AD5748 datasheet(29/32 Pages) AD

29 / 32 page

AD5748

Rev. A | Page 29 of 32

CURRENT SETTING RESISTOR

Referring to Figure 1, RSET is an internal sense resistor as part of

the voltage-to-current conversion circuitry. The nominal value

of the internal current sense resistor is 15 kΩ. To allow for over-

range capability in current mode, the user can also select the

internal current sense resistor to be 14.7 kΩ, giving a nominal

2% overrange capability. This feature is available in the 0 mA

to 21 mA and 4 mA to 21 mA current ranges.

The stability of the output current value over temperature is

dependent on the stability of the value of RSET. As a method of

improving the stability of the output current over temperature,

an external low drift resistor can be connected to the REXT1

and REXT2 pins of the AD5748, which can be used instead of

the internal resistor. The external resistor is selected via the

input shift register. If the external resistor option is not used,

the REXT1 and REXT2 pins should be left floating.

PACKET ERROR CHECKING

To verify that data has been received correctly in noisy environ-

ments, the AD5748 offers the option of error checking based on

an 8-bit (CRC-8) cyclic redundancy check. The device controlling

the AD5748 should generate an 8-bit frame check sequence

using the following polynomial:

C(x) = x8 + x2 + x1 + 1

This is added to the end of the data-word, and 24 data bits are

sent to the AD5748 before taking SYNC high. If the AD5748

receives a 24-bit data frame, it performs the error check when

SYNC goes high. If the check is valid, then the data is written

to the selected register. If the error check fails, the FAULT pin

goes low and Bit D3 of the status register is set. After reading

this register, this error flag is cleared automatically and the

FAULT pin goes high again.

SCLK

SDIN

SYNC

UPDATE ON SYNC HIGH

D15

(MSB)

(LSB)

16-BIT DATA

16-BIT DATA TRANSER—NO ERROR CHECKING

SCLK

SDIN

SYNC

FAULT

UPDATE AFTER SYNC HIGH

ONLY IF ERROR CHECK PASSED

FAULT GOES LOW IF

ERROR CHECK FAILS

D23

(MSB)

(LSB)

16-BIT DATA

8-BIT FCS

16-BIT DATA TRANSER WITH ERROR CHECKING

Figure 53. PEC Error Checking Timing

AD5748 Datasheet(PDF) 29 Page - Analog Devices

AD5748 Datasheet(HTML) 29 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL