PWM Output and Current

Accuracy

Offset

The PWM output is quantized to 1024 levels. Therefore, the

duty cycle can change only in increments of 1/1024.

There is a one-half (0.5) quantization cycle delay in the out-

put of the PWM circuitry. That is to say that instead of a duty

The quantization error can be corrected for if a more precise

result is desired. To correct for this error, simply subtract

1/2048 from the measured duty cycle.

bit if it is not corrected for. This is approximately 1.0 mA for

1.0 Amp parts, and 10 mA for 2.0 Amp parts.

Jitter

In addition to quantization, the duty cycle will contain some

jitter. The jitter is quite small (for example, the standard de-

viation of jitter is only 0.1% for the LM3822-1.0). Statistically

the jitter can cause an error in a current sample. Because the

jitter is a random variable, the mean and standard deviation

are used. The mean, or average value, of the jitter is zero.

The standard deviation (0.1%) can be used to define the

peak error caused from jitter.

The “crest factor” has often been used to define the maxi-

mum error caused by jitter. The crest factor defines a limit

within which 99.7% of the samples fall. The crest factor is de-

Since the jitter is a random variable, averaging multiple out-

puts will reduce the effective jitter. Obeying statistical laws,

the jitter is reduced by the square root of the number of read-

ings that are averaged. For example, if four readings of the

duty cycle are averaged, the resulting jitter (and crest factor)

are reduced by a factor of two.

Jitter and Noise

Jitter in the PWM output appears as noise in the current

measurement. The Electrical Characteristics show noise

measured in current RMS (root mean square). Arbitrarily one

could specify PWM jitter, as opposed to noise. In either case

the effect results in a random error in an individual current

measurement.

Noise, just like jitter, can be reduced by averaging many

readings. The RMS value of the noise corresponds to one

standard deviation. The “crest factor” can be calculated in

noise).

Noise will also be reduced by averaging multiple readings,

and follows the statistical laws of a random variable.

Accuracy versus Noise

The graph shown in

Figure 5 illustrates the typical response

axis indicates time, and the vertical axis indicates measured

current (the PWM duty cycle has been converted to current).

The graph was generated for an actual current of 500 mA.

The difference between successive readings manifests itself

as jitter in the PWM output or noise in the current measure-

ment (when duty cycle of the PWM output is converted to

current).

The accuracy of the measurement depends on the noise in

the current waveform. The accuracy can be improved by av-

eraging several outputs. Although there is variation in suc-

cessive readings, a very accurate measurement can be ob-

tained by averaging the readings. For example, on

averaging the readings shown in this example, the average

current measurement is 502.3 mA (

Figure 5). This value is

very close to the actual value of 500 mA. Moreover, the ac-

curacy depends on the number of readings that are

averaged.

Low Current Measurements

The DC offset of the LM3822-1.0 is typically under 1 mA.

This low offset allows accurate low current measurements.

Even currents in the 10 mA range can be measured with ac-

DS101249-26

FIGURE 5. Typical Response of LM3822

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