Datasheet

9
Signal Chain Design Guide
DC Voltage and Current Measurement
DC voltage and current measurement can be easily done
by using low speed high resolution Delta Sigma ADC such
as MCP3421 and MCP3422 family devices. The MCP3421
is a single channel device while the MCP3422 is a dual
channel device, which can measure the voltage and
current using the same device.
The following circuits show simple example of Battery
voltage and current measurement using the MCP3421.
The MCP3421 uses internal reference voltage of 2.048V.
If the input voltage is greater than the reference, it needs
a voltage divider to bring down the input full scale range
below the reference voltage. This example is shown in
example circuit (a). In the current measurement, the ADC
is simply connected across a simple shunt current sensor
as shown in the figure. The current is calculated using the
measured voltage value and a known shunt’s resistance
value. The MCP3421 has a differential input and the MSb
in the output bit represents the direction of the current.
Voltage Measurement Using MCP3421 Device
Current Measurement Using MCP3421 Device
(a) If V
REF
< V
BAT
ADC
V BAT
–
+
R
2
R 1
(b) If V
REF
> V
BAT
R2
V
IN =
(
________
)
● ( V BAT)
R1 + R2
R2
V
Measured
= ADC Output Codes
●
LSB
●
_________
●
_____
(R1 + R2)
PGA
1
2
N
–1
LSB =
________________
Reference Voltage
2.048V
2
17
2
N
–1
LSB of 18-bit ADC =
________________
=
______
= 15.625 µV
Reference Voltage
ADC
V BAT
–
+
MCP3421
MCP3421
MCU
MCU
V
IN
Current = (Measured Voltage)/(Known Resistance Value of Current Sensor)
Direction of current is determined by sign bit (Msb bit) of the ADC output code.
ADC
Battery
Charging
Current
Discharging
Current
Current Sensor
To Load
–
+
MCP3421
MCU
Voltage and Current Measurement
Battery Fuel Management by Measuring
Battery Voltage and Current
By measuring the battery voltage and current, an intelligent
battery fuel management algorithm can be developed.
The figure below shows an example of battery fuel
management circuit. The MCP3422 measures both voltage
and current draws of the battery, and the system tracks
how much the battery fuel has been used and remained.
The MCU controls the MCP73831 for the recharging of the
single cell Li-Ion battery
Example Circuit for Battery Fuel Management by
measuring Battery Voltage and Current
Battery
MCU
MCP73831
MCP3422
CH B
R2
CH A
Charging Current
Decharging
Current
Battery Fuel Measurement
Battery Fuel Charger
V
BAT
S2
S1
R1
STAT PROG
R3
Load