AD8229
Data Sheet
Rev. B | Page 20 of 24
R
AD8229
+VS
+IN
–IN
0.1F
10F
0.1F
REF
VOUT
–VS
RG
CD
10nF
CC
1nF
CC
1nF
4.02k
0
94
12-
06
3
Figure 63. RFI Suppression
CD affects the difference signal, and CC affects the common-mode
signal. Values of R and CC should be chosen to minimize RFI. A
mismatch between R × CC at the positive input and R × CC at the
negative input degrades the CMRR of the
AD8229. By using a
value of CD one magnitude larger than CC, the effect of the
mismatch is reduced, and performance is improved.
Resistors add noise; therefore, the resistor and capacitor values
chosen depend 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
The total noise of the amplifier front end depends on much
more than the 1 nV/√Hz headline 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 multiple the RTI noise by the
gain of the instrumentation amplifier.
R2
RG
R1
SENSOR
AD8229
09
41
2-
06
4
Protection Resistors
Source Resistance Noise
Any sensor connected to th
e AD8229 has some output resistance.
There may also be resistance placed in series with inputs for
protection from either overvoltage or radio frequency interference.
This combined resistance is labeled R1 and R2 i
n Figure 64. Any
resistor, no matter how well made, has a minimum 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).
For example, assuming that the combined sensor and protection
resistance on the positive input is 4 kΩ, and on the negative
input is 1 kΩ, the total noise from the input resistance is
2
)
1
4
(
)
4
(
=
16
64
= 8.9 nV/ Hz
Voltage Noise of the Instrumentation Amplifier
The voltage noise of the instrumentation amplifier is calculated
using three parameters: the part input noise, output noise, and
the RG resistor noise. It is calculated as follows:
Total Voltage Noise =
2
)
(
)
(
)
/
(
Resistor
R
of
Noise
Input
G
Noise
Output
G
For example, for a gain of 100, the gain resistor is 60.4 Ω. Therefore,
the voltage noise of the in-amp is
2
)
0604
.
0
4
(
1
)
100
/
45
(
= 1.5 nV/√Hz
Current Noise of the Instrumentation Amplifier
Current noise is calculated by multiplying the source resistance
by the current noise.
and the R2 source resistance is 1 k Ω, the total effect from the
current noise is calculated as follows:
)
5
.
1
(
)
5
.
1
4
((
2
= 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.
R2 source resistance is 1 k Ω, and the gain of the in-amps is 100,
the total noise, referred to input, is
)
2
.
6
5
.
1
9
.
8
2
= 11.0 nV/√Hz