Data Sheet

MCP3422/3/4
DS22088C-page 16 © 2009 Microchip Technology Inc.
4.9.2 CONVERTING THE DEVICE
OUTPUT CODE TO INPUT SIGNAL
VOLTAGE
When the user gets the digital output codes from the
device as described in Section 4.9.1 “Digital output
code from device”, the next step is converting the
digital output codes to a measured input voltage.
Equation 4-4 shows an example of converting the
output codes to its corresponding input voltage.
If the sign indicator bit (MSB) is ‘0’, the input voltage
is obtained by multiplying the output code with the LSB
and divided by the PGA setting.
If the sign indicator bit (MSB) is ‘1’, the output code
needs to be converted to two’s complement before
multiplied by LSB and divided by the PGA setting.
Table 4-4 shows an example of converting the device
output codes to input voltage.
EQUATION 4-4: CONVERTING OUTPUT
CODES TO INPUT
VOLTAGE
TABLE 4-4: EXAMPLE OF CONVERTING OUTPUT CODE TO VOLTAGE (WITH 18 BIT SETTING)
If MSB = 0 (Positive Output Code):
If MSB = 1 (Negative Output Code):
Where:
LSB = See Table 4-1
2’s complement = 1’s complement + 1
Input Voltage (Output Code)
LSB
PGA
------------
• =
Input Voltage
(2
′ s complement of Output Code)
LSB
PGA
------------
• =
Input Voltage
[CHn+ - CHn-]
• PGA]
Digital Output Code MSB Example of Converting Output Codes to Input Voltage
≥ V
REF
011111111111111111
0
(2
16
+2
15
+2
14
+2
13
+2
12
+2
11
+2
10
+2
9
+2
8
+2
7
+2
6
+2
5
+2
4
+2
3
+2
2
+2
1
+2
0
)
x LSB(15.625μV)/PGA = 2.048 (V) for PGA = 1
V
REF
- 1 LSB 011111111111111111
0
(2
16
+2
15
+2
14
+2
13
+2
12
+2
11
+2
10
+2
9
+2
8
+2
7
+2
6
+2
5
+2
4
+2
3
+2
2
+2
1
+2
0
)
x LSB(15.625μV)/PGA = 2.048 (V) for PGA = 1
2LSB 000000000000000010
0
(0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+2
1
+0)x LSB(15.625μV)/PGA
= 31.25 (μV) for PGA = 1
1LSB 000000000000000001
0
(0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+2
0
)x LSB(15.625μV)/PGA
= 15.625 (μV)for PGA = 1
0 000000000000000000
0
(0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0)x LSB(15.625μV)/PGA
= 0 V (V) for PGA = 1
-1 LSB 111111111111111111
1
-(0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+2
0
)x LSB(15.625μV)/PGA
= - 15.625 (μV)for PGA = 1
-2 LSB 111111111111111110
1
-(0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+2
1
+0)x LSB(15.625μV)/PGA
= - 31.25 (μV)for PGA = 1
- V
REF
100000000000000000
1
-(2
17
+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0) x
LSB(15.625μV)/PGA = - 2.048 (V) for PGA = 1
≤ -V
REF
100000000000000000
1
-(2
17
+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0+0) x
LSB(15.625μV)/PGA = - 2.048 (V) for PGA = 1