AN929 datasheet(8/22 Pages) MICROCHIP

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AN929

DS00929A-page 8

 2004 Microchip Technology Inc.

EMI/ESD Protection

EMI and ESD filters function as both a noise filter and

a protective device to the circuit on the PCB. An IC

input pin should never be connected directly to a

remote sensor because EMI or ESD overvoltage

failures will likely occur. Ferrite beads, capacitive feed-

through filters, RC filters and transient-voltage-sup-

pressor (TVS) zener diodes are popular devices that

can be used to provide protection for the sensor circuit.

Ferrite beads, capacitive feed-through filters and RC

filters function as filters and only limit the slew rate of a

transient-input voltage. A voltage-clamping device

(such as a TVS zener diode) is required to limit the

input voltage to a safe value that will not damage the IC

amplifier. Though a TVS device is similar to a standard

zener diode, they are designed to turn on fast and

dissipate a short duration, high-peak energy voltage

transient. In contrast, a zener diode is designed to

clamp a steady-state voltage for a long duration.

In many applications, a combination of different EMI/

ESD filter devices are often used. One option is to use

a capacitive feed-through filter that is located inside the

connector, in addition to TVS and RC filters which are

placed on the PCB board, as shown in Figure 13.

Feed-through capacitors are typically mounted on a

conductive chassis, with the mechanical mounting

forming the ground connection. The noise signal is

filtered at the connector before the signal reaches the

PCB. The effectiveness of the filter is usually very good

because the inductance associated with the ground

connection is minimized. The TVS diodes on the PCB

ensure that the transient voltage is limited to a safe

value, while the RC filters provide additional filtering to

the instrumentation amplifier.

FIGURE 13:

Remote Sensors May

Require Multiple EMI Filter Devices.

Figure 14 provides the design equations for a RC filter

which can be used with differential and instrumentation

amplifiers. The RC combinations of R1/C1 and R2/C2

are used to form common mode filters and reduce the

noise which is common to both input lines. The com-

mon-mode resistors and capacitors should be matched

as close as possible and the resistors should have a

tolerance of 1% or better, while the capacitors should

be at least 5%. Capacitor C3 forms a differential mode

filter that attenuates the signal with respect to the differ-

ence in the voltage potentials of the two inputs. C3 also

compensates for any mismatch of R1/C1 and R2/C2,

which is important because the difference in the R/C

combinations degrades an amplifier’s CMRR.

FIGURE 14:

Providing EMI/ESD

Overvoltage Protection with Resistors and

Capacitors.

Fault Detection Capability

It is often necessary to identify a failed sensor,

especially in remote-sensing applications. Differential

amplifiers can be used to implement a Built-In-Test

(BIT) circuit that can determine an open or short failure

at the sensor. Figure 15 shows circuits that can be

used to detect a failed RTD and thermocouple. A logic

inverter gate can be used to monitor the voltage divider

network of a RTD. Another approach to provide BIT to

a sensor is to add pull-up or pull-down resistors, or

both, as shown in Figure 15.

The typical failure mode of RTDs and thermocouples is

an open-circuit failure. Wire wound RTDs are con-

structed from a relatively small gauge wire and are

prone to vibration failures. Thermocouple wires can

also fail due to vibration because the wires get brittle

over time when exposed to high temperatures. Also,

the voltage at the amplifier inputs resulting from noise

can

equal

magnitude

functional

thermocouple.

Instrumentation

Amplifier

Ferrite Bead

Equivalent Circuit

Feed-through

Capacitor

TVS Diode

Sensor

Connector

PCB

R1 = R2

C1 = C2

C3 >> C1 and C2

Common Mode Filters

f-3dB = 1/(2 π R1C1)

f-3dB = 1/(2 π R2C2)

Differential Mode Filter

Instrumentation

Amplifier

3db

–

π R

()

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