MCP6D11 datasheet(40/60 Pages) MICROCHIP

40 / 60 page

MCP6D11

DS20006162A-page 40

 2019 Microchip Technology Inc.

EQUATION 5-8:

The noise contribution from resistors RG1, RF1, RG2

and RF2 can be calculated based on the Johnson noise

equation: enR = √4kTR, where k is Boltzmann's

constant (1.38065 x 10-23J/K), T is the resistor's

absolute temperature in Kelvin, and R is the resistor

value in ohms (

).

By using the noise gain (GN =1+RF/RG) the two

resistor noise terms can be combined into a single term

of (4kTRFGN) resulting in a much simplified equation

for the amplifier's differential output noise:

EQUATION 5-9:

The first term of Equation 5-9 is the differential input

noise times the noise gain. The second term is the

input current noise times the feedback resistor - twice,

since there are two uncorrelated current noise terms.

The last term is the output noise resulting from both the

RF and RG resistors, at again twice the value for the

output noise power of each side added together.

The input referred noise of the MCP6D11 can be

equated to that of a 1.6 k

 resistor. The recommended

value for the feedback resistor RF is 1k, which results

in the total output referred noise to be dominated by the

amplifier's voltage noise. While there is flexibility in

selecting different values for RF (and similarly for RG),

lowering the feedback resistor value in order to lower

its noise contribution will increase the amplifier's total

output load and eventually result in an increase in

distortion. Scaling the resistor value up will have the

opposite effect of potentially improving distortion at the

expense of higher noise contribution. However,

because the feedback resistor interacts with the

amplifier's input capacitance large values can lead to a

noticeable reduction in phase margin and cause

stability issues. A typical approach is to start with the

recommended feedback resistor value and set the

desired gain by scaling the gain resistor (RG)

accordingly; Table 5-2 shows some example resistor

values and corresponding noise results.

5.2.2

FACTORS AFFECTING HARMONIC

DISTORTION

In general, an amplifier's output harmonic distortion

mainly relates to the open loop linearity in the output

stage corrected by the loop gain at the fundamental

frequency. Reducing the total load impedance,

including the effect of the feedback resistor as

discussed previously, the output stage open loop

linearity degrades, causing an increase in harmonic

distortion. Secondly, harmonic distortion will degrade

as a function of the amplifier's output swing due to fine

scale open loop output stage nonlinearities. A nominal

swing of 2Vpp is typically used for harmonic distortion

testing where Figure 3-29 illustrates the effect of going

up to an 8Vpp differential swing that is more common

with SAR-type ADC converters. An increase in the

amplifiers' gain correspondingly reduces the available

loop gain to correct errors resulting in an increase in

harmonic distortion terms.

The MCP6D11 has a nearly constant distortion level

when the VOCM operating point is moved within the

allowed range; see Figure 3-30 and Figure 3-31.

Driving the VOCM voltage beyond this range or the

output voltages close to the supply rails will rapidly

degrade the distortion performance.

The device characterization used primarily resistors

with a 1% tolerance. The resulting imbalance of the

feedback factors does not directly degrade the

distortion performance of the amplifier, but rather DC

errors

(see

section

Section 5.1.3

“Mismatches and DC Errors”).

eno

eni 1

--------





 2

2inRF



24kTRG

--------





 2

24kTRF



Where: 4kT = 1.64-20J at 298K (25°C)

eno

eniGN



2inRF



24kTRFGN



TABLE 5-2:

EXAMPLE OUTPUT NOISE RESULTS FOR THE SINGLE-ENDED INPUT

CONFIGURATION WITH 50

 INPUT MATCHING PER Figure 5-4

Ideal Gain

(V/V)

Act. Gain

(V/V)

RF1, RF2

(

)

RG1

(

)

RT1

(

)

RG2

(

)

ZIN

(

)

Diff-Out Noise

eno (nV/√Hz)

Noise RTI

(nV/√Hz)

0.997

1000

52.3

1020

50.3

12.90

1.988

1020

499

52.3

523

48.9

18.02

9.06

5.057

1000

187

59.0

215

50.2

31.99

6.33

10.009

1020

88.7

68.1

118

50.6

52.60

5.26

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