AD8421ARMZ-R7 datasheet(24/29 Pages) AD

24 / 29 page

Data Sheet

AD8421

Rev. 0 | Page 23 of 28

Overvoltage performance is shown in Figure 14, Figure 15,

Figure 16, and Figure 17. The AD8421 inputs can withstand

a current of 40 mA at room temperature for at least a day. This

time is cumulative over the life of the device. If long periods of

overvoltage are expected, the use of an external protection method

is recommended. Under extreme input conditions, the output

of the amplifier may invert.

RADIO FREQUENCY INTERFERENCE (RFI)

RF rectification is often a problem when amplifiers are used in

applications that have strong RF signals. The problem is intensified

if long leads or PCB traces are required to connect the amplifier

to the signal source. The disturbance can appear as a dc offset

voltage or a train of pulses.

High frequency signals can be filtered with a low-pass filter

network at the input of the instrumentation amplifier, as shown

in Figure 68.

AD8421

+VS

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–VS

10nF

1nF

33Ω

*CHIP FERRITE BEAD.

Figure 68. RFI Suppression

The choice of resistor and capacitor values depends on the

desired trade-off between noise, input impedance at high

frequencies, CMRR, signal bandwidth, and RFI immunity. An

RC network limits both the differential and common-mode

bandwidth, as shown in the following equations:

)

(

DIFF

uency

FilterFreq

uency

FilterFreq

where CD ≥ 10 CC.

CD affects the differential signal, and CC affects the common-

mode signal. A mismatch between R × CC at the positive input

and R × CC at the negative input degrades the CMRR of the

AD8421. By using a value of CD that is one order of magnitude

larger than CC, the effect of the mismatch is reduced and CMRR

performance is improved near the cutoff frequencies.

To achieve low noise and sufficient RFI filtering, the use of chip

ferrite beads is recommended. Ferrite beads increase their impe-

dance with frequency, thus leaving the signal of interest unaffected

while preventing RF interference to reach the amplifier. They also

help to eliminate the need for large resistor values in the filter,

thus minimizing the system’s input-referred noise. The selection

of the appropriate ferrite bead and capacitor values is a function

of the interference frequency, input lead length, and RF power.

For best results, place the RFI filter network as close as possible

to the amplifier. Layout is critical to ensure that RF signals are

not picked up on the traces after the filter. If RF interference is

too strong to be filtered sufficiently, shielding is recommended.

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

for input protection.

CALCULATING THE NOISE OF THE INPUT STAGE

The total noise of the amplifier front end depends on much more

than the 3.2 nV/√Hz specification of this data sheet. The three

main contributors to noise are: the source resistance, the voltage

noise of the instrumentation amplifier, and the current noise of

the instrumentation amplifier.

In the following calculations, noise is referred to the input (RTI).

In other words, all sources of noise are calculated as if the source

appeared at the amplifier input. To calculate the noise referred

to the amplifier output (RTO), multiply the RTI noise by the

gain of the instru-mentation amplifier.

Source Resistance Noise

Any sensor connected to the AD8421 has some output resistance.

There may also be resistance placed in series with inputs for pro-

tection from either overvoltage or radio frequency interference.

This combined resistance is labeled R1 and R2 in Figure 69. Any

resistor, no matter how well made, has an intrinsic level of noise.

This noise is proportional to the square root of the resistor value.

At room temperature, the value is approximately equal to

4 nV/√Hz × √(resistor value in kΩ).

SENSOR

AD8421

Figure 69. Source Resistance from Sensor and Protection Resistors

For example, assume that the combined sensor and protection

resistance is 4 kΩ on the positive input and 1 kΩ on the negative

input. Then the total noise from the input resistance is

(

) (

)

8.9 nV/√Hz

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