How NOT to Design Active Filters

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How NOT to Design Active Filters
Active Filter Design Softwareis flexible,
inexpensive and easy to use But practical aspects
of hardware designfrequently degrade the
performance ofthe very best theoretical
circuits This article talks about hardware not
software
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How NOT to Design Active Filters
An Ideal Operational Amplifierhas infinite gain,
bandwidth, slew rate, CMRR PSRRand zero
offset, drift, bias current, noise, crosstalk (in
duals quads) and output impedance Real op amps
(even Analog Devices ones) are more messy!
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How NOT to Design Active Filters
A Real Operational Amplifierhas finite gain,
bandwidth, slew rate, CMRR PSRRand appreciable
offset, drift, bias current, noise, crosstalk (in
duals quads) and output impedance Active filter
designs must take account of these realities
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How NOT to Design Active Filters
The gain of a real op amp is not infiniteNeither
is the bandwidth Typical values of Aol for
general purpose op amps are 105 - 107Gain
bandwidth products of such op amps rarely exceed
10 - 20 MHzand are frequently much lower in high
precision parts If Aol is 106 and GBP is
1 MHzOpen Loop Gain at 20 KHz is only 50Which
is far from infinite!
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How NOT to Design Active Filters
High-speed op amps have wider bandwidthBut,
usually, much lower Aol Typical values of Aol for
high-speed op amps are 103 - 105but are
sometimes even lower GBP of high-speed voltage
feedback op amps may reach 350 MHz Current-feedbac
k (transimpedance) op ampsDO NOT HAVE a gain
bandwidth productTo a first approximation their
BW is not affected by the gain for a given
RfbTheir bandwidths can reach 1 GHz but they are
not suitable for active filters Active filters
made with op amps should not be used at over 20
MHz --above this frequency passive LC filters
with amplifiers providinginterstage gain and
isolation will give more reliable performance
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How NOT to Design Active Filters
When designing active filtersit is best to treat
an op amp as an integrator  
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How NOT to Design Active Filters
But some op amps have a second poleat HF which
can cause instability if ignored         (His
name is Frederic Chopin)
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How NOT to Design Active Filters
Spice macro-models sometimesomit HF poles
zeros  This is partly to permit reasonably rapid
convergenceand partly because too complete a
model enablesour competitors to deduce how the
amplifier is designed
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How NOT to Design Active Filters
Think about large-signal bandwidthas well as
small-signal bandwidth For an op amp with a
slew-rate of SR (V/Sec)the relationship between
full power bandwidth(FPBW) and pk-pk output
swing 2Vpk is-  
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How NOT to Design Active Filters
Zout of an op amp reacts with Cload to producean
additional pole which may cause instability Op
amps exist which are designed to drive capacitive
loads withoutinstability, but such amplifiers
are still slowed by load capacitance.
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How NOT to Design Active Filters
Transimpedance or current-feedbackop amps
oscillate with capacitive feedback Current-feedbac
k or transimpedance op ampsare a relatively new
architecture of HF op amp They have a
low-impedance current input at theirinverting
input and oscillate with capacitive
feedback Therefore they cannot be used in
manyclassical active filter configurations
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How NOT to Design Active Filters
Adequate supply decoupling is essential this
means that supplies must be short-circuitedat
all frequencies above DC   At low frequencies
decoupling capacitors may be shared between
several ICs,but at HF each op amp must have its
own decoupling. HF decoupling capacitors must be
low inductance types (ideally surface mount)and
must have short, wide, low inductance leads and
PC tracks.       ( DC short-circuits are
inadvisable.)
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How NOT to Design Active Filters
Most people remember the offset voltagemany
forget the bias current When Ib flows in a
resistanceit increases the effective Vos When
designing active filters there is a temptation to
use largeresistances so that one can use small
capacitors (which arecheaper and more readily
available at high accuracies) This can cause high
offsets Sometimes this matters sometimes it
doesnt
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How NOT to Design Active Filters
Bias compensation can helpbut only if Ib Ib-
are equal       Bias compensation resistor Rbc
has the same resistance as the parallel
combination of Rin and Rfb(Decoupling Rbc
ensures HF stability)
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How NOT to Design Active Filters
Modern single supply rail-to-railop amps
often have higher bias currentthan previous
generations of op amps This is because techniques
to reduce Ib do not work if the inputcommon-mode
range must include one or both supplies op amps
with FET Inputsdo not have this problem
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How NOT to Design Active Filters
NEVERTHELESSthe use of FET input op amps do
notallow the use of very high resistancesbecause
high resistance is associatedwith high Johnson
noise All resistances have Johnson noise of   T
is the temperature in Kelvin, R is the
Resistance,k is Boltzmanns Constant (1.38 x
10-23 Joules/K)It is rarely profitable to
reduce the temperature, one can reduce the
resistance,but it is not possible to change
Boltzmanns Constant as Boltzmann is dead
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How NOT to Design Active Filters
AMPLIFIER NOISE Every op amp contains
threeuncorrelated noise sources  Voltage
noise   VnCurrent noise in the non-inverting
input InCurrent noise in the inverting
input In-
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How NOT to Design Active Filters
When calculating the noise of an amplifierit is
necessary to consider the effectsof all three
amplifier noise sources andalso the Johnson
noise of all resistors used The op amp current
noise In generates voltage noisewhen it flows in
any impedance, resistive or reactive But only
resistances have Johnson noise  The diagram on
the next slide shows only resistancesbut a more
general (and complex) diagram would showreactive
and resistive impedances
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How NOT to Design Active Filters
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How NOT to Design Active Filters
TO SUMMARIZEDo not assume all Op Amp parameters
are either zero or infinitebut actually consider
the effects of finite non-zero Aol, GBP, Ib,slew
rate, crosstalk, noise (voltage Current), CMRR,
PSRR and Zout RTFDS Read The Friendly Data
Sheet(See article at http//www.analog.com/analog
_root/static/raq/raq_caveat.html)