Datasheet

ADM1276 Data Sheet
Rev. C | Page 32 of 48
VOLTAGE AND CURRENT CONVERSION USING
LSB VALUES
The direct format voltage and current values returned by the
READ_VIN, READ_VOUT, and READ_IOUT commands, and
the corresponding peak versions, are the data output directly by
the ADM1276 ADC. Because the voltages and currents are 12-bit
ADC output codes, they can also be converted to real-world values
when there is knowledge of the size of the LSB on the ADC.
The m, b, and R coefficients defined for the PMBus conversion
are required to be whole integers by the standard and have,
therefore, been rounded slightly. Using this alternative method,
with the exact LSB values, can provide somewhat more accurate
numerical conversions.
To co nvert an ADC code to current in amperes, the following
formulas can be used:
V
SENSE
= LSB
25 mV
× (I
ADC
− 2048)
I
OUT
= V
SENSE
/(R
SENSE
× 0.001)
where:
V
SENSE
= (V
SENSE+
) − (V
SENSE−
).
LSB
25 mV
= 12.4 µV.
I
ADC
is the 12-bit ADC code.
I
OUT
is the measured current value in amperes.
R
SENSE
is the value of the sense resistor in milliohms.
To convert an ADC code to a voltage, the following formula can
be used:
V
M
= LSB
xV
× (V
ADC
+ 0.5)
where:
V
M
is the measured value in volts.
V
ADC
is the 12-bit ADC code.
LSB
xV
values are based on the voltage range (see Table 8).
Table 8. Voltage Ranges and LSB Values
Voltage Range, LSB
xV
LSB Magnitude
0 V to 6 V 1.488 mV
0 V to 20 V 5.208 mV
To convert a current in amperes to a 12-bit value, the following
formula can be used (round the result to the nearest integer):
V
SENSE
= I
A
× R
SENSE
× 0.001
I
CODE
= 2048 + (V
SENSE
/LSB
25 mV
)
where:
V
SENSE
= (V
SENSE+
) − (V
SENSE−
).
I
A
is the current value in amperes.
R
SENSE
is the value of the sense resistor in milliohms.
I
CODE
is the 12-bit ADC code.
LSB
25 mV
= 12.4 µV.
To convert a voltage to a 12-bit value, the following formula can
be used (round the result to the nearest integer):
V
CODE
= (V
A
/LSB
xV
) − 0.5
where:
V
CODE
is the 12-bit ADC code.
V
A
is the voltage value in volts.
LSB
xV
values are based on the voltage range (see Table 8).