ADM3082/ADM3085/ADM3088

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PRELIMINARY TECHNICAL DATA

CURRENT LIMIT AND THERMAL SHUTDOWN

The ADM3082/ADM3085/ADM3088 incorporate two

protection mechanisms to guard the drivers against short-

circuits, bus contention or other fault conditions. The first

is a foldback current-limited output stage that protects the

driver against short-circuits over the entire common-mode

voltage range. The second is a thermal shutdown circuit

that puts the driver outputs into a high-impedance state if

the die temperature exceeds a safe limit.

REDUCED EMI AND REFLECTIONS

The ADM3082 and ADM3085 incorporate slew-rating lim-

iting in the drivers. This reduces reflections due to incor-

rect cable termination and minimizes electromagnetic

interference (EMI).

Figures 9 to 11 show driver output waveforms and Fourier

analyses of 20kHz signals for the three different slew-rate

settings. It can be seen that the harmonic content is greatly

reduced for the ADM3085 and still further for the

ADM3082 (Figures 10 and 9 respectively).

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Figure 9 ADM3082 Driver Output Waveform and FFT Plot

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Figure 10. ADM3085 Driver Output Waveform and FFT Plot

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Figure 11. ADM3088 Driver Output Waveform and FFT Plot

The length of an unterminated stub that can be driven

with only minor reflections depends on the rise time of

the transmitter. A conservative estimate for this is given

by the following equation:

sult by 3.28 for answer in feet).

For example, the rise time of the ADM3082 is typically

1320ns, which results in acceptable waveforms with stub

lengths up to 27 metres. This is not the ultimate limit on

unterminated stub length, as a system can still work if the

waveformis allowed to settle before sampling the data.

The RS-485/RS-422 standard covers line lengths up to

4000 feet (1219 metres). Driver output and receiver out-

put waveforms for the three slew rate settings, driving a

4000 foot cable, are shown in figures 12 to 14.

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Figure 12. Driver Input, Driver Output and Receiver Output

Waveforms of ADM3082, Driving 4000ft (1219m) of Cable

at 50kHz

REV. PrA 02/02