AD8422ARZ-R7 datasheet(21/24 Pages) AD

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Data Sheet

AD8422

Rev. 0 | Page 21 of 24

A ground plane layer is helpful to reduce parasitic inductances.

This minimizes voltage drops with changes in current. The area

of the current path is directly proportional to the magnitude of

parasitic inductances and, therefore, the impedance of the path

at high frequencies. Large changes in currents in an inductive

decoupling path or ground return create unwanted effects due

to the coupling of such changes into the amplifier inputs.

Because load currents flow from the supplies, connect the load at

the same physical location as the bypass capacitor grounds.

Reference Pin

The output voltage of the AD8422 is developed with respect to

the potential on the reference terminal. Ensure that REF is tied

to the appropriate local ground.

INPUT BIAS CURRENT RETURN PATH

The input bias current of the AD8422 must have a dc return

path to ground. When using a floating source without a current

return path, such as a thermocouple, create a current return

path, as shown in Figure 61.

THERMOCOUPLE

+VS

REF

–VS

AD8422

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8422

TRANSFORMER

+VS

REF

–VS

AD8422

INCORRECT

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8422

fHIGH-PASS = 2πRC

THERMOCOUPLE

+VS

REF

–VS

10MΩ

AD8422

TRANSFORMER

+VS

REF

–VS

AD8422

CORRECT

Figure 61. Creating an Input Bias Current Return Path

INPUT VOLTAGES BEYOND THE SUPPLY RAILS

Many instrumentation amplifiers specify excellent CMRR and

input impedance, but in a real system, the performance suffers

because of the external components required for input protection.

The AD8422 has very robust inputs. It typically does not need

additional input protection. Input voltages can be up to 40 V from

the opposite supply rail without damage to the part. For example,

with a +5 V positive supply and a 0 V negative supply, the part can

safely withstand voltages from −35 V to +40 V. Unlike some other

instrumentation amplifiers, the part can handle large differential

input voltages even when the part is in high gain.

MOST APPLICATIONS

+VS

AD8422

–VS

VIN+

–

VIN+

–

Figure 62. Input Overvoltage Protection with no External Components

For input voltages less than 40 V from the opposite rail, no input

protection is required.

Keep the rest of the AD8422 terminals within the supplies. All

terminals of the AD8422 are protected against ESD.

Input Voltages Beyond the Maximum Ratings

For applications where the AD8422 encounters voltages beyond

the limits in the Absolute Maximum Ratings section, external

protection is required. This external protection depends on the

duration of the overvoltage event and the noise performance

required.

For short-lived events, transient protectors such as metal oxide

varistors (MOVs) may be all that is required.

For longer events, use resistors in series with the inputs combined

with diodes. To avoid worsening bias current performance, low

leakage diodes, such as the BAV199 or FJH1100s, are recommended.

The diodes prevent the voltage at the input of the amplifier from

exceeding the maximum ratings, while the resistors limit the

current into the diodes. Because most external diodes can easily

handle 100 mA or more, resistor values do not have to be large.

Therefore, the protection resistance has minimal impact on

noise performance.

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