OPA690 datasheet(18/23 Pages) TI

18 / 23 page

OPA691

SBOS226A

www.ti.com

dynamic range does not decrease significantly. For two

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

Ω load (i.e., 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 48dBc 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

Wideband current feedback op amps generally have a higher

output noise than comparable voltage feedback op amps. The

OPA691 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 10 shows the op amp noise analysis 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.

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

will give the equivalent input-referred spot noise voltage at

the noninverting input, as shown in Equation 6.

(6)

kTR

NNI

BN SS

BI F

(

) ++ 







Evaluating these two equations for the OPA691 circuit and

component values (see Figure 1) will give a total output spot

noise voltage of 8.0nV/

√Hz and a total equivalent input spot

noise voltage of 4.0nV/

√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. If the feedback resistor is reduced in high

gain configurations (as suggested previously), the total input-

referred voltage noise given by Equation 5 will approach just

the 1.7nV/

√Hz of the op amp itself. For example, going to a

gain of +10 using RF = 180Ω will give a total input-referred

noise of 2.1nV/

√Hz.

DC ACCURACY AND OFFSET CONTROL

A current feedback op amp like the OPA691 provides excep-

tional bandwidth in high gains, giving fast pulse settling but

only moderate DC accuracy. The Typical Specifications show

an input offset voltage comparable to high-speed voltage

feedback amplifiers. However, the two input bias currents are

somewhat higher and are unmatched. Whereas bias current

cancellation techniques are very effective with most voltage

feedback op amps, they do not generally reduce the output DC

offset for wideband current feedback op amps. Since 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 ineffec-

tive. 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 • 2.5mV) + (35µA • 25Ω • 2) ± (402Ω • 25µA)

±5mV + 1.75mV ± 10.05mV

= –13.3mV

→ +16.8mV

A fine-scale, output offset null, or DC operating point adjust-

ment, is sometimes required. Numerous techniques are

available for introducing DC offset control into an op amp

circuit. Most simple adjustment techniques do not correct for

temperature drift. It is possible to combine a lower speed,

precision op amp with the OPA691 to get the DC accuracy

of the precision op amp along with the signal bandwidth of

the OPA691. See Figure 11 for a noninverting G = +10 circuit

that holds an output offset voltage less than

±7.5mV over-

temperature with > 150MHz signal bandwidth.

This DC-coupled circuit provides very high signal bandwidth

using the OPA691. At lower frequencies, the output voltage

is attenuated by the signal gain and compared to the original

FIGURE 10. Op Amp Noise Analysis Model.

4kT

OPA691

4kT = 1.6E –20J

at 290

°K

√4kTR

The total output spot noise voltage can be computed as the

square root of the sum of all squared output noise voltage

contributors. Equation 5 shows the general form for the

output noise voltage using the terms shown in Figure 10.

(5)

kTR

I R

kTR NG

BN SS

BI F

(

) +









(

) +

OPA690 Datasheet(PDF) 18 Page - Texas Instruments

OPA690 Datasheet(HTML) 18 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL