LM6211MF National Semiconductor, LM6211MF Datasheet - Page 13

• Manufacturer Part Number: LM6211MF
• Description: IC,Operational Amplifier,SINGLE,CMOS,TSOP,5PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM6211MF

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Typical Applications
ACTIVE LOOP FILTER FOR PLLs
A typical phase locked loop, or PLL, functions by creating a
negative feedback loop in terms of the phase of a signal. A
simple PLL consists of three main components: a phase
detector, a loop filter and a voltage controlled oscillator
(VCO). The phase detector compares the phase of the out-
put of the PLL with that of a reference signal, and feeds the
error signal into the loop filter, thus performing negative
feedback. The loop filter performs the important function of
averaging (or low-pass filtering) the error and providing the
VCO with a DC voltage, which allows the VCO to modify its
frequency such that the error is minimized. The performance
of the loop filter affects a number of specifications of the PLL,
like its frequency range, locking time and phase noise.
Since a loop filter is a very noise sensitive application, it is
usually suggested that only passive components be used in
its design. Any active devices, like discrete transistors or op
amps, would add significantly to the noise of the circuit and
would hence worsen the in-band phase noise of the PLL. But
newer and faster PLLs, like National’s LMX2430, have a
power supply voltage of less than 3V, which limits the phase-
detector output of the PLL. If a passive loop filter is used with
such circuits, then the DC voltage that can be provided to the
VCO is limited to couple of volts. This limits the range of
frequencies for which the VCO, and hence the PLL, is func-
tional. In certain applications requiring a wider operating
range of frequencies for the PLL, like set-top boxes or base
stations, this level of performance is not adequate and re-
quires active amplification, hence the need for active loop
filters.
An active loop filter typically consists of an op amp, which
provides the gain, accompanied by a three or four pole RC
filter. The non-inverting input of the op amp is biased to a
fixed value, usually the mid-supply of the PLL, while a feed-
back network provides the gain as well as one, or two, poles
for low pass filtering. Figure 5 illustrates a typical active loop
filter.
FIGURE 5. A Typical Active Loop Filter
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Certain performance characteristics are essential for an op
amp if it is to be used in a PLL loop filter. Low input referred
voltage and current noise are essential, as they directly
affect the noise of the filter and hence the phase noise of the
PLL. Low input bias current is also important, as bias current
affects the level of ‘reference spurs’, artifacts in the fre-
quency spectrum of the PLL caused by mismatch or leakage
at the output of the phase detector. A large input and output
swing is beneficial in terms of increasing the flexibility in
biasing the op amp. The op amp can then be biased such
that the output range of the PLL is mapped efficiently onto
the input range of the VCO.
With a CMOS input, ultra low input bias currents (2 pA) and
low input referred voltage noise (5.5 nV/
is an ideal op amp for using in a PLL active loop filter. The
LM6211 has a ground sensing input stage, a rail-to-rail out-
put stage, and an operating supply range of 5V - 24V, which
makes it a versatile choice for the design of a wide variety of
active loop filters.
Figure 7 shows the LM6211 used with the LMX2430 to
create an RF frequency synthesizer. The LMX2430 detects
the PLL output, compares it with its internal reference clock
and outputs the phase error in terms of current spikes. The
LM6211 is used to create a loop filter which averages the
error and provides a DC voltage to the VCO. The VCO
generates a sine wave at a frequency determined by the DC
voltage at its input. This circuit can provide output signal
frequencies as high as 2 GHz, much higher than a compara-
tive passive loop filter. Compared to a similar passive loop
filter, the LM6211 doesn’t add significantly to the phase noise
of the PLL, except at the edge of the loop bandwidth, as
shown in Figure 6. A peaking of loop gain is expected, since
the loop filter is deliberately designed to have a wide band-
width and a low phase margin so as to minimize locking time.
FIGURE 6. Effect of LM6211 on Phase Noise of PLL
), the LM6211
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