Datasheet

OPA695
19
SBOS293G
www.ti.com
FIGURE 9. Cable Modem Upstream Driver.
WIDEBAND CABLE DRIVING
APPLICATIONS
The high slew rate and bandwidth of the OPA695 can be
used to meet the most demanding cable driving applications.
CABLE MODEM RETURN PATH DRIVER
The standard cable modem upstream driver is typically
required to drive high power over a 5MHz to 65MHz band-
width while delivering < –50dBc distortion. Highly-integrated
solutions (including programmable gain stages) often fall
short of this target due to high losses from the amplifier
output to the line. The higher gain operating capability of the
OPA695, along with its very high slew rate, provides a low-
cost solution for delivering this signal with the required
spurious-free dynamic range. Figure 9 shows one example
of using the OPA695 as an upstream driver for a cable
modem return path. In this case, the input impedance of the
driver is set to 75Ω by the gain resistor (R
G
). The required
input level from the adjustable gain stage is significantly
reduced by the 15.5dB gain provided by the OPA695. In this
example, the physical 75Ω output matching resistor, along
with the 3dB loss in the diplexer, will attenuate the output
swing by 9dB on the line. In this example, a single +12V
supply was used to achieve the lowest harmonic distortion
for the 6V
PP
output pin voltage through 65MHz. Measured
performance for this example gave 600MHz small-signal
bandwidth and < –54dBc distortion through 65MHz for a
6V
PP
output pin voltage swing.
OPA695
R
F
450Ω
R
G
75Ω
75Ω
6kΩ
6kΩ
20Ω
75Ω
Diplexer
–3dB
Receive Channel
Supply decoupling
not shown
67dBmV
+12V
PGA Output
58dBmV
0.1µF1000pF
0.01µF
0.1µF
51.5dBmV
DIS
1000pF
1000pF
An alternative to this circuit, giving even lower distortion, is a
differential driver using two OPA695s driving into an output
transformer. This can be used either to double the available
line power, or to improve distortion by cutting the required
output swing in half for each stage. The channel disable
required by the MCNS specification should be implemented
by using the PGA disable feature. The MCNS disable speci-
fication requires that an output impedance match be main-
tained with the signal channel shut off. The disable feature of
the OPA695 is intended principally for power savings and
puts the output and inverting input pins into a high imped-
ance mode. This will not maintain the required output imped-
ance matching. Turning off the signal at the input of Figure
9, while keeping the OPA695 active, will maintain the imped-
ance matching while putting very little noise on the line. The
line noise in disable for the circuit of Figure 9 (with the PGA
source turned off, but still presenting a 75Ω source imped-
ance) will be a very low 4nV/
√Hz (–157dBm/Hz) due to the
low input noise of the OPA695.
RGB VIDEO LINE DRIVER
The extremely high bandwidth of the OPA695 operating at a
gain of +2 will support the fastest RAMDAC outputs for
applications such as auxiliary monitor driving. The front page
of this data sheet shows measured performance for a
0 → +1V input square wave at 125MHz. As a general rule,
the required full-power bandwidth for the amplifier must be at
least one-half the pixel rate. With its noninverting gain of +2,
slew rate of 2900V/µs, and a 1.4V
PP
output pin voltage swing
for standard RGB video levels, the OPA695 will give a