DS1862 datasheet(19/42 Pages) MAXIM

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XFP Laser Control and Digital Diagnostic IC

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Internal Calibration

The DS1862 has two means for scaling an analog input

to a digital result. The two devices alter the gain and

offset of the signal to be calibrated. All of the inputs

except internal temperature have unique registers for

both the gain and the offset that can be found in Table

04h. See the table below for a complete description of

internal calibration capabilities including right-shifting

for all monitor channels.

To scale a specific input’s gain and offset, the relation-

ship between the analog input and the expected digital

result must be known. The input that would produce a

corresponding digital result of all zeroes is the null

value (normally this input is GND). The input that would

produce a corresponding digital result of all ones is the

full-scale (FS) value minus one LSB. The FS value is

also found by multiplying an all ones digital value by

the weighted LSB. For example, a digital reading is 16

bits long, assume that the LSB is known to be 50µV,

then the FS value would be 216 x 50µV = 3.2768V.

A binary search can be used to find the appropriate

gain value to achieve the desired FS of the converter.

Once the gain value is determined, then it can be

loaded into the appropriate channels’ Gain register.

This requires forcing two known voltages on to the

monitor input pin. For best results, one of the forced

voltages should be the NULL input and the other

should be 90% of FS. Since the LSB of the least signifi-

cant bit in the digital reading register is known, the

expected digital results are also known for both the null

and FS value inputs. Figure 9 describes the hysteresis

built into the DS1862’s LUT functionality.

With the exception of BMD, which can source or sink

current, all monitored channels are high impedance

and are only capable of directly measuring a voltage. If

other measured quantities are desired, such as: light,

frequency, power, current etc., they must be converted

to a voltage. In this situation the user is not interested in

voltage measurement on the monitored channel, but

the measurement of the desired parameter. Only the

relationship between the indirect measured quantity

(light, frequency, power, current, etc.) to the expected

digital result must be known.

An example of gain scaling using the recommended

binary search procedure is provided with the following

pseudo-code.

To help will the computation, two integers need to be

defined: count 1 and count 2. CNT1 = NULL / LSB and

CNT2 = 90%FS / LSB. CLAMP is the largest result that

can be accommodated.

Table 8. Temperature Conversion

Examples

MSB (BIN)

LSB (BIN)

TEMPERATURE (°C)

01000000

00000000

+64

01000000

00001000

+64.03215

01011111

00000000

+95

11110110

00000000

-10

11011000

00000000

-40

Table 9. Internal Calibration Capabilities

SIGNAL

INTERNAL

SCALING

INTERNAL

OFFSET

RIGHT-

SHIFTING

Temperature

—x

—

VCC2/3

—

IBIASMON

RSSI (RX-P)

AUX1MON

AUX2MON

BMD (TX-P)

TEMPERATURE (

°C)

INCREASING

TEMPERATURE

DECREASING

TEMPERATURE

Figure 9. Look-Up Table Hysteresis

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