AD8422ARMZ-R7 datasheet(24/27 Pages) AD

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AD8422

Data Sheet

Rev. B | Page 24 of 27

+VS

AD8422

–VS

VIN+

–

VIN–

–

+VS

AD8422

RPROTECT

–VS

VIN+

–

VIN–

–

+VS

AD8422

RPROTECT

–VS

VIN+

–

VIN–

–

+VS

–VS

+VS

AD8422

RPROTECT

–VS

VIN+

–

VIN–

–

SIMPLE CONTINUOUS PROTECTION

TRANSIENT PROTECTION

LOW NOISE CONTINUOUS

OPTION 2

LOW NOISE CONTINUOUS

OPTION 1

Figure 62. Input Protection Options for Input Voltages Beyond Absolute

Maximum Ratings

At the expense of some noise performance, another solution is

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

inputs of the AD8422 is internally limited to a safe value for the

amplifier. Although the AD8422 inputs must still be kept within

the Absolute Maximum Ratings, the I × R drop across the

protection resistor increases the maximum voltage that the

system can withstand to the following values:

For positive input signals,

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

For negative input signals,

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

Overvoltage performance is shown in Figure 15, Figure 16,

Figure 17, and Figure 18. With gains greater than 100 and

supply voltages less than ±2.5 V, overdrive voltages beyond

the rails may cause the output to invert as far as the REF pin

voltage.

RF 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 63.

AD8422

+VS

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–VS

10nF

1nF

2kΩ

Figure 63. RFI Suppression

The filter limits the input signal bandwidth, according to the

following relationship:

)

(

DIFF

uency

FilterFreq

uency

FilterFreq

where CD ≥ 10 CC.

CD affects the difference signal, and CC affects the common-mode

signal. Choose values of R and CC that minimize RFI. A mismatch

between R × CC at the positive input and R × CC at the negative

input degrades the CMRR of the AD8422. By using a value of CD

that is one order of magnitude larger than CC, the effect of the

mismatch is reduced, and performance is improved.

Resistors add noise; therefore, the choice of the resistor and

capacitor values depends on the desired tradeoff between noise,

input impedance at high frequencies, and RF immunity. The

resistors used for the RFI filter can be the same as those used for

input protection.

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