CN-0385

Circuit Note

Rev. 0 | Page 4 of 13

System Noise Analysis

One of the key design goals in precision data acquisition

systems is achieving a high SNR, which can be achieved by

increasing the full-scale signal amplitude and/or by decreasing

the noise power generated by the components in the system.

The total noise power present in the system can be found by

taking the root sum square (rss) of the noise power contributed

by its individual components, referred to the input of the

AD4003:

2

2

,

2

,

2

5

,

,

AD4003

n,

AD8475

n

AD8251

n

207

ADG

n

TOTAL

n

v

v

v

v

v

+

+

+

=

found using

















=

TOTAL

n

REF

EXPECTED

v

V

SNR

,

2

log

20

The expected noise contributions for each component in the

system and the resulting expected SNR performance of the

whole system is shown in Table 2. The total system noise

calculation ignores thermal noise contributed by the passive

components in the system.

Noise Due to the AD4003 ADC

The noise of the AD4003 ADC is a function of both its inherent

quantization error and noise caused by internal components

(such as passive components producing thermal noise).

The rms input voltage noise of the AD4003 can be calculated

from its specified SNR using













×

=

20

,

4003

10

2

AD

SNR

REF

AD4003

n

V

v

98 dB for a 4.096 V reference.

The single-pole RC filter at the input of the AD4003 limits the

wideband noise from the upstream components. A smaller filter

bandwidth improves SNR by further limiting noise power;

however, its time constant must also be sufficiently short to

settle voltage kickbacks due to charge injections that occur as

the AD4003 inputs reconnect to the front-end circuitry during

the acquisition phase. The appropriate bandwidth for the system

is at least 5 MHz (for more information, see the Analog

Dialogue article, Front-End Amplifier and RC Filter Design for a

Precision SAR Analog-to-Digital Converter).

Noise Due to the AD8475 Funnel Amplifier

RC

AD8475

AD8475

n,

BW

e

v

×

π

×

=

2

Noise Due to the AD8251 Instrumentation Amplifier

The AD8251 functions as a gain stage that improves SNR for

small amplitude signals by boosting their amplitude to more

the system gain increases by a factor of G, the SNR (in dB) of

the input signal improves by

This level of improvement is not achievable in reality, however,

because wideband noise is also amplified by the noise gain of

the circuit. Fortunately, this degradation is not as large as the

improvement due to signal gain.

The rms noise contributed by the AD8251 is a function of its

bandwidth at the input of the AD4003:

RC

AD8475

AD8251

AD

n

BW

G

G

e

v

×

π

×

×

×

=

2

8251

AD

8251

,

Noise Due to the ADG5207 Multiplexer

The NSD and resulting rms noise contributed by the ADG5207

can be found by using the Johnson/Nyquist noise equation,

because the device acts like a series resistance between the

source and the rest of the analog front end:

ON

B

207

ADG

n,

R

T

k

e

×

×

×

= 4

5

and

RC

AD8475

AD8251

207

ADG

n,

207

ADG

n,

BW

G

G

e

v

×

π

×

×

×

=

2

5

5

ADG5207 data sheet.

A summary of the calculated noise performance of the system is

shown in Table 2. The largest contributors to the total noise are

the AD8251 in-amp and the AD4003 ADC.