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AD8428

Data Sheet

Rev. 0 | Page 16 of 20

RADIO FREQUENCY INTERFERENCE (RFI)

Because of its high gain and low noise properties, the AD8428 is

a highly sensitive amplifier. Therefore, RF rectification can be a

problem if the AD8428 is used in applications that have strong

RF signal sources present. 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 44.

AD8428

+VS

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–VS

10nF

1nF

33Ω

*CHIP FERRITE BEAD.

Figure 44. RFI Suppression

The filter limits both the differential and common-mode band-

width, 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. 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 AD8428. By using

a value of CD 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 resistor and capac-

itor values depends on the desired trade-off between noise, input

impedance at high frequencies, and RFI immunity. To achieve

low noise and sufficient RFI filtering, the use of inductive ferrite

beads is recommended (see Figure 44). Using inductive ferrite

beads allows the value of the resistors to be reduced, which helps

to minimize the noise at the input.

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, shielding is recommended.

Note that the resistors used for the RFI filter can be the same

as those used for input protection (see the Input Protection

section).

CALCULATING THE NOISE OF THE INPUT STAGE

The total noise of the amplifier front end depends on much

more than the specifications in this data sheet. The three main

contributors to noise are as follows: 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);

that is, all sources of noise are calculated as if the source appeared

at the amplifier input. To calculate the noise referred to the ampli-

fier output (RTO), simply multiply the RTI noise by the gain of

the instrumentation amplifier.

Source Resistance Noise

Any sensor connected to the AD8428 has some output resistance.

There may also be resistance placed in series with the inputs for

protection from either overvoltage or radio frequency interference.

This combined resistance is labeled R1 and R2 in Figure 45. 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

AD8428

Figure 45. Source Resistance from Sensor and Protection Resistors

For example, assuming that the combined sensor and protec-

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

negative input, the total noise from the input resistance is

(

) (

)

nV/

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