MCP3421 datasheet(29/42 Pages) MICROCHIP

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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.

Figure 6-5 shows a circuit example measuring the

battery voltage. When the input voltage is greater than

the internal reference voltage (VREF = 2.048V), it needs

a voltage divider circuit to prevent the output code from

being saturated. In the example, R1 and R2 form a volt-

age divider. The R1 and R2 are set to yield VIN to be

less

than

the

internal

reference

voltage

(VREF = 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

circuit

example

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.

FIGURE 6-6:

Battery Current

Measurement.

Output Code

LSB

-------------------

PGA

-----------

Battery

R1 and R2 = Voltage Divider

-------------------

BAT

(V)

To Load

VIN+

VIN-

VDD

Input Voltage Calculation from Output Code:

Measured Analog Input Voltage

VBAT

MCP3421

Battery

(V)

To Load

VIN-

VDD

VIN+

Current Sensor

Discharging Current

Charging

Current

Current Calculation from Output Code:

Current

Output Code

LSB

RSensor

()

----------------------------------------------------

PGA

------------

()

MCP3421

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