OP4008B Datasheet, PDF (3/6 Pages) RF Monolithics, Inc – Quartz SAW Stabilized and Filtered “Diff Sine” Technology

Load Recommendations for the OP4008B
The Q and Q outputs of RFM's OPB-series of SAW clocks are AC-
coupled internally. This allows customers to set the DC level of the
outputs with a simple resistor pair on each output to drive most any
application.
V_DC = V_LOAD*R1/(R1 + R2)
and
50 = R1*R2/(R1 + R2)
The values of the resistors R2 and R1 are given directly as:
R2 = 50*V_LOAD/V_DC
R1 = 1/(0.02 - (1/R2))
Vdc is the DC level desired on the outputs.
V_load is the DC supply on the load resistors (typically 3.3 V).
3.3V
V_TUNE
OP4005B
V_LOAD
R2
R2
V _DC
V _DC
Load
R1
R1
The equations describe the voltage divider created by R1 & R2 and the equilivent load impedence set by the parrellel combination of R1 & R2.
A given load type will have specifications that set Vdc. Choose an available V_load that is 2x to 3x larger than the desired Vdc. This will allow R1 to be
expressed in terms of R2.
The following table has some typical values for Vdc & V_load for various ECL loads along with R1 & R2 values that are standard.
Load Type
10K 3.3 V PECL
100K 3.3 V PECL
10K 5 V PECL
100k 5 V PECL
10K -5 V NECL
100K -5 V NECL
Vdc
R1
1.95
120
1.88
120
3.65
180
3.58
180
-1.30
240
-1.42
240
R2
V_load
91
3.3 V
91
3.3 V
68
5.0 V
68
5.0 V
62
-5.0 V
62
-5.0 V
Note that the OPB parts will drive negative ECL loads by applying negative voltage to the V_load points in the circuit.
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