AD8429

Rev. 0 | Page 18 of 20

R

R

AD8429

+IN

–IN

0.1µF

10µF

10µF

0.1µF

REF

10nF

1nF

1nF

4.02kΩ

4.02kΩ

Figure 53. RFI Suppression

The filter limits the input signal bandwidth, according to the

following relationship:

)

2

(

π

2

1

C

D

DIFF

C

C

R

uency

FilterFreq

+

=

C

CM

RC

uency

FilterFreq

π

2

1

=

input degrades the CMRR of the AD8429. By using a value of

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

R1

SENSOR

AD8429

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 kΩ).

For example, assuming that the combined sensor and protec-

tion resistance on the positive input is 4 kΩ, and on the negative

input is 1 kΩ, the total noise from the input resistance is

(

) (

)

=

+

=

×

+

×

16

64

1

4

4

4

2

2

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,

Total Voltage Noise =

(

) (

)(

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. There-

fore, the voltage noise of the in-amp is

()

(

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 54 is 4 kΩ,

and the R2 source resistance is 1 kΩ, the total effect from the

current noise is calculated as follows:

()

(

)

(

2

5

.

1

1

5

.

1

4

×

+

×

= 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 kΩ, the

R2 source resistance is 1 kΩ, and the gain of the in-amps is 100,

the total noise, referred to input, is

2

2

2

2

.

6

5

.

1

9

.

8

+

+

= 11.0 nV/√Hz