AD8428 Data Sheet
Rev. A | 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 as 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.
R
R
AD8428
+
S
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
OUT
–V
S
C
D
10nF
C
C
1nF
C
C
1nF
33Ω
33Ω
09731-049
L*
L*
*CHIP FERRITE BEAD.
Figure 44. RFI Suppression
The filter limits both the differential and common-mode band-
width, as shown in the following equations:
)2(π2
1
C
D
DIFF
CCR
uencyFilterFreq
+
=
C
CM
RC
uencyFilterFreq
π2
1
=
where C
D
≥ 10 C
C
.
C
D
affects the differential signal, and C
C
affects the common-
mode signal. Choose values of R and C
C
that minimize RFI. A
mismatch between R × C
C
at the positive input and R × C
C
at
the negative input degrades the CMRR of the AD8428. By using
a value of C
D
one order of magnitude larger than C
C
, 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 to the
amplifier as possible. 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:
• Source resistance
• Voltage noise of the instrumentation amplifier
• 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), 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 k).
R2
R1
SENSO
AD8428
09731-050
Figure 45. Source Resistance from Sensor and Protection Resistors
For example, assuming that the combined sensor and protec-
tion resistance is 4 k on the positive input and 1 k on the
negative input, the total noise from the input resistance is
)
)
HznV/9.816641444
22
=+=×+×
