OPA2832 datasheet(24/32 Pages) BURR-BROWN

24 / 32 page

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DISTORTION PERFORMANCE

ENI

IBN RS

4kTRS NG

IBI RF

4kTRFNG

(1)

BNRS

4kTR

BIRF

4kTR

(2)

NOISE PERFORMANCE

DC ACCURACY AND OFFSET CONTROL

4kT

1/3

OPA3832

4kT=1.6E

20J

at290 K

4kTR

OPA3832

SBOS370 – DECEMBER 2006

The total output spot noise voltage can be computed

as the square root of the sum of all squared output

The OPA3832 provides good distortion performance

noise voltage contributors. Equation 1 shows the

into a 150

Ω load. Relative to alternative solutions, it

general form for the output noise voltage using the

provides exceptional performance into lighter loads

terms shown in Figure 55:

and/or

operating

single

+3.3V

supply.

Generally, until the fundamental signal reaches very

high frequency or power levels, the 2nd-harmonic will

dominate

the

distortion

with

negligible

Dividing

this

expression

the

noise

gain

3rd-harmonic component. Focusing then on the

(NG

RF/RG))

gives

the

equivalent

2nd-harmonic,

increasing

the

load

impedance

input-referred spot noise voltage at the noninverting

improves distortion directly. Remember that the total

input, as shown in Figure 55:

load

includes

the

feedback

network;

the

noninverting configuration (see Figure 47) this is the

sum of RF + RG, while in the inverting configuration,

only RF needs to be included in parallel with the

actual load.

Evaluating these two equations for the circuit and

component values shown in Figure 46 gives a total

output spot noise voltage of 18.8nV/

√Hz and a total

High slew rate, unity-gain stable, voltage-feedback

equivalent input spot noise voltage of 9.42nV/

√Hz.

op amps usually achieve their slew rate at the

This total includes the noise added by the resistors.

expense of a higher input noise voltage. The

This total input-referred spot noise voltage is not

9.2nV/

√Hz input voltage noise for the OPA3832,

much higher than the 9.2nV/

√Hz specification for the

however, is much lower than comparable amplifiers.

op amp voltage noise alone.

The

input-referred

voltage

noise

and

the

two

input-referred

current

noise

terms

(2.2pA/

√Hz)

combine to give low output noise under a wide

The

balanced

input

stage

wideband

variety

operating

conditions.

voltage-feedback op amp allows good output dc

Figure 55 shows the op amp noise analysis model

accuracy in a wide variety of applications. The

with all the noise terms included. In this model, all

power-supply current trim for the OPA3832 gives

noise terms are taken to be noise voltage or current

even tighter control than comparable products.

density terms in either nV/

√Hz or pA/√Hz.

Although the high-speed input stage does require

relatively high input bias current (typically 5

µA out of

each input terminal), the close matching between

them may be used to reduce the output dc error

caused by this current. This configuration matches

the dc source resistances appearing at the two

inputs. Evaluating the configuration of Figure 48

(which has matched dc input resistances), using

worst-case +25

°C input offset voltage and current

specifications, gives a worst-case output offset

voltage equal to:

• (NG = noninverting signal gain at dc)

• ±(NG × V

OS(MAX)) + RF × IOS(MAX))

• = ±(2 × 80mV) + (400Ω × 1.5µA)

• = –15.4mV to +16.6mV

Figure 55. Noise Analysis Model

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