AND8066 datasheet(2/8 Pages) ONSEMI

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AND8066/D

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VBB Reference

For a standard differential receiver with two input pins –

D and D – only one of two inputs is suitably selected to

receive the signal while the non–driven input must be biased

to a (DC) reference voltage, VBB (see Figure 4).

VEE

VCC

VEE

Figure 4. Standard SE Receivers with VBB

True

VBB

VCC

VTT

The VBB value is designed to be maintained midway

(50%) between the HIGH and LOW levels of the received

signal, that is, the crosspoint voltage of a differential signal

pair, to preserve the duty cycle and signal integrity (see

Figure 5).

VBB or Crosspoint

Figure 5. VBB Crosspoint Voltage

True

Invert

LOW

HIGH

If VBB shifts, due to drift or noise, above the input signal

50% crosspoint, the device output signal will shift the duty

cycle away from a pure 50% point to a decreased, narrowing

pulse width (see Figure 6).

Figure 6.

DVBB

Input

HIGH

LOW

Output Shift Narrower

If VBB shifts below the input signal 50% crosspoint, the

device output signal will shift the duty cycle away from a

pure 50% point to an increased, widening pulse width (see

Figure 7).

Figure 7.

DVBB

Input

HIGH

LOW

Output Shift Wider

Obviously, any error voltage present on the VBB reference

level injects jitter directly into the signal.

VBB: Voltage Reference Sources

A VBB reference voltage output source pin may be available

on the receiver device. When present, VBB is an internally

generated voltage supply and available only to that device’s

inputs. Current demand on the VBB pin should be limited to 0.5

mA. Bypass (0.01

mF) VBB to the quietest plane, usually VCC,

since noise on VBB will inject jitter and corrupt duty cycle. The

VBB voltage is derived from referencing the VCC supply and

will track changes in VCC 100% or 1:1. If VCC shifts 1 mV,

then VBB also changes 1 mV. Changes in VEE also affect the

VBB voltage and will track at the rate of 0 to 20%, typically 5%.

If VEE shifts 100 mV, then VBB follows with a 0 mV to 20 mV

shift of the same polarity, typically 5 mV.

A VBB reference voltage may be generated off–device and

supplied to the input pins. Ripple content must be kept as low

as possible on VCC since it transfers to the signal as jitter and

phase error. A VBB voltage reference level may be supplied

from a VBB generator, as shown in Figure 8. Any of the “16”

type buffers are recommended to produce a high current gain

VBB buffer. For example, the E416, EL16, LVEL16, EP16,

LVEP16, EL17, LVEL17, etc. type devices have a VBB pin

available. A 1 K

W resistor may be needed the feedback path

to stabilize higher gain buffers.

Figure 8. VBB Voltage Reference Generator

VBB(out)

VTT

VCC or VTT

VBB

0.01

1 K

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