d) Connections to other wideband devices on the board
may be made with short direct traces or through onboard
transmission lines. For short connections, consider the
trace and the input to the next device as a lumped capacitive
load. Relatively wide traces (50mils to 100mils) should be
used, preferably with ground and power planes opened up
around them. Estimate the total capacitive load and set RS
from the plot of Recommended RS vs Capacitive Load. Low
parasitic capacitive loads (< 5pF) may not need an RS, since
the OPA846 is nominally compensated to operate with a 2pF
parasitic load. Higher parasitic capacitive loads without an RS
are allowed, as the signal gain increases (increasing the
unloaded phase margin). If a long trace is required, and the
6dB signal loss intrinsic to a doubly-terminated transmission
line is acceptable, implement a matched impedance trans-
mission line using microstrip or stripline techniques (consult
an ECL design handbook for microstrip and stripline layout
techniques). A 50Ω environment is normally not necessary
onboard and, in fact, a higher impedance environment im-
proves distortion, as shown in the distortion versus load
plots. With a characteristic board trace impedance defined
based on board material and trace dimensions, a matching
series resistor into the trace from the output of the OPA846
is used, as well as a terminating shunt resistor at the input of
the destination device. Remember also that the terminating
impedance is the parallel combination of the shunt resistor
and input impedance of the destination device; this total
effective impedance should be set to match the trace imped-
ance. If the 6dB attenuation of a doubly-terminated transmis-
sion line is unacceptable, a long trace can be series-termi-
nated at the source end only. Treat the trace as a capacitive
load in this case and set the series resistor value as shown
in the plot of Recommended RS vs Capacitive Load. This
does not preserve signal integrity as well as a doubly-
terminated line. If the input impedance of the destination
device is low, there will be some signal attenuation due to the
voltage divider formed by the series output into the terminat-
ing impedance.
e) Socketing a high-speed part like the OPA846 is not
recommended. The additional lead length and pin-to-pin
capacitance introduced by the socket can create an ex-
tremely troublesome parasitic network, which can make it
almost impossible to achieve a smooth, stable frequency
response. Best results are obtained by soldering the OPA846
onto the board.
INPUT AND ESD PROTECTION
The OPA846 is built using a very high-speed complementary
bipolar process. The internal junction breakdown voltages are
relatively low for these very small geometry devices. These
breakdowns are reflected in the Absolute Maximum Ratings
table. All device pins are protected with internal ESD protec-
tion diodes to the power supplies, as shown in Figure 17.
+VCC
External
Pin
Internal
Circuitry
–VCC
FIGURE 17. Internal ESD Protection.
These diodes provide moderate protection to input overdrive
voltages above the supplies as well. The protection diodes
can typically support 30mA continuous current. Where higher
currents are possible (e.g., in systems with ±15V supply parts
driving into the OPA846), current-limiting series resistors
should be added into the two inputs. Keep these resistor
values as low as possible, since high values degrade both
noise performance and frequency response.
OPA846
SBOS250C
19
www.ti.com
TI [ TEXAS INSTRUMENTS ]
