AND8020 Datasheet, PDF (18/18 Pages) Analog Devices – Termination of ECL Logic Devices with EF (Emitter Follower) OUTPUT Structure

AND8020/D
Combining the high impedance and impedance matching
networks results in an input voltage scheme shown in
Figure 24. This creates the proper input voltage D, VBIAS,
using fewer components.
V_CC
0.001 mF
IN
INb
0.001 mF
R1
Receiver
R_Z
R2
R_t
OUT
OUTb
R_t
V_EE
V_TT
Figure 24. VBIAS and Auto−Oscillation Suppression
with Thevenin Parallel Network
For a 3.3 V V_CC, the values of R1 and R2 provide a
Thevenin parallel network divider voltage with V_IH in the
V_IHCMR of the receiver. Current through the divider
develops the default offset across R_z and can be adjusted as
needed. For example, in Z₀ = 50 traces, a 30 mV default
offset difference will be created if V_CC = 3.3 V and the DC
bias voltage is 2.0 V (typical V_BB) when:
R1 + 4.22 kW
R2 + 6.34 kW
RZ + 100 W
The 0.001 coupling cap may need to be adjusted to
frequency and Vpp amplitude of the receiver input signal.
A similar single−ended network may be used with only
one coupling cap and sufficient bypass capacitance on the
non−driven resistor to preserve a DC level.
Output Level Shifting
Receiver inputs may be level shifted using capacitive
coupling and adjusting VBIAS within the acceptable
common mode range for V_IH. Output levels may also be
changed independent of input levels. The driver device may
be operated with both V_CC and V_EE at shifted values. This
is used at the factory to evaluate devices and conveniently
port signals directly into standard 50 W impedance
equipment modules. The V_CC is fixed to +2.0 V above Test
System chassis ground and the test equipment internal 50 W
impedance constitutes a proper signal termination. Thus, the
split V_EE supply is adjusted to a negative value.
|V_CC − V_EE|
3.0
3.3
5.0
5.5
Split V_CC
+2.0
Split V_EE
−1.0
Unit
V
+2.0
−1.3
V
+2.0
−3.0
V
+2.0
−3.5
V
Output levels may be shifted to symmetrically cross 0.0 V
by a similar method although the advantage of conveniently
directly connecting into standard test equipment is no longer
available.
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AND8020/D