AD8421ARMZ-R7 datasheet(23/29 Pages) AD

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AD8421

Data Sheet

Rev. 0 | Page 22 of 28

Reference Pin

The output voltage of the AD8421 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 AD8421 must have a 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 65.

THERMOCOUPLE

+VS

REF

–VS

AD8421

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8421

TRANSFORMER

+VS

REF

–VS

AD8421

INCORRECT

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8421

fHIGH-PASS = 2πRC

THERMOCOUPLE

+VS

REF

–VS

10MΩ

AD8421

TRANSFORMER

+VS

REF

–VS

AD8421

CORRECT

Figure 65. Creating an Input Bias Current Return Path

INPUT VOLTAGES BEYOND THE SUPPLY RAILS

The AD8421 has very robust inputs. It typically does not need

additional input protection, as shown in Figure 66.

MOST APPLICATIONS

+VS

AD8421

–VS

VIN+

–

VIN+

–

Figure 66. Typical Application; No Input Protection Required

The AD8421 inputs are current limited; therefore, input voltages

can be up to 40 V from the opposite supply rail, with no input

protection required at all gains. For example, if +VS = +5 V and

−VS = −8 V, the part can safely withstand voltages from −35 V to

+32 V.

The remaining AD8421 terminals should be kept within the

supplies. All terminals of the AD8421 are protected against ESD.

Input Voltages Beyond the Maximum Ratings

For applications where the AD8421 encounters voltages beyond the

limits in the Absolute Maximum Ratings table, external protection

is required. This external protection depends on the duration of

the overvoltage event and the noise performance that is required.

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

varistors (MOVs)), may be all that is required.

+VS

AD8421

–VS

VIN+

–

VIN–

–

+VS

AD8421

RPROTECT

–VS

VIN+

–

VIN–

–

+VS

AD8421

RPROTECT

–VS

VIN+

–

VIN–

–

+VS

–VS

+VS

AD8421

RPROTECT

–VS

VIN+

–

VIN–

–

SIMPLE CONTINUOUS PROTECTION

TRANSIENT PROTECTION

LOW NOISE CONTINUOUS

OPTION 2

LOW NOISE CONTINUOUS

OPTION 1

Figure 67. Input Protection Options for Input Voltages Beyond Absolute

Maximum Ratings

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

with diodes. To avoid degrading bias current performance, low

leakage diodes such as the BAV199 or FJH1100 are recommended.

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

exceeding the maximum ratings, and the resistors limit the current

into the diodes. Because most external diodes can easily handle

100 mA or more, resistor values do not need to be large and,

therefore, have a minimal impact on noise performance.

At the expense of some noise performance, another solution is

to use series resistors. In the case of overvoltage, current into

the AD8421 inputs is internally limited. Although the AD8421

inputs must be kept within the limits defined in the Absolute

Maximum Ratings section, the I × R drop across the protection

resistor increases the maximum voltage that the system can

withstand, as follows:

For positive input signals

VMAX_NEW = (40 V + Negative Supply) + IIN × RPROTECT

For negative input signals

VMIN_NEW = (Positive Supply − 40 V) − IOUT × RPROTECT

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