OPA875 datasheet(12/18 Pages) TI

12 / 18 page

www.ti.com

+(R I )xG

(V ) x10

± |

OSO_envelope

OSO

S+

PSRR+

PSRR-

± |

(V ) x10

S-

- -

NOISE PERFORMANCE

14mV+75

18 Ax2

6 x10

( 6) x10

= ±22.7mV

(5)

DISTORTION PERFORMANCE

+5V

Channel

Select

OPA875

e =2

e +(i R ) +4kTR

(6)

OPA875

SBOS340 – DECEMBER 2006

Leaving both temperature and gain error parameters

are extremely low at low output power levels. The

aside, the output offset voltage envelope can be

output stage continues to hold them low even as the

described as shown in Equation 4:

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

(4)

decrease significantly. For two tones centered at

20MHz, with 4dBm/tone into a matched 50

Ω load

With:

(that is, 1VPP for each tone at the load, which

VOSO: Output offset voltage

requires 4VPP for the overall 2-tone envelope at the

RS: Input resistance seen by R0, R1, G0, G1,

output pin), the Typical Characteristics show a 82dBc

B0, or B1.

difference between the test-tone power and the

Ib: Input bias current

3rd-order intermodulation spurious levels.

G: Gain

VS+: Positive supply voltage

VS–: Negative supply voltage

The OPA875 offers an excellent balance between

voltage and current noise terms to achieve low

PSRR+: Positive supply PSRR

output noise. As long as the AC source impedance

PSRR–: Negative supply PSRR

looking out of the noninverting node is less than

Evaluating

the

front-page

schematic,

using

100

Ω, this current noise will not contribute

worst-case, +25

°C offset voltage, bias current and

significantly to the total output noise. The device

PSRR specifications and operating at

±6V, gives a

input voltage noise and the input current noise terms

worst-case output equal to Equation 5:

combine to give low output noise under a wide

variety of operating conditions. Figure 30 shows this

device 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.

The OPA875 provides good distortion performance

into a 100

Ω load on ±5V supplies. Relative to

alternative

solutions,

provides

exceptional

performance into lighter loads. Generally, until the

fundamental signal reaches very high frequency or

power

levels,

the

2nd-harmonic

dominates

the

distortion with a negligible 3rd-harmonic component.

Focusing then on the 2nd-harmonic, increasing the

load impedance improves distortion directly. Also,

providing an additional supply decoupling capacitor

(0.01

µF) between the supply pins (for bipolar

operation) improves the 2nd-order distortion slightly

Figure 30. Noise Model

(3dB to 6dB).

In most op amps, increasing the output voltage

The total output spot noise voltage can be computed

swing increases harmonic distortion directly. The

as the square root of the sum of all squared output

Typical

Characteristics

show

the

2nd-harmonic

noise voltage contributors. Equation 6 shows the

increasing at a little less than the expected 2X rate

general form for the output noise voltage using the

while the 3rd-harmonic increases at a little less than

terms shown in Figure 30.

the expected 3X rate. Where the test power doubles,

the 2nd-harmonic increases only by less than the

expected 6dB, whereas the 3rd-harmonic increases

by less than the expected 12dB. This also shows up

in the two-tone, 3rd-order intermodulation spurious

(IM3) response curves. The 3rd-order spurious levels

Submit Documentation Feedback

OPA875 Datasheet(PDF) 12 Page - Texas Instruments

OPA875 Datasheet(HTML) 12 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Datasheet Download

Link URL