How NOT to Design Active Filters

1. 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

2. 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!

3. 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

4. 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!

5. 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-feedback (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

6. How NOT to Design Active Filters When designing active filtersit is best to treat an op amp as an integrator

7. 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)

8. 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

9. 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:-

10. 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.

11. How NOT to Design Active Filters 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

12. 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.)

13. 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 doesn’t

14. 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)

15. 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 inputcommon-mode range must include one or both supplies op amps with FET Inputsdo not have this problem

16. 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 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]

17. How NOT to Design Active Filters AMPLIFIER NOISE Every op amp contains threeuncorrelated noise sources  Voltage noise   VnCurrent noise in the non-inverting input In+Current noise in the inverting input In-

18. 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 flowsin 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

19. How NOT to Design Active Filters

20. 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)