MCP3421 datasheet(30/42 Pages) MICROCHIP

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MCP3421

DS22003E-page 30

6.2.3

PRESSURE MEASUREMENT

Figure 6-7 shows an example of measuring the

pressure

using

NPP301

(manufactured

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 VDD of 5V (sensor excitation

voltage). Equation 6-1 shows an example of calculat-

ing the number of output code for the full scale output

of the NPP301.

FIGURE 6-7:

Example of Pressure

Measurement.

EQUATION 6-1:

EXPECTED NUMBER OF

OUTPUT CODE FOR

NPP301 PRESSURE

SENSOR

6.2.4

WHEATSTONE BRIDGE TYPE

SENSORS WITH SIGNAL

CONDITIONING

Wheatstone bridge is one of the most common

configurations in the sensor applications. Strain

gauges and pressure sensors are the common

examples. When the sensor output signal is small and

the common mode noise level is large, it needs a signal

conditioning circuit between the sensor and the

MCP3421. Figure 6-8 and Figure 6-9 show examples

of using the MCP6V01 (high precision auto-zeroed

Op Amp) for the sensor signal conditioning. Figure 6-8

shows the interface circuit with a minimum of compo-

nents between the sensor and the MCP3421, but it is

not symmetric, and therefore, the ADC input becomes

a single ended. On the other hand, the Figure 6-9 has

a symmetric and differential output, but requires more

components.

FIGURE 6-8:

Simple Signal Conditioning

Design with Asymmetric Circuit.

FIGURE 6-9:

High Performance Signal

Conditioning Design with Symmetric Circuit.

NPP301

MCP3421

VIN+

VIN-

VDD

VSS

SCL

SDL

10 µF

0.1 µF

VDD

TO MCU

(MASTER)

VDD

Expected

log

100 mV

15.625

μV

PGA

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

⎝⎠

⎜⎟

⎛⎞

Number of Output Code =

Where:

1 LSB

15.625 µV with 18 Bit configuration.

= 12.64 bits for PGA

= 13.64 bits for PGA

= 14.64 bits for PGA

= 15.64 bits for PGA

VDD

100R

MCP6V01

VDD

0.2R

MCP3421

20 k

1µF

20 k

1µF

VDD

1/2 MCP6V02

200

1µF

VDD

10 nF

200

MCP3421

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