AN246 데이터 시트보기 (PDF) - Philips Electronics

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AN246

Transmission lines and terminations with Philips Advanced Logic families

Philips Electronics 

Philips Semiconductors

Transmission lines and terminations

with Philips Logic families

Application Note

AN246

Figure 9. LVT16244 I-V curves

SH00102

Based on the I-V curves, all families have good drive current in the

logic high state, and ABT(16), ALVT, and LVT(16) BiCMOS families

have considerably stronger drive than the other families in the logic

low state. The BiCMOS families have been optimized to drive

backplanes. The other CMOS families are suitable for local buses

and driving point-to-point loads. The following table shows

recommended minimum line impedances that can be driven by the

buffer/drivers of various Philips logic families:

Table 2. Minimum line impedance for logic

families

Logic Family

Minimum ZO

ABT(16)

35 Ω

AC/ACT

50 Ω

ALS(–1)

65 Ω

FAST

50 Ω

LVT(16)

35 Ω

ALVT

35 Ω

LVC(16)

50 Ω

ALVC

50 Ω

REFLECTIONS FROM IMPEDANCE MISMATCHES

Since a driver has non-zero output impedance, its impedance along

with the line impedance form a voltage divider. The incident wave

launched down the line is a portion of the driver’s voltage. When the

wave encounters an impedance change from either the line or a

receiver input, a portion of the wave is reflected back towards the

driver (Vreflected) which is determined by the reflection coefficient r.

The reflected portion is also added to the incident wave which

continues propagating down the line (Vtransmitted). The relationship

of these voltages are shown in the following equations:

Eq. 8

ò

+

(Zload

(Zload

*

)

ZO)

ZO)

Eq. 9

Vreflected + Vincident

ò

Eq. 10

Vtransmitted + Vincident ) Vreflected

Since driver and line impedances are usually mismatched, a

reflection occurs at the driver and travels back towards the load.

The reflection coefficient at the driver is determined by Equation 11:

Eq. 11

ò

+

(Zdriver

(Zdriver

*

)

ZO)

ZO)

This volley of wave reflections continues, with reflections getting

smaller as the signal waveform settles.

During the reflection period, the waveform may have a stairstep

response––in the case of a driver’s impedance higher than the

line’s––or it may have a “ringy” response––in the case of a driver’s

impedance lower than the line’s. To predict the signal integrity of a

waveform you can use reflection charts or Bergeron plots, but they

can be cumbersome.

1998 Feb 05

7

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