Datasheet

Low-PassFilter
Scaling
Amplifier
PhaseDectorInputSignal
OPA827
OPA211
Current
Source
Divider
LevelAdjustmentand
BufferAmplifier
OutputSignal
Current
Source
OffsetVoltageGenerator
(FrequencyAdjustment)
VCO
1/N
OPA827
www.ti.com SBOS376H –NOVEMBER 2006–REVISED MAY 2012
PHASE-LOCK LOOP Operational amplifiers used in PLL applications are
often required to have low voltage offset. As with
The OPA827 is well-suited for phase-lock loop (PLL)
other dc levels generated in the loop, a voltage offset
applications because of the low voltage offset, low
applied to the VCO is interpreted as a phase error.
noise, and wide gain bandwidth. Figure 48 illustrates
An operational amplifier with inherently low voltage
an example of the OPA827 in this application. The
offset helps reduce this source of error. Also, any
first amplifier (OPA827) provides the loop low-pass,
noise produced by the operational amplifiers
active filter function, while the second amplifier
modulates the voltage applied to the VCO and limits
(OPA211) serves as a scaling amplifier. This second
the spectral purity of the oscillator output. The VCO
stage amplifies the dc error voltage to the appropriate
generates noise-related, random phase variations of
level before it is applied to the voltage-controlled
its own, but this characteristic becomes worse when
oscillator (VCO).
the input voltage source noise is included. This noise
appears as random sideband energy that can limit
system performance. The very low flicker noise (1/f)
and current noise (In) of the OPA827 help to
minimize the operational amplifier contribution to the
phase noise.
Figure 48. PLL Application
Copyright © 2006–2012, Texas Instruments Incorporated 17