THS4281 Datasheet, PDF (30/43 Pages) Texas Instruments – VERY LOW-POWER, HIGH-SPEED, RAIL-TO-RAIL INPUT AND OUTPUT VOLTAGE-FEEDBACK OPERATIONAL AMPLIFIER

THS4281
SLOS432B – APRIL 2004 – REVISED OCTOBER 2015
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Layout Guidelines (continued)
device. Remember also that the terminating impedance is 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 6-dB 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 use a
series resistor (R_(ISO) = 10 Ω to 100 Ω, as noted Driving Capacitive Loads) to isolate the capacitive load. If
the input impedance of the destination device is low, there is signal attenuation due to the voltage divider
formed by R_(ISO) into the terminating impedance. A 50-Ω environment is normally not necessary onboard,
and in fact a higher impedance environment improves distortion as shown in the distortion versus load plots.
5. Socketing a high-speed part like the THS4281 is not recommended. The additional lead length and pin-
to-pin capacitance introduced by the socket can create a troublesome parasitic network which can make it
almost impossible to achieve a smooth, stable frequency response. Best results are obtained by soldering
the THS4281 onto the board.
10.2 Layout Examples
TOP
Figure 76. THS4281EVM Layout (Top Layer and Silkscreen
Layer)
Layer 2 − GND
Figure 77. THS4281EVM Board Layout
30
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