MCP3421 datasheet(13/42 Pages) MICROCHIP

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DS22003E-page 13

MCP3421

4.9

Digital Output Codes and

Conversion to Real Values

4.9.1

DIGITAL OUTPUT CODE FROM

DEVICE

The digital output code is proportional to the input

voltage and PGA settings. The output data format is a

binary two’s complement. With this code scheme, the

MSB can be considered a sign indicator. When the

MSB is a logic ‘0’, the input is positive. When the MSB

is a logic ‘1’, the input is negative. The following is an

example of the output code:

for

negative

full

scale

input

voltage:

100...000

Example: (VIN+- VIN-) •PGA = -2.048V

for a zero differential input voltage: 000...000

Example: (VIN+- VIN-) = 0

for

positive

full

scale

input

voltage:

011...111

Example: (VIN+- VIN-) • PGA = 2.048V

The MSB (sign bit) is always transmitted first through

the I2C serial data line. The resolution for each

conversion is 18, 16, 14, or 12 bits depending on the

conversion rate selection bit settings by the user.

The output codes will not roll-over even if the input volt-

age exceeds the maximum input range. In this case,

the code will be locked at 0111...11 for all voltages

greater than (VREF - 1 LSB)/PGA and 1000...00 for

voltages less than -VREF/PGA. Table 4-2 shows an

example of output codes of various input levels for 18

bit conversion mode. Table 4-3 shows an example of

minimum and maximum output codes for each

conversion rate option.

The number of output code is given by:

EQUATION 4-2:

The LSB of the data conversion is given by:

EQUATION 4-3:

Table 4-1 shows the LSB size of each conversion rate

setting. The measured unknown input voltage is

obtained by multiplying the output codes with LSB. See

the following section for the input voltage calculation

using the output codes.

TABLE 4-1:

RESOLUTION SETTINGS VS.

LSB

TABLE 4-2:

EXAMPLE OF OUTPUT CODE

FOR 18 BITS (NOTE 1,NOTE 2)

TABLE 4-3:

MINIMUM AND MAXIMUM

OUTPUT CODES (NOTE)

Number of Output Code

Maximum Code

() PGA

IN+VIN-

–

()

2.048V

-----------------------------------

Where:

See Table 4-3 for Maximum Code.

LSB

REF

----------------------

2.048V

--------------------------

Where:

N = User programmable bit resolution:

12,14,16, or 18

Resolution Setting

LSB

12 bits

1 mV

14 bits

250 µV

16 bits

62.5 µV

18 bits

15.625 µV

Input Voltage:

[VIN+-VIN-] • PGA

Digital Output Code

≥ V

REF

011111111111111111

VREF - 1 LSB

011111111111111111

2LSB

000000000000000010

1LSB

000000000000000001

000000000000000000

-1 LSB

111111111111111111

-2 LSB

111111111111111110

- VREF

100000000000000000

< -VREF

100000000000000000

Note 1: MSB is a sign indicator:

0: Positive input (VIN+>VIN-)

1: Negative input (VIN+< VIN-)

2: Output data format is binary two’s

complement.

Resolution

Setting

Data Rate

Minimum

Code

Maximum

Code

240 SPS

-2048

2047

60 SPS

-8192

8191

15 SPS

-32768

32767

3.75 SPS

-131072

131071

Note:

Maximum n-bit code = 2N-1 - 1

Minimum n-bit code = -1 x 2N-1

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