OPA2832 datasheet(25/37 Pages) NSC

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DISTORTION PERFORMANCE

O +

ENI

) IBNRS

) 4kTRS NG

2 ) I

BIRF

) 4kTRFNG

(1)

N +

) IBNRS

) 4kTRS )

BIRF

)

4kTR

(2)

NOISE PERFORMANCE

DC ACCURACY AND OFFSET CONTROL

4kT

1/2

OPA2832

4kT = 1.6E

− 20J

at 290

4kTR

√

4kTR

√

OPA2832

www.ti.com ............................................................................................................................................. SBOS327C – FEBRUARY 2005 – REVISED AUGUST 2008

The OPA2832 provides good distortion performance

The total output spot noise voltage can be computed

into a 150

Ω load. Relative to alternative solutions, it

as the square root of the sum of all squared output

provides exceptional performance into lighter loads

noise voltage contributors. Equation 1 shows the

and/or operating on a single +3.3V supply. Generally,

general form for the output noise voltage using the

until the fundamental signal reaches very high

terms shown in Figure 71:

frequency or power levels, the 2nd-harmonic will

dominate the distortion with a negligible 3rd-harmonic

component. Focusing then on the 2nd-harmonic,

increasing the load impedance improves distortion

Dividing

this

expression

the

noise

gain

directly. Remember that the total load includes the

(NG

RF/RG)) will give the equivalent

feedback network; in the noninverting configuration

input-referred spot noise voltage at the noninverting

(see Figure 62) this is sum of RF + RG, while in the

input, as shown in Figure 71:

inverting configuration, only RF needs to be included

in parallel with the actual load. Running differential

suppresses the 2nd-harmonic, as shown in the

differential typical characteristic curves.

Evaluating these two equations for the circuit and

component values shown in Figure 61 will give a total

output spot noise voltage of 19.3nV/

√Hz and a total

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

equivalent input spot noise voltage of 9.65nV/

√Hz.

amps usually achieve their slew rate at the expense

This is including the noise added by the resistors.

of a higher input noise voltage. The 9.2nV/

√Hz input

This total input-referred spot noise voltage is not

voltage noise for the OPA2832, however, is much

much higher than the 9.2nV/

√Hz specification for the

lower than comparable amplifiers. The input-referred

op amp voltage noise alone.

voltage noise and the two input-referred current noise

terms (2.8pA/

√Hz) combine to give low output noise

under

wide

variety

operating

conditions.

Figure 71 shows the op amp noise analysis model

The

balanced

input

stage

wideband

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

voltage-feedback op amp allows good output DC

noise terms are taken to be noise voltage or current

accuracy in a wide variety of applications. The

density terms in either nV/

√Hz or pA/√Hz.

power-supply current trim for the OPA2832 gives

even

tighter

control

than

comparable

products.

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 is done by matching the

DC source resistances appearing at the two inputs.

Evaluating the configuration of Figure 63 (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 × 7.5mV) + (400Ω × 1.5µA)

• = –14.4mV to +15.6mV

Figure 71. Noise Analysis Model

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