Figure 11 shows the extinction ratios (expressed in
decibels) as a function of N for several A
M
/A
D
ratios.
Note the actual extinction ratio value is determined by
the ratio A
M
/A
D
in addition to N. If the A
M
/A
D
ratio
varies due to voltage, temperature, or aging effects, it
becomes an additional error source when determining
the overall extinction ratio variance. The DS1861 is
meant to be used with a monolithic laser driver, where
A
M
and A
D
are generated on the same chip, so
changes in the ratio of A
M
to A
D
are minimal.
The N value itself is determined by the extinction ratio
register. There are two components of the ER register
used to calculate N. The range select bit (RSEL), which
selects the high (RSEL = 1) or low (RSEL = 0) range of
N, is the MSB of the register. The lower seven bits of
the register (ER<6:0>) determine the value of N within
the selected range. The value of N is given by:
The ER<6:0> value should be programmed to values
between 28 and 104 (decimal), regardless if the high or
low range is selected, to ensure that the N value
remains accurate and constant as intended. These lim-
its allow the N values of the high and low range to over-
lap to ensure that all N values can be attained, but
prevents potential errors that can be caused by using
the extremes of each RSEL range. Figure 12 shows the
N values as a function of the ER register setting.
The Convergence Algorithm and
Overshoot Control
The DS1861 uses a tiered slew-rate control system that
adjusts the DAC update rate and the number of LSBs it
increases/decreases per update cycle when the control
loop is seeking to converge to its steady state value. For
the APC loop, it makes its decision on the required con-
vergence rate based on the percent error between the
present BMD current and the BMD current-set point.
The modulation current slew rate is adjusted based
solely on the difference between its present code and
the code-set point that is determined by the AERC cir-
cuitry. Both update rates are designed to prevent any
overshoot during large set-point changes in excess of
10%, which assumes (see the following) the ratio of the
laser diode’s threshold current (I
TH
) to the average
power current (I
APC
) is below 7:1. Most systems do not
exhibit any overshoot when using the DS1861.
