ADA4895-2ARMZ-R7 datasheet(19/24 Pages) AD

19 / 24 page

Data Sheet

ADA4895-2

Rev. 0 | Page 19 of 24

NOISE CONSIDERATIONS

Figure 41 illustrates the primary noise contributors for the

typical gain configurations. The total rms output noise is

the root mean square of all the contributions.

iep

ien

+ vout_en –

ven

4kT × RS

vn _ RS =

4kT × RG

vn _ RG =

4kT × RF

vn _ RF =

Figure 41. Noise Sources in Typical Gain Configurations

The output noise spectral density can be calculated as follows:

vout_en =

(6)

[

]

ien

kTR

ven

iep

kTR

























where:

k is Boltzmann’s constant.

T is the absolute temperature (degrees Kelvin).

RF and RG are the feedback network resistances, as shown in

Figure 41.

RS is the source resistance, as shown in Figure 41.

iep and ien represent the amplifier input current noise spectral

density (pA/√Hz).

ven is the amplifier input voltage noise spectral density (nV/√Hz).

Source resistance noise, amplifier voltage noise (ven), and the

voltage noise from the amplifier current noise (iep × RS) are all

subject to the noise gain term (1 + RF/RG). Note that with a

1 nV/√Hz input voltage noise and a 2.7 pA/√Hz input current

noise, the noise contributions of the amplifier are relatively

small for source resistances from approximately 50 Ω to 700 Ω.

Figure 42 shows the total RTI noise due to the amplifier vs. the

source resistance. In addition, the value of the feedback resistors

used affects the noise. It is recommended that the value of the

feedback resistors be maintained between 250 Ω and 1 kΩ to

keep the total noise low.

500

SOURCE RESIST

ANCE (Ω)

0.5

500

50k

AMPLIFIER NOISE

AMPLIFIER AND

RESISTOR NOISE

SOURCE

RESISTANCE NOISE

Figure 42. RTI Noise vs. Source Resistance

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