Datasheet
AD8429
Rev. 0 | Page 18 of 20
R
R
AD8429
+
V
S
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
V
OUT
–V
S
R
G
C
D
10nF
C
C
1nF
C
C
1nF
4.02k
4.02k
09730-063
Figure 53. RFI Suppression
The filter limits the input signal bandwidth, according to the
following relationship:
)2(π2
1
C
D
DIFF
CCR
uencyFilterFreq
+
=
C
CM
RC
uencyFilterFreq
π2
1
=
where C
D
≥ 10 C
C
.
C
D
affects the difference 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 AD8429. By using a value of
C
D
that is one 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 capacitor
values depends on the desired tradeoff between noise, input
impedance at high frequencies, and RFI immunity. The resistors
used for the RFI filter can be the same as those used for input
protection.
CALCULATING THE NOISE OF THE INPUT STAGE
R2
R
G
R1
SENSO
R
AD8429
09730-064
Figure 54. Source Resistance from Sensor and Protection Resistors
The total noise of the amplifier front end depends on much
more than the 1 nV/√Hz specification of this data sheet. There
are three main contributors: 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, everything is calculated as if it appeared
at the amplifier input. To calculate the noise referred to the
amplifier output (RTO), simply multiply the RTI noise by the
gain of the instrumentation amplifier.
Source Resistance Noise
Any sensor connected to the AD8429 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 54. 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).
For example, assuming that the combined sensor and protec-
tion resistance on the positive input is 4 k, and on the negative
input is 1 k, the total noise from the input resistance is
(
)
(
)
=+=×+× 16641444
22
8.9 nV/√Hz
Voltage Noise of the Instrumentation Amplifier
The voltage noise of the instrumentation amplifier is calculated
using three parameters: the device input noise, output noise,
and the R
G
resistor noise. It is calculated as follows:
Total Voltage Noise =
(
)
(
)( )
222
/ ResistorRofNoiseNoiseInputGNoiseOutput
G
++
For example, for a gain of 100, the gain resistor is 60.4 . There-
fore, the voltage noise of the in-amp is
()
(
)
2
22
0604.041100/45 ×++
= 1.5 nV/√Hz
Current Noise of the Instrumentation Amplifier
Current noise is calculated by multiplying the source resistance
by the current noise.
For example, if the R1 source resistance in Figure 54 is 4 k,
and the R2 source resistance is 1 k, the total effect from the
current noise is calculated as follows:
()()
(
)
22
5.115.14 ×+× = 6.2 nV/√Hz
Total Noise Density Calculation
To determine the total noise of the in-amp, referred to input,
combine the source resistance noise, voltage noise, and current
noise contribution by the sum of squares method.
For example, if the R1 source resistance in Figure 54 is 4 k, the
R2 source resistance is 1 k, and the gain of the in-amps is 100,
the total noise, referred to input, is
222
2.65.19.8 ++ = 11.0 nV/√Hz