OPA830 Datasheet, PDF (27/37 Pages) Burr-Brown (TI) – Low-Power, Single-Supply, Wideband Operational Amplifier

OPA830
www.ti.com
significant time constants that can degrade performance.
Good axial metal film or surface-mount resistors have
approximately 0.2pF in shunt with the resistor. For resistor
values > 1.5kΩ, this parasitic capacitance can add a pole
and/or zero below 500MHz that can effect circuit
operation. Keep resistor values as low as possible
consistent with load driving considerations. The 750Ω
feedback used in the Typical Characteristics is a good
starting point for design.
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 R_S from the typical characteristic curve
Recommended R_S vs Capacitive Load. Low parasitic
capacitive loads (< 5pF) may not need an R_S since the
OPA830 is nominally compensated to operate with a 2pF
parasitic load. Higher parasitic capacitive loads without an
R_S 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 transmission 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 will improve
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 OPA830 is
used as well as a terminating shunt resistor at the input of
the destination device. Remember also that the
terminating impedance will be the parallel combination of
the shunt resistor and the input impedance of the
destination device; this total effective impedance should
be set to match the trace impedance. If the 6dB attenuation
of a doubly-terminated transmission line is unacceptable,
a long trace can be series-terminated 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 typical
characteristic curve Recommended R_S vs Capacitive
Load. This will 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 terminating impedance.
SBOS263F − AUGUST 2004 − REVISED AUGUST 2008
e) Socketing a high-speed part is not recommended.
The additional lead length and pin-to-pin capacitance
introduced by the socket can create an extremely
troublesome parasitic network which can make it almost
impossible to achieve a smooth, stable frequency
response. Best results are obtained by soldering the
OPA830 onto the board.
INPUT AND ESD PROTECTION
The OPA830 is built using a very high-speed complemen-
tary 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 protection diodes to the power supplies, as
shown in Figure 11.
+V_CC
External
Pin
Internal
Circuitry
− V_CC
Figure 11. 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 (that is, in
systems with ±15V supply parts driving into the OPA830),
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.
27