AD8229

Data Sheet

Rev. B | Page 20 of 24

R

R

AD8229

+IN

–IN

0.1µF

10µF

10µF

0.1µF

REF

10nF

1nF

1nF

4.02kΩ

4.02kΩ

Figure 63. RFI Suppression

negative input degrades the CMRR of the AD8229. By using a

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

R1

SENSOR

AD8229

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

2

2

)

1

4

(

)

4

4

(







=

16

64



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

Total Voltage Noise =

2

2

2

)

(

)

(

)

/

(

Resistor

R

of

Noise

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

2

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.

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

.

1

1

(

)

5

.

1

4

((

2

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.

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

.

6

5

.

1

9

.

8

2

2

2





= 11.0 nV/√Hz