AD8229HDZ datasheet(19/24 Pages) AD

19 / 24 page

AD8229

Rev. 0 | Page 19 of 24

INPUT BIAS CURRENT RETURN PATH

The input bias current of the AD8229 must have a return path to

ground. When using a floating source without a current return

path, such as a thermocouple, a current return path should be

created, as shown in Figure 59.

THERMOCOUPLE

+VS

REF

–VS

AD8229

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8229

TRANSFORMER

+VS

REF

–VS

AD8229

INCORRECT

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8229

fHIGH-PASS = 2πRC

THERMOCOUPLE

+VS

REF

–VS

10M

Ω

AD8229

TRANSFORMER

+VS

REF

–VS

AD8229

CORRECT

Figure 59. Creating an Input Bias Current Return Path

INPUT PROTECTION

The inputs to the AD8229 should be kept within the ratings

stated in the Absolute Maximum Ratings section of this data

sheet. If this cannot be done, protection circuitry can be added

in front of the AD8229 to limit the current into the inputs to a

maximum current, IMAX.

Input Voltages Beyond the Rails

If voltages beyond the rails are expected, an external resistor

should be used in series with each input to limit current during

overload conditions. The limiting resistor at the input can be

computed from

MAX

SUPPLY

PROTECT

−

≥

Noise-sensitive applications may require a lower protection

resistance. Low leakage diode clamps, such as the BAV199, can be

used at the inputs to shunt current away from the AD8229 inputs

and therefore allow smaller protection resistor values. To ensure

current flows primarily through the external protection diodes,

place a small value resistor, such as a 33 Ω, between the diodes and

the AD8229.

SIMPLE METHOD

LOW NOISE METHOD

+VS

AD8229

RPROTECT

–VS

VIN+

–

VIN–

–

+VS

AD8229

RPROTECT

Ω

RPROTECT

–VS

VIN+

–

VIN–

–

+VS

–VS

Figure 60. Protection for Voltages Beyond the Rails

Large Differential Input Voltage at High Gain

If large differential voltages at high gain are expected, an

external resistor should be used in series with each input to limit

current during overload conditions. The limiting resistor at each

input can be computed from













−

≥

MAX

DIFF

PROTECT

Noise-sensitive applications may require a lower protection

resistance. Low leakage diode clamps, such as the BAV199, can be

used across the inputs to shunt current away from the AD8229

inputs and therefore allow smaller protection resistor values.

AD8229

RPROTECT

VDIFF

–

AD8229

RPROTECT

VDIFF

–

SIMPLE METHOD

LOW NOISE METHOD

Figure 61. Protection for Large Differential Voltages

IMAX

The maximum current into the AD8229 inputs, IMAX, depends

both on time and temperature. At room temperature, the part

can withstand a current of 10 mA for at least a day. This time is

cumulative over the life of the part. At 210°C, current should be

limited to 2 mA for the same period of time. The part can

withstand 5 mA at 210°C for an hour, cumulative over the life of

the part.

RADIO FREQUENCY INTERFERENCE (RFI)

RF rectification is often a problem when amplifiers are used in

applications that have strong RF signals. The disturbance can

appear as a small dc offset voltage. High frequency signals can

be filtered with a low-pass RC network placed at the input of

the instrumentation amplifier, as shown in Figure 62. The filter

limits the input signal bandwidth, according to the following

relationship:

)

(

DIFF

uency

FilterFreq

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