AND8066 Datasheet, PDF (2/8 Pages) ON Semiconductor – Interfacing with ECLinPS

AND8066/D
V_BB 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, V_BB (see Figure 4).
V_CC
Q
V_EE
True
V_TT
D
V_CC
Q
V_{BB D}
Q
V_EE
Figure 4. Standard SE Receivers with V_BB
The V_BB 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).
Invert
True
HIGH
V_BB or Crosspoint
LOW
Figure 5. V_BB Crosspoint Voltage
If V_BB 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).
DV_BB
Input
HIGH
DV_BB
Input
HIGH
Output Shift Wider
Figure 7.
LOW
Obviously, any error voltage present on the V_BB reference
level injects jitter directly into the signal.
V_BB: Voltage Reference Sources
A V_BB reference voltage output source pin may be available
on the receiver device. When present, V_BB is an internally
generated voltage supply and available only to that device’s
inputs. Current demand on the V_BB pin should be limited to 0.5
mA. Bypass (0.01 mF) V_BB to the quietest plane, usually V_CC,
since noise on V_BB will inject jitter and corrupt duty cycle. The
V_BB voltage is derived from referencing the V_CC supply and
will track changes in V_CC 100% or 1:1. If V_CC shifts 1 mV,
then V_BB also changes 1 mV. Changes in V_EE also affect the
V_BB voltage and will track at the rate of 0 to 20%, typically 5%.
If V_EE shifts 100 mV, then V_BB follows with a 0 mV to 20 mV
shift of the same polarity, typically 5 mV.
A V_BB reference voltage may be generated off–device and
supplied to the input pins. Ripple content must be kept as low
as possible on V_CC since it transfers to the signal as jitter and
phase error. A V_BB voltage reference level may be supplied
from a V_BB generator, as shown in Figure 8. Any of the “16”
type buffers are recommended to produce a high current gain
V_BB buffer. For example, the E416, EL16, LVEL16, EP16,
LVEP16, EL17, LVEL17, etc. type devices have a V_BB pin
available. A 1 KW resistor may be needed the feedback path
to stabilize higher gain buffers.
1 KW
LOW
Output Shift Narrower
Figure 6.
If V_BB 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).
16
V_BB
V_BB(out)
RT
0.01 mF
V_TT
V_CC or V_TT
Figure 8. V_BB Voltage Reference Generator
http://onsemi.com
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