Datasheet

AD8229 Data Sheet
Rev. B | Page 20 of 24
R
R
AD8229
+
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
4.02kΩ
4.02kΩ
09412-063
Figure 63. RFI Suppression
C
D
affects the difference signal, and C
C
affects the common-mode
signal. Values of R and C
C
should be chosen to 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 AD8229. By using a
value of C
D
one magnitude larger than C
C
, 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
R
G
R1
SENSOR
AD8229
09412-064
Figure 64. AD8229 with Source Resistance from Sensor and
Protection Resistors
Source Resistance Noise
Any sensor connected to the 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 in 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
22
)14()44( 
=
1664 
= 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 R
G
resistor noise. It is calculated as follows:
Total Voltage Noi se =
222
)()()/( ResistorRofNoiseNoiseInputGNoiseOutput
G

For example, for a gain of 100, the gain resistor is 60.4 Ω. Therefore,
the voltage noise of the in-amp is
222
)0604.04(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.
For example, if the R1 source resistance in Figure 64 is 4 kΩ,
and the R2 source resistance is 1 k Ω, the total effect from the
current noise is calculated as follows:
))5.11()5.14((
22

= 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 64 is 4 kΩ, the
R2 source resistance is 1 k Ω, and the gain of the in-amps is 100,
the total noise, referred to input, is
)2.65.19.8
222

= 11.0 nV/√Hz