OPA830IDBVT datasheet(27/35 Pages) TI

27 / 35 page

OPA830

SBOS263F − AUGUST 2004 − REVISED AUGUST 2008

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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 > 1.5k

Ω, 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 750

Ω

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 RS from the typical characteristic curve

Recommended RS vs Capacitive Load. Low parasitic

capacitive loads (< 5pF) may not need an RS since the

OPA830 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

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 50

Ω

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 RS 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.

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.

External

−V

Internal

Circuitry

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.

OPA830IDBVT Datasheet(PDF) 27 Page - Texas Instruments

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