Техническая Спецификация для Microchip Technology MCP3421DM-WS
© 2009 Microchip Technology Inc.
DS22003E-page 29
MCP3421
6.2
Application Examples
6.2.1
VOLTAGE MEASUREMENT
The MCP3421 device can be used in a broad range of
sensor and data acquisition applications.
sensor and data acquisition applications.
shows a circuit example measuring the
battery voltage. When the input voltage is greater than
the internal reference voltage (V
the internal reference voltage (V
REF
= 2.048V), it needs
a voltage divider circuit to prevent the output code from
being saturated. In the example, R
being saturated. In the example, R
1
and R
2
form a volt-
age divider. The R
1
and R
2
are set to yield V
IN
to be
less than the internal reference voltage
(V
(V
REF
= 2.048V).
If the input voltage range is much less than the internal
reference voltage, the voltage divider at the input pin is
not needed, and the user may use the internal PGA
with a gain of up to 8.
reference voltage, the voltage divider at the input pin is
not needed, and the user may use the internal PGA
with a gain of up to 8.
When the voltage divider or internal PGA is used for the
input signal, these factor must be taken into account
when the user converts the output codes to the actual
input voltage.
input signal, these factor must be taken into account
when the user converts the output codes to the actual
input voltage.
Find the Microchip Application Note AN1156 for the
input voltage and current measurement using the
MCP342X device family. The MCU firmware is well
documented in the reference.
input voltage and current measurement using the
MCP342X device family. The MCU firmware is well
documented in the reference.
FIGURE 6-5:
Battery Voltage
Measurement.
6.2.2
CURRENT MEASUREMENT
shows a circuit example of current
measurement. For the current measurement, the
device measures the voltage across the current sensor,
and converts it to current by dividing the measured
voltage by a known resistance value of the current
sensor. The voltage drops across the sensor is waste.
Therefore, the current measurement often prefers to
use a current sensor with smaller resistance value,
which, in turn, requires high resolution ADC device.
The high precision MCP342x devices from Microchip
Technology Inc. are suitable for the current
measurement with low resistive current sensors. These
devices can measure the input voltage as low as 2 µV
range (or current in ~ µA range) with 18 bit resolution
and PGA = 8 settings. The MSB (= sign bit) of the
output code indicates the direction of the current.
device measures the voltage across the current sensor,
and converts it to current by dividing the measured
voltage by a known resistance value of the current
sensor. The voltage drops across the sensor is waste.
Therefore, the current measurement often prefers to
use a current sensor with smaller resistance value,
which, in turn, requires high resolution ADC device.
The high precision MCP342x devices from Microchip
Technology Inc. are suitable for the current
measurement with low resistive current sensors. These
devices can measure the input voltage as low as 2 µV
range (or current in ~ µA range) with 18 bit resolution
and PGA = 8 settings. The MSB (= sign bit) of the
output code indicates the direction of the current.
FIGURE 6-6:
Battery Current
Measurement.
Output Code
LSB
R
1
R
2
+
R
2
-------------------
×
×
1
PGA
-----------
×
=
R
1
Battery
R
1
and R
2
= Voltage Divider
V
IN
R
2
R
1
R
2
+
-------------------
V
BAT
×
=
(V)
To Load
V
IN
+
V
IN
-
V
DD
Input Voltage Calculation from Output Code:
R
2
Measured Analog Input Voltage
V
BAT
MCP3421
Battery
(V)
To Load
V
IN
-
V
DD
V
IN
+
Current Sensor
Discharging Current
Charging
Current
Current Calculation from Output Code:
Current
Output Code
LSB
×
R
Sensor
(
)
----------------------------------------------------
1
PGA
------------
A
( )
×
=
MCP3421