MSP430 datasheet(26/88 Pages) TI

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The DAC_OFFSET is stored in the DAC_OFFSET register using 2's-complement encoding. The DAC_OFFSET

value can be programmed from –2(N–1) to 2(N–1) – 1 using Equation 3.

DAC_OFFSET=−OFFSETMSB×2N−1 +∑i=0N−2OFFSETi×2i

(3)

where

• N = DAC_OFFSET resolution in bits: 16 for the AFE88101 and 14 for the AFE78101.

• OFFSETMSB = MSB bit of the DAC_OFFSET register.

• OFFSETi = The rest of the bits of the DAC_OFFSET register.

• i = Position of the bit in the DAC_OFFSET register.

• OFFSET data are left justified; the last two LSBs in the DAC_OFFSET register are ignored for the device.

The most significant bit determines the sign of the number and is called the sign bit. The sign bit has the weight

of –2(N–1) as shown in Equation 3.

The example DAC_OFFSET settings for the AFE88101 are shown in Table 7-4.

Table 7-4. DAC_OFFSET Setting vs OFFSET Code

DAC_OFFSET

OFFSET (HEX)

32767

0x7FFF

0x0001

0x0000

–1

0xFFFF

–2

0xFFFE

–32768

0x8000

The following transfer function is applied to the DAC_DATA.DATA based on the DAC_GAIN and DAC_OFFSET

values: DAC_CODE= DATA×DAC_GAIN +DAC_OFFSET

(4)

where

• DAC_CODE is the internal signal applied to the DAC.

• DATA is the decimal value of the DAC_DATA register.

• DAC_GAIN and DAC_OFFSET are the user calibration settings.

• DATA data are left justified; the last two LSBs in the DAC_DATA register are ignored for the AFE78101.

Substituting DAC_GAIN and DAC_OFFSET in Equation 4 with Equation 2 and Equation 3 results in:

DAC_CODE= DATA× 12+GAIN2N −OFFSETMSB×2N−1+∑i=0N−2OFFSETi×2i

(5)

The multiplier is implemented using truncation instead of rounding. This truncation can cause a difference of one

LSB if rounding is expected. Figure 7-2 shows the DAC calibration path.

DAC_DATA

DAC_GAIN

DAC_OFFSET

DAC_CODE

Gain = ½ + DAC_GAIN / 2

< 8000h: Gain < 1

= 8000h: Gain = 1

> 8000h: Gain > 1

Offset in 2’s-complement

< 0000h: Negative offset

0000h: No offset

> 0000h: Positive offset

Figure 7-2. DAC Calibration Path

AFE78101, AFE88101

SLASF21 – DECEMBER 2022

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MSP430 Datasheet(PDF) 26 Page - Texas Instruments

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