Power source ripple

In the circuit of the power supply of linear stabilized voltage of complete wave commutate, 100Hz ripple is main power source noise, put circuit to carry, 100Hz noise n asks control is in normally 10nV-100nV (RTI) inside, this depends on 3 elements: The power source when carry is put in 100Hz is restrained than (PSRR) , the ripple of manostat controls the bulk that compares the electric capacity of input filter wave that reachs manostat.

 

Graph the 1 PSRR- frequency curve that is OP77 of amplifier of ADI high pressure, can see, OP77 is when 100Hz PSRR is 76dB about, should obtain do not be more than 100nV (RTI) function, the ripple of power supply source must be less than 0.6mV. Commonly used 3 upright stabilized voltage can be offerred commonly about the ripple of 60dB controls capacity, below this kind of circumstance, the capacitance of input filter wave of manostat must enough big, with will inputting the ripple of end limitation is under 0.6V.

 

 

Power source goes Ou

The bag in the power source of power supply of typical series connection manostat contains extent to be 150uV, frequency limits is the noise of 100Hz-100KHz, switch power source is more serious, the PSRR that carry puts is in high frequency when reduce with the speed of 20dB/Decade, go through RC or LC are being added in power source foot Ou network, can filter divides major noise, if circuit form pursues 3. Go in use RC when Ou, the change that should note load current can bring about the modulation that presses to electrify of power source foot.

 

Graph 3: The RC that carry puts power supply goes Ou

 

Power source regulation

The change of any power source voltage can cause carry put an input to slant the change of buy electric current, graph 1 in the PSRR of OP77 is 126dB when DC (0.5uV/V) , the change of power source voltage is a source of potential low frequency noise. In the application that in low noise carry puts, the ripple that reduces power supply and the regulation that increase power supply are very important, inadequacy of power source regulation can cause the low frequency noise that be fed up with normally.

 

Switch power source

Switch power source is a very serious noise source, the switch power source that next graphs are a model outputs the voltage weaveform of end:

 

Graph 4. Output of switch power source carries voltage weaveform

 

Can see, noise spectrum includes switch frequency and its harmonic part already, still include the high frequency part of the damp oscillation that switch loop resonance causes, continue to a few MHz all the time from a few KHz, and the PSRR when common use is put in above of hundreds of Hz begins to drop quickly, to it is almost when hundreds of KHz 0, right now, the power source noise that appears in output end will be very serious.

 

Affect way and the way to deal with a situation:

Choose besides what notice to put PSRR or CMRR parameter to carry and strengthen carry to put power supply to go Ou (if use RC to go Ou) outside, in design of power supply of switch power source, still should note following a few respects:

● The noise in power source may adopt fiducial cause or the direct coupling input to amplifier carries the leakage of electricity of PCB. Want to notice the filter wave that fiducial to voltage source outputs, to PCB leakage of electricity, down-lead and power source can be inputted to walk along space to add ground wire to defend in signal;

● Noise may take the distributinging electric capacity between the line through PCB direct coupling inputs end to amplifier, cause interference. When PCB wiring, want to notice power supply cord and weak line do not want parallel of press close to to take a line, the line is apart from completely be more than a line wide 3 times (3W principle) , add ground wire segregation between small line of power supply cord or digital line and imitate;

● Ground connection processing is undeserved, noise affects sensitive circuit part through communal impedance. Introduce power source noise signal loop to prevent communal impedance, want when to notice as follows: Sheet of; of ground of small signal of the class before the big electricity that avoids to take noise on ground connection has flowed nods ground connection, cloth of; of ground connection of departure of circuit of power source, imitate, number board use n facing, the smallest change output of power source of switch of; of ground wire impedance to carry land of derivative power source from the ground of capacitance of wave of the last filter, avoid to be carried from the ground of the capacitance before filter wave inductance derivative.

 

Graph 5: In all sketch map of coupling of modular impedance noise

 

Switch provides the displacement electricity of drive of voltage of leakage pole switch, pass first sub distributinging capacitance, sub circuit, sub pair of earths and stray capacitance, the earth and the primary stray capacitance between form annulus way, midstream of sub imitate circuit passes in all modular electric current has shed lopsided impedance to be changed into wrong standard, disturb to magnifying circuit is caused (if pursue 6) . In all the interference that modular way introduces is the high frequency heft in switch noise commonly (several MHz above) .

 

Measure basically has as follows at 3 o’clock:

1, the channel of bypass of low impedance noise that offers to return primary land subly from switch power source, use 1000p~2200p normally bring compasses electric capacity;

2, use in all of the output that modular choke strengthens switch power source in all modular filter wave;

3, use segregation technology, the smallest in influencing return in all modular electric current.

 

Graph 6. In switch power source in all loop of modular electric current

 

Arrive through coupling of dimensional magnetic field in the signal return that has area of certain annulus road or ground wire annulus, create the effect of didymous signal. The high frequency interference that comes from switch power source or town cable network additionally may pass dimensional stray capacitance direct coupling arrives signal loop.

 

The consideration in the design includes

● Equitable distribution, adjust inductance coil or transformer to set direction, optimize wiring, reduce the loop area of crucial signal, avoid to form ground annulus road to be able to reduce interference;

● Wiring of double-faced or odd face plate, notice line and ground wire, power supply cord and ground wire must parallel of press close to walks along line; to use ground contact of much spot of 1000p capacitance radio frequency, can EMC of give attention to two or morethings and the demand that low frequency believes a confusion of voices to compare;

● Add screen to sensitive circuit, notice screen layer join arrives to be protected the referenced ground of signal;

● Notice power supply cord is not together with line bundle up on design taking a line.

 

Brief summary: The main measure that power source noise drops in carrying discharge road is designed includes

● Through going the measure such as wave of Ou, filter reduces ingredient of the ripple that power source outputs and noise

● Improve a design, increase regulation of power source voltage

● Wiring of PCB of reasonable circuit structure, exquisite, sound technology taking a line

● The choice is in sensitive noise frequency paragraph the parts of an apparatus with PSRR or taller CMRR

(origin: Yadenuo semiconductor)