OPA4872MDREP datasheet(17/24 Pages) TI

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NOISE PERFORMANCE

E =

+(I

R ) +4kTR

S NG

(I R )

+4kTR NG

Ö(

)

E =

+(I

R ) +4kTR

S +

I R

4kTR

)

(

(10)

4kT

OPA4872

4kT=1.6

- 20

at290K

Ö4kTR

OPA4872-EP

www.ti.com........................................................................................................................................................................................... SBOS444 – DECEMBER 2008

The OPA4872 offers an excellent balance between

(9)

voltage and current noise terms to achieve low output

noise. The inverting current noise (19 pA/

√Hz) is

Dividing this expression by the noise gain (NG = (1 +

significantly lower than earlier solutions, while the

RF/RG)) gives the equivalent input-referred spot noise

input voltage noise (4.5 nV/

√Hz) is lower than most

voltage at the noninverting input, as shown in

unity-gain stable, wideband, voltage-feedback op

Equation 10.

amps. As long as the ac source impedance looking

out of the noninverting node is less than 100

Ω, this

current noise will not contribute significantly to the

total output noise. The op amp input voltage noise

Evaluating these two equations for the OPA4872

and the two input current noise terms combine to give

circuit and component values (see Figure 27) gives a

low output noise under a wide variety of operating

total output spot noise voltage of 14.2 nV/

√Hz and a

conditions. Figure 34 shows the OPA4872 noise

total equivalent input spot noise voltage of 7.1

analysis model with all the noise terms included. In

nV/

√Hz. This total input-referred spot noise voltage is

this model, all noise terms are taken to be noise

higher than the 4.5-nV/

√Hz specification for the

voltage or current density terms in either nV/

√Hz or

OPA4872 voltage noise alone. This voltage reflects

pA/

√Hz.

the noise added to the output by the inverting current

noise times the feedback resistor. If the feedback

resistor is reduced in high-gain configurations, the

total

input-referred

voltage

noise

given

Equation 10 approaches only the 4.5 nV/

√Hz of the

op amp itself. For example, going to a gain of +10

using RF = 178 Ω gives a total input-referred noise of

4.7 nV/

√Hz.

Figure 34. Op Amp Noise Analysis Model

The total output spot noise voltage can be computed

as the square root of the sum of all squared output

noise voltage contributors. Equation 9 shows the

general form for the output noise voltage using the

terms shown in Figure 35.

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