MCP3425 datasheet(11/38 Pages) MICROCHIP

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DS22072B-page 11

MCP3425

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 volt-

age 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 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

voltage 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 16 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 16 BITS

TABLE 4-3:

MINIMUM AND MAXIMUM

CODES

Number of Output Code

Maximum Code

()

PGA

IN+VIN-

–

()

2.048V

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

Where:

See Table 4-3 for Maximum Code

LSB

REF

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

2.048V

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

Where:

User programmable bit resolution:

12,14, or 16

Resolution Setting

LSB

12 bits

1 mV

14 bits

250 µV

16 bits

62.5 µV

Input Voltage:

[VIN+-VIN-] • PGA

Digital Output Code

≥ V

REF

0111111111111111

VREF - 1 LSB

0111111111111111

2LSB

0000000000000010

1LSB

0000000000000001

0000000000000000

-1 LSB

1111111111111111

-2 LSB

1111111111111110

- VREF

1000000000000000

< -VREF

1000000000000000

Note 1:

MSB is a sign indicator:

0: Positive input (VIN+ > VIN-)

1: Negative input (VIN+ > VIN-)

Output data format is binary two’s

complement.

Number

of Bits

Data Rate

Minimum

Code

Maximum

Code

240 SPS

-2048

2047

60 SPS

-8192

8191

15 SPS

-32768

32767

Note:

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

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

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