input of the transformer,
then down to 5V
PP
maximum at the 50Ω load at the output of
the transformer. This step-down approach reduces the peak
output current to 14V
P
/(200Ω) = 70mA.