how not to design active filters active filter design software is flexible, inexpensive and easy to...

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How NOT to Design Active Filters Active Filter Design Software is flexible, inexpensive and easy to use But practical aspects of hardware design frequently degrade the performance of the very best theoretical circuits This article talks about hardware – not software

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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 of

the very best theoretical circuits

This article talks about hardware – not software

How NOT to Design Active Filters

An Ideal Operational Amplifierhas infinite gain, bandwidth, slew rate, CMRR & PSRR

and zero offset, drift, bias current, noise, crosstalk (in duals & quads) and output impedance

Real op amps (even Analog Devices ones) are more messy!

How NOT to Design Active Filters

A Real Operational Amplifierhas finite gain, bandwidth, slew rate, CMRR & PSRR

and appreciable offset, drift, bias current, noise, crosstalk (in duals & quads) and output impedance

Active filter designs must take account of these realities

The gain of a real op amp is not infiniteNeither is the bandwidth

Typical values of Aol for general purpose op amps are 105 - 107

Gain 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 MHz

Open Loop Gain at 20 KHz is only 50Which is far from infinite!

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 - 105

but are sometimes even lower

GBP of high-speed voltage feedback op amps may reach 350 MHz

Current-feedback (transimpedance) op ampsDO NOT HAVE a gain bandwidth product

To a first approximation their BW is not affected by the gain for a given R fb

Their 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

How NOT to Design Active Filters

When designing active filtersit is best to treat an op amp as an integrator

 

How NOT to Design Active Filters

But some op amps have a second pole*

at HF which can cause instability if ignored

 

 

 

 

(*His name is Frederic Chopin)

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 enables

our competitors to deduce how the amplifier is designed

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:-

 

How NOT to Design Active Filters

pkV2SRFPBW

How NOT to Design Active Filters

Zout of an op amp reacts with Cload to produce

an additional pole which may cause instabilityOp amps exist which are designed to drive capacitive loads withoutinstability, but such amplifiers are still slowed by load capacitance.

Transimpedance or current-feedbackop amps oscillate with capacitive feedback

Current-feedback 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

How NOT to Design Active Filters

Adequate supply decoupling is essential –this means that supplies must be short-circuited

at 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.)

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 are

cheaper and more readily available at high accuracies)

This can cause high offsets

Sometimes this matters – sometimes it doesn’t

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)

How NOT to Design Active Filters

Modern “single supply” & “rail-to-rail”op amps often have higher bias currentthan previous generations of op amps

This is because techniques to reduce Ib do not work if the input

common-mode range must include one or both supplies

op amps with FET Inputsdo not have this problem

How NOT to Design Active Filters

NEVERTHELESSthe use of FET input op amps do not

allow the use of very high resistancesbecause high resistance is associated

with high Johnson noise

All resistances have Johnson noise of

 T is the temperature in Kelvin, R is the Resistance,

k is Boltzmann’s 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 Boltzmann’s Constant as Boltzmann is dead]

How NOT to Design Active Filters

kTR4 HzV

AMPLIFIER NOISEEvery op amp contains threeuncorrelated noise sources

 Voltage noise   Vn

Current noise in the non-inverting input In+

Current noise in the inverting input In-

How NOT to Design Active Filters

When calculating the noise of an amplifierit is necessary to consider the effects

of all three amplifier noise sources andalso the Johnson noise of all resistors used

The op amp current noise In generates voltage noise

when 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 show

reactive and resistive impedances

How NOT to Design Active Filters

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)

How NOT to Design Active Filters