Editor’s Notice: This can be a four-part sequence of DIs proposing enhancements within the efficiency of a “conventional” PWM—one whose output is an obligation cycle-variable rectangular pulse which requires filtering by a low-pass analog filter to supply a DAC. The primary half suggests mitigations and eliminations of widespread PWM error varieties. The second discloses circuits pushed from numerous Vprovide voltages to energy rail-rail op amps and allow their output swings to incorporate floor and Vprovide. The third pursues the optimization of post-PWM analog filters. This fourth half pursues the optimization of post-PWM analog filters.
Half 1 might be discovered right here.
Half 2 might be discovered right here.
Half 3 might be discovered right here.
Not too long ago, there was a spate of design concepts (DIs) revealed (see Associated Content material) which offers with microprocessor-generated pulse width modulators driving low-pass filters to supply DACs. Approaches have been launched which handle ripple attenuation, settling time minimization, and limitations in accuracy. That is the fourth in a sequence of DIs proposing enhancements in general PWM-based DAC efficiency. Every of the sequence’ suggestions is implementable independently of the others. This DI addresses PWM sequence modifications which ease low move analog filtering necessities.
Wow the engineering world along with your distinctive design: Design Concepts Submission Information
The tyranny of decision vs response time
The mixture of PWM clock frequency Fclk Hz and the variety of bits b of PWM decision dictates the bottom frequency (Fclk·2-b Hz) output part of an ordinary PWM. Over all of the doable obligation cycles, this part can also be the most important and due to this fact essentially the most difficult for an analog filter to suppress. For a given Fclk, the extra bits of decision, the longer the settling time will likely be of a filter which gives satisfactory suppression. However there’s a means round this limitation.
Suppose an ordinary 8-bit PWM whose output is both 0 or 1 is configured for an obligation cycle of (arbitrarily) 121/256. The primary 121 states in a 256-state cycle could be 1 and the remaining 135 could be 0’s. However what if the primary 128 states began with 60 ones and the final 128 states began with 61 ones? Let’s name this the “split-in-two” PWM. These two sequences have been offset in amplitude barely in order that they are often clearly seen on a graph proven in Determine 1.
Determine 1 Output sequences of ordinary and split-in-two 8-bit PWMs with the identical clock frequency, interval, and obligation cycle (121/256).
The blue waveform represents the usual PWM and the orange one is the split-in-two PWM. Why would possibly the latter be advantageous? Think about the spectra of the 2 PWMs seen in Determine 2.
Determine 2 Frequency content material of ordinary and split-in-two 8-bit PWMs with the identical clock frequency, interval, and obligation cycle (121/256).
The vitality within the first harmonic of the split-in-two PWM is negligible compared with that of the usual PWM. The mandatory attenuation for the primary harmonic has been considerably lessened, and that which was required is now utilized to the harmonic at double the frequency. A much less aggressive attenuation-with-frequency analog filter can now be employed, leading to a shorter settling time in response to a change in obligation cycle.
One other means to take a look at that is to double the split-in-two PWM interval to 512 states to supply a 9-bit PWM. As proven in Determine 3, the spectra of the 2 PWMs are nearly equivalent as a result of the time area waveforms are nearly equivalent—they differ solely in that each different 256-bit sequence, one further one-state replaces a zero-state. The upper decision 9-bit PWM produces a small quantity of vitality (lower than 1%) at half the frequency of the 8-bit’s elementary. Any analog low move filter with satisfactory suppression of the 8-bit elementary frequency will greater than sufficiently attenuate the sign at half that frequency.
Determine 3 Frequency content material of an ordinary 8-bit PWM of obligation cycle 121/256 and a split-in-two 9-bit PWM of obligation cycle (121.5/256). They share the identical clock, however the split-in-two’s interval is twice the usual PWM’s.
The super-cycle
We will consider the split-in-two as producing a “super-cycle” consisting of two cycles of twob states, every having no less than S one-states, with 0 ≤ S < 2b. In a single cycle, one zero-state could possibly be swapped for a one-state if the whole variety of ones within the super-cycle is odd. This can be a (b+1)-bit PWM with a interval of twob+1 states. However there isn’t any motive to cease at two. There could be a super-cycle of twon cycles the place n is any integer. With every cycle able to optionally swapping one zero-state for a one-state, this results in a PWM super-cycle with a decision of twob+n bits. However not like commonplace, non-super-cycle PWMs whose most spectral vitality part is at fclk/2b+n Hz, the super-cycle’s is at a a lot increased fclk/2b Hz. As with the particular case of the split-in-two, this eases analog filtering necessities and ends in a shorter settling time.
It’s price pondering of a super-cycle as consisting of the sum of two completely different sequences. One is the S-sequence through which each cycle consists of an equivalent sequence of S contiguous one-states. The opposite is the X-sequence the place every cycle optionally swaps the primary zero-state following the final one-state with one other one-state. The X-sequence has X one-states the place 0 ≤ X < 2n. The obligation cycle of the super-cycle is then (2n·S + X)/2b+n.
When n = 1 for a super-cycle, there is just one cycle the place an additional one-state can reside. However when n > 1, X can also be higher than one and the query turns into the best way to distribute the X ones among the many 2n cycles in order to attenuate the super-cycle’s vitality at low frequencies. The nice of us at Microchip who manufacture the SAM D21 microcontroller not solely have figured this out for us, however they’ve additionally carried out it in {hardware} [1]! For this IC, it’s needed solely to write down the values of X and S to separate registers to implement a super-cycle PWM; the {hardware} does the remainder unsupervised. Thankfully, it’s easy for nearly any microprocessor to enhance an ordinary PWM to implement a super-cycle. For every PWM cycle, the obligation cycle rely have to be modified in order that instantly after the sequence of S ones, the primary zero will get modified to a one if and provided that the next C expression is true for that cycle:
MASK & (cycleNbr * X) > MASK – X
Right here, MASK = 2n– 1, X is as earlier than, and cycleNbr is the numeric place of the cycle within the super-cycle. Determine 4 is a graph of the magnitudes of the bottom 32 harmonics of an n = 4, b = 8 super-cycle PWM. The graph gives proof of the good thing about this strategy.
Determine 4 First 32 harmonics of an n=4, b=8 super-cycle PWM. Spectra are displayed for X=1 by way of 8. (Spectra of X=9 by way of 15 are the identical as these proven.)
The X-sequence’s vitality is comparatively low, having solely 0 by way of 2n-1 one-states, nevertheless it additionally presents the bottom frequency part, fclk/2n+b Hz. The S-sequence typically comprises essentially the most vitality by far (aside from cases of very small obligation cycles), however its smallest frequency part is noticeably increased at Fclk/2b Hz. Among the many X sequences, X = 1 offers the most important amplitude for its first harmonic: 2-11 at fclk/2n+b Hz. The S sequence’s spectrum begins on the X sequence’s harmonic quantity 24 = 16 and produces its largest amplitude of two/π for that harmonic when S = 211. If this have been an ordinary PWM (an n = 0 super-cycle—no super-cycle in any respect that’s, only a regular PWM), then that amplitude of two/π would seem at frequency which is 16 instances decrease. The usual PWM presents a way more extreme filtering downside. Its filter would take loads longer to settle in response to an obligation cycle change due to the a lot bigger quantity of low frequency attenuation required.
Evaluating the filters for (n+b)-bit commonplace and super-cycle PWMs
The filtered AC regular state time-domain contributions of each the usual and the super-cycle (with its X and S sequences) PWMs needs to be lower than some fraction α of the voltage of the PWMs’ one-state. An affordable worth of α is 2-(n+b+1), ½ LSB. This interprets to an attenuation issue of 1/4 on the first harmonic of the X sequence. It’s lucky that even a easy two-component R-C filter assembly this requirement will sufficiently attenuate all increased X sequence harmonics, so there aren’t any further constraints to fulfill to suppress them. The 16th X harmonic frequency is that of the primary S harmonic. Its PWM vitality requires an attenuation issue of (π/2)·2-(n+b+1) at a 50% obligation cycle. Once more, any low move filter assembly this requirement will adequately attenuate the remaining S-sequence harmonics. For an Fclk = 20 MHz, Determine 5 and Determine 6are graphs of the frequency and time area step responses of threerd order filters (two op-amps, 3 resistors, and three capacitors) assembly these necessities for traditional 12-bit and super-cycle n = 4, b = 8 (12-bit) PWMs.
Determine 5 The frequency responses of filters for traditional and super-cycle n = 4 bit PWMs with 12 bits of decision. The maxima of the peaked waveforms are the utmost responses allowed for the filters on the peaked frequencies. The filters be certain that the regular state time area vitality at their outputs is lower than ½ LSB of Full Scale.
Determine 6 The log of absolutely the worth of time responses of filters for traditional and super-cycle n = 4 bit PWMs with 12 bits of decision. The a lot shorter settling time of the super-cycle PWM is clearly evident.
Easing low move analog filter necessities
When partnered with an acceptable analog filter, an strategy to PWM embodiment obtainable in {hardware} in an present microprocessor [1] presents considerably shorter settling instances than does an ordinary PWM. This strategy might be carried out with assistance from a small quantity of software program in nearly any microcontroller.
Christopher Paul has labored in numerous engineering positions within the communications business for over 40 years.
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