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DS1886_V1 Datasheet - Page 25

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Delta-Sigma Output and Reference
One delta-sigma output (DAC) is provided. This provides
a 10-bit resolution output. The maximum voltage output is
set by the input REFIN. An inexpensive shunt reference is
recommended to generate the voltage applied to REFIN,
as shown in
Figure
8. The output includes the ability to
compensate the APD bias for temperature as given by
the following formula:
DAC_INT = TINDEX[6:0] + DAC OFFSET
If INV_DAC = 0, then DAC[9:0] = DAC_INT/DACFS x
V
.
REFIN
If INV_DAC = 1, then DAC[9:0] = [3FF - (DAC_INT/
DACFS)] x V
.
REFIN
where:
1) INV_DAC is at
A2h Table 02h, Register
2) TINDEX is at
A2h Table 02h, Register
3) DAC OFFSET is an 8-bit value, representing the 8
MSBs of a 10-bit value. The two LSBs are 0.
4) DACFS
(A2h Table 02h, Register
value, representing the 8 MSBs of a 10-bit value. The
two LSBs are 0.
5) DAC is a 10-bit value.
6) The DAC[9:0] is clamped at DACFS.
7) DAC_INT is an internal signal.
The delta-sigma output uses pulse-density modulation.
It provides much lower output ripple than a standard
O
1
2
3
4
5
6
7
Figure 9. Delta-Sigma Output
SFP and PON ONU Controller
with Digital LDD Interface
digital PWM output given the same clock rate and filter
components. An RC filter is required on the DAC output
as suggested in
nents are chosen based on ripple requirements, output
load, delta sigma frequency, and desired response time.
Before t
INIT
The reference input, REFIN, is the supply voltage for the
DAC’s output buffer. The voltage source connected to
REFIN must be able to support the edge rate require-
ments of the delta sigma outputs. In a typical application,
a 0.1uF capacitor should be connected between REFIN
and ground.
The DS1886’s delta-sigma output is 10 bits. For illustra-
tive purposes, a 3-bit example is provided in
8Dh, bit 7.
81h.
88h) is an 8-bit
DS1886
Figure 8. Recommended Shunt Reference and RC Filter for
DAC Output
DS1886
Figure
8. The external RC filter compo-
, the DAC output is high impedance.
V
CC
1kΩ
0201
2.5V
REFIN
0.1µF
ZTL431A
0201
SOT23
39.2kΩ
0201
68.1kΩ
0201
CONNECT TO
CONTROL INPUT
DAC
1µF
ON DC-DC
20kΩ
0402
0201
Figure
9.
25

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