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DS20006162A-page 39

MCP6D11

5.2

Noise and Distortion

5.2.1

NOISE ANALYSIS

The MCP6D11 features very low noise with the voltage

noise density at 5.0 nV/√Hz and the current noise

density at 0.6 pA/√Hz. When analyzing the total output

surrounding resistor network contributions need to be

considered. Shown in Figure 5-5 is the general noise

model of a differential I/O amplifier and its resistor

feedback components.

FIGURE 5-5:

Differential I/O Amplifier Noise Analysis Model.

The analysis starts by identifying each voltage- and

current-noise term and its corresponding multiplication

factor (noise gain) in order to derive its contribution to

the

total

output

noise

output-referred noise terms are then squared to

combine noise as powers and are subsequently

combined as a root-sum-of-squares (RSS). For the

Differential I/O amplifier the voltage- and current-noise

terms from each feedback path result in a 2 x

contribution to the total noise as shown in Equation 5-7

below. One additional term is the common-mode

output as a common-mode term, unless the two

feedback ratios are mismatched. Then a conversion

the gain multiplier when referred to the output is:

EQUATION 5-7:

If the feedback ratios,

siderably to an expression very similar to a common

voltage-feedback op amp's noise equation, as shown

in Equation 5-8. Also, the current noise terms are

+

-

+

-

eno

2eni



2

2in Req1





2

2in Req2





2

–







+

2

–





2

–





2

++

+

+



1



2

+



2

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

enRF1



2

enRF2



2

++

=

Where:

Req1

RG1 RF2



=

Req2

RG2 RF2



=