OPA690 datasheet(15/27 Pages) TI

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OPA3692

SBOS228E

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harmonic increases by less than the expected 12dB. This also

shows up in the 2-tone, 3rd-order intermodulation spurious (IM3)

response curves. The 3rd-order spurious levels are extremely

low at low output power levels. The output stage continues to

hold them low even as the fundamental power reaches very high

levels. As the Typical Characteristics show, the spurious

intermodulation powers do not increase as predicted by a

traditional intercept model. As the fundamental power level

increases, the dynamic range does not decrease significantly.

For two tones centered at 20MHz, with 10dBm/tone into a

matched 50

Ω load (that is, 2Vp-p for each tone at the load, which

requires 8Vp-p for the overall 2-tone envelope at the output pin),

the Typical Characteristics show a 58dBc difference between the

test-tone power and the 3rd-order intermodulation spurious

levels. This exceptional performance improves further when

operating at lower frequencies.

NOISE PERFORMANCE

The OPA3692 offers an excellent balance between voltage

and current noise terms to achieve low output noise. The

inverting current noise (15pA/

√Hz) is significantly lower than

earlier solutions while the input voltage noise (1.7nV/

√Hz) is

lower than most unity-gain stable, wideband, voltage-feedback

op amps. This low input voltage noise was achieved at the

price of higher noninverting input current noise (12pA/

√Hz). 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 and the two input current noise terms

combine to give low output noise under a wide variety of

operating conditions. Figure 7 shows the op amp noise analy-

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

noise terms are taken to be noise voltage or current density

terms in either nV/

√Hz or pA/√Hz.

The total output spot noise voltage can be computed as the

square root of the sum of all squared output noise voltage

contributors. Equation 1 shows the general form for the output

noise voltage using the terms shown in Figure 7.

(1)

kTR

I R

kTR NG

BN SS

BI F

(

) +









(

) +

Dividing this expression by the noise gain (NG = (1+RF/RG))

gives the equivalent input-referred spot noise voltage at the

noninverting input as shown in Equation 2.

(2)

kTR

NNI

BN SS

BI F

(

) ++ 







Evaluating these two equations for the OPA3692 circuit and

component values shown in Figure 1 gives a total output spot

noise voltage of 8nV/

√Hz and a total equivalent input spot

noise voltage of 4nV/

√Hz. This total input-referred spot noise

voltage is higher than the 1.7nV/

√Hz specification for the op

amp voltage noise alone. This reflects the noise added to the

output by the inverting current noise times the feedback

resistor. This inverting node current noise is modeled as

internal to the OPA3692 with RF set internally as well.

DC ACCURACY

The OPA3692 provides exceptional bandwidth in high gains,

giving fast pulse settling but only moderate DC accuracy. The

Electrical Characteristics show an input offset voltage com-

parable to high-speed voltage-feedback amplifiers. However,

the two input bias currents are somewhat higher and are

unmatched. Bias current cancellation techniques do not

reduce the output DC offset for OPA3692. As the two input

bias currents are unrelated in both magnitude and polarity,

matching the source impedance looking out of each input to

reduce their error contribution to the output is ineffective.

Evaluating the configuration of Figure 1, using worst-case

+25

°C input offset voltage and the two input bias currents,

gives a worst-case output offset range equal to:

±(NG • V

OS(MAX)) + (IBN • RS/2 • NG) ± (IBI • RF)

where NG = noninverting signal gain

±(2 • 3mV) + (35µA • 25Ω • 2) ± (402Ω • 25µA)

±6mV + 1.75mV ± 10.05mV

= –14.3mV

→ +17.8mV

Minimizing the resistance seen by the noninverting input will

give the best DC offset performance.

4kT

OPA3692

4kT = 1.6E –20J

at 290

°K

√4kTR

FIGURE 7. Noise Model.

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