ADA4895-1ARJZ-R7 datasheet(18/24 Pages) AD

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ADA4895-1/ADA4895-2

Data Sheet

NOISE CONSIDERATIONS

Figure 48 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 48. 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 48.

RS is the source resistance, as shown in Figure 48.

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 1.7 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 49 shows the total RTI noise due to the amplifier vs. the

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

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 49. RTI Noise vs. Source Resistance

Rev. B | Page 18 of 24

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