MCP6421 datasheet(19/34 Pages) MICROCHIP

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DS25165A-page 19

MCP6421

The easiest way to reduce surface leakage is to use a

guard ring around sensitive pins (or traces). The guard

ring is biased at the same voltage as the sensitive pin.

An example of this type of layout is shown in

Figure 4-6.

FIGURE 4-6:

Example Guard Ring Layout

for Inverting Gain.

Non-inverting Gain and Unity-Gain Buffer:

Connect the non-inverting pin (VIN+) to the

input with a wire that does not touch the

PCB surface.

Connect the guard ring to the inverting input

pin (VIN–). This biases the guard ring to the

Common mode input voltage.

Inverting

Gain

and

Transimpedance

Gain

Amplifiers (convert current to voltage, such as

photo detectors):

Connect the guard ring to the non-inverting

input pin (VIN+). This biases the guard ring

to the same reference voltage as the op

amp (e.g., VDD/2 or ground).

Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB

surface.

4.6

Electromagnetic Interference

Rejection Ratio (EMIRR)

Definitions

The electromagnetic interference (EMI) is the distur-

bance that affects an electrical circuit due to either elec-

tromagnetic induction or electromagnetic radiation

emitted from an external source.

The parameter which describes the EMI robustness of

an op amp is the Electromagnetic Interference

Rejection Ratio (EMIRR). It quantitatively describes the

effect that an RF interfering signal has on op amp

performance. Internal passive filters make EMIRR

better compared with older parts. This means that, with

good PCB layout techniques, your EMC performance

should be better.

EMIRR is defined as :

EQUATION 4-1:

4.7

Application Circuits

4.7.1

CO GAS SENSOR

A CO gas detector is a device which detects the

presence of carbon monoxide gas level. Usually this is

battery powered and transmits audible and visible

warnings.

The sensor responds to CO gas by reducing its

resistance proportionaly to the amount of CO present in

the air exposed to the internal element. On the sensor

module, this variable is part of a voltage divider formed

by the internal element and potentiometer R1. The

output of this voltage divider is fed into the non-

inverting inputs of the MCP6421 op amp. The device is

configured as a buffer with unity gain and is used to

provide a non-loaded test point for sensor sensitivity.

Because this sensor can be corrupted by parasitic elec-

tromagnetic signals, the MCP6421 op amp can be

used for conditioning this sensor.

In Figure 4-7, the variable resistor is used to calibrate

the sensor in different environments.

FIGURE 4-7:

CO Gas Sensor Circuit.

Guard Ring

VIN–VIN+

VSS

EMIRR dB

 20

V

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





log



Where:

VRF = Peak Amplitude of

RF Interfering Signal (VPK)

V

OS = Input Offset Voltage Shift (V)

VDD

VOUT

MCP6421

VREF

VDD

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