OPA827AI datasheet(17/24 Pages) TI

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PHASE-LOCK LOOP

Low-PassFilter

Scaling

Amplifier

PhaseDector

InputSignal

OPA827

OPA211

Current

Source

Divider

LevelAdjustmentand

BufferAmplifier

OutputSignal

Current

Source

OffsetVoltageGenerator

(FrequencyAdjustment)

VCO

1/N

OPA827

www.ti.com ..................................................................................................................................... SBOS376E – NOVEMBER 2006 – REVISED SEPTEMBER 2008

An operational amplifier with inherently low voltage

offset helps reduce this source of error. Also, any

The OPA827 is well-suited for phase-lock loop (PLL)

noise

produced

the

operational

amplifiers

applications because of the low voltage offset, low

modulates the voltage applied to the VCO and limits

noise, and wide gain bandwidth. Figure 48 illustrates

the spectral purity of the oscillator output. The VCO

an example of the OPA827 in this application. The

generates noise-related, random phase variations of

first amplifier (OPA827) provides the loop low-pass,

its own, but this characteristic becomes worse when

active filter function, while the second amplifier

the input voltage source noise is included. This noise

(OPA211) serves as a scaling amplifier. This second

appears as random sideband energy that can limit

stage amplifies the dc error voltage to the appropriate

system performance. The very low flicker noise (1/f)

level before it is applied to the voltage-controlled

and current noise (In) of the OPA827 help to

oscillator (VCO).

minimize the operational amplifier contribution to the

phase noise.

Operational amplifiers used in PLL applications are

often required to have low voltage offset. As with

other dc levels generated in the loop, a voltage offset

applied to the VCO is interpreted as a phase error.

Figure 48. PLL Application

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