Rainbow Electronics DS1876 - Manual (Page 16)

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DS1876
SFP Controller with Dual LDD Interface
For all device addresses sourced from EEPROM (Table
02h, Register 8Bh), the default device addresses are
A2h and B2h until V
CC exceeds POA allowing the device
address to be recalled from the EEPROM.
Delta-Sigma Outputs
Four delta-sigma outputs are provided: MOD1, MOD2,
APC1, and APC2. With the addition of an external RC
filter, these outputs provide 10-bit resolution analog
outputs with the full-scale range set by the input REFIN.
Each output is either manually controlled or controlled
using a temperature-indexed LUT.
A delta-sigma DAC has a digital output using pulse-
density modulation. It provides much lower output ripple
than a standard digital PWM output given the same clock
rate and filter components. Before t
INIT , the DAC outputs
are high impedance. The external RC filter components
are chosen based on ripple requirements, output load,
delta-sigma frequency, and desired response time.
Figure 6 shows a recommended filter.
For illustrative purposes, a 3-bit example is provided in
Figure 7.
In LUT mode the DACs are each controlled by an LUT
with high-temperature resolution and an OFFSET LUT
with lower temperature resolution. The high-resolution
LUTs each have 2NC resolutions. The OFFSET LUTs
are located in the upper eight registers (F8h
-FFh) of
the table containing each high-resolution LUT. The DAC
values are determined as follows:
DAC value = LUT + 4 x (OFFSET LUT)
An example calculation for MOD1 DAC is as follows:
Assumptions:
1) Temperature is +43NC
2) Table 04h (MOD1 OFFSET LUT), Register FCh = 2Ah
3) Table 04h (MOD1 LUT), Register AAh = 7Bh
Because the temperature is +43NC, the MOD1 LUT index
is AAh and the MOD1 OFFSET LUT index is FCh.
MOD1 DAC = 7Bh + 4 x 2Ah = 123h = 291
Figure 6. Recommended RC Filter for DAC Outputs in Voltage
Mode and Current Sink Mode
Figure 5. Low-Voltage Hysteresis Example
DS1876
DAC
3.24k Ω 3.24k Ω
0.01µF 0.01µF
VOLTAGE OUTPUT
DS1876
DAC
1k Ω 1k Ω
0.1µF 0.1µF
CURRENT SINK
2kΩ
V POA
V POD
V CC
SEE RECALLED VALUE RECALLED VALUE
PRECHARGED
TO 0
PRECHARGED
TO 0
PRECHARGED TO 0
SEE RECALL
SEE RECALL