Datasheet

© 2009 Microchip Technology Inc. DS22072B-page 27
MCP3425
6.2 Application Examples
6.2.1 VOLTAGE MEASUREMENT
The MCP3425 device can be used in a broad range of
sensor and data acquisition applications.
Figure 6-5 shows a circuit example measuring the bat-
tery voltage. When the input voltage is greater than 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
1
and R
2
form a
voltage divider. The R
1
and R
2
are set to yield V
IN
to be less than the internal reference voltage
(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.
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.
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.
FIGURE 6-5: Battery Voltage
Measurement.
6.2.2 CURRENT MEASUREMENT
Figure 6-6 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. The
MSB (= sign bit) of the output code indicates the direc-
tion of the current.
FIGURE 6-6: Battery Current
Measurement.
6.2.3 PRESSURE MEASUREMENT
Figure 6-7 shows an example of measuring the
pressure using NPP301 (manufactured by
GE NovaSensor). No external signal conditioning
circuit is needed by utilizing its internal PGA. The
pressure sensor output is 20 mV/V. This gives 100 mV
of full scale output for V
DD
of 5V (sensor excitation
voltage).
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
-
MCP3425
V
DD
Input Voltage Calculation from Output Code:
R
2
Measured Input Voltage
V
BAT
Battery
(V)
To Load
V
IN
-
MCP3425
V
DD
V
IN
+
Current Sensor
Discharging Current
Charging
Current
Current Calculation from Output Code:
Current
Output Code LSB
×
RSensor()
----------------------------------------------------
1
PGA
------------
A()
× =