Microchip Technology MCP3422, Manual

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©
2008 Microchip Technology Inc. DS22088B-page 15
MCP3422/3/4
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:
(a) for a negative full-scale input voltage: 100...000
Example: (CHn+ - CHn-)
•
PGA = -2.048V
(b) for a zero differential input voltage: 000...000
Example: (CHn+ - CHn-) = 0
(c) for a positive full-scale input voltage: 011...111
Example: (CHn+ - CHn-)
•
PGA = 2.048V
The MSB (sign bit) is always transmitted first through
the I
2
C serial data line. The resolution for each conver-
sion is 18, 16, 14, or 12 bits depending on the conver-
sion 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 (V
REF
- 1 LSB)/PGA and
1000...00
for
voltages less than -V
REF
/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 conver-
sion 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 1 +
( )
PGA
CHn + CHn - –
( )
2.048V
-----------------------------------------
× ×
=
Where:
See Table 4-3 for Maximum Code
LSB
2 V
REF
×
2
N
----------------------
2 2.048 V
×
2
N
-------------------------- = =
Where:
N = Resolution, which is programmed in
the Configuration Register.
Resolution Setting LSB
12 bits 1 mV
14 bits 250 µV
16 bits 62.5 µV
18 bits 15.625 µV
Input Voltage:
[CHn+ - CHn-] • PGA
Digital Output Code
≥
V
REF
011111111111111111
V
REF
- 1 LSB
011111111111111111
2 LSB
000000000000000010
1 LSB
000000000000000001
0
000000000000000000
-1 LSB
111111111111111111
-2 LSB
111111111111111110
- V
REF
100000000000000000
< -V
REF
100000000000000000
Note 1: MSB is a sign indicator:
0: Positive input (CHn+ > CHn-)
1: Negative input (CHn+ < CHn-)
2: Output data format is binary two’s
complement.
Resolution
Setting
Data Rate
Minimum
Code
Maximum
Code
12 240 SPS -2048 2047
14 60 SPS -8192 8191
16 15 SPS -32768 32767
18 3.75 SPS -131072 131071
Note: Maximum n-bit code = 2
N-1
- 1
Minimum n-bit code = -1 x 2
N-1