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APPLICATION INFORMATIONUNDERSTANDING INPUT SIGNAL VARIATION
in Sensor ICs with SolidSpeed Digital Architecture
By Shaun Veilleux, Allegro MicroSystems
Modern Allegro MicroSystems speed sensors use SolidSpeed
Digital Architecture technology that moves most signal processing into the digital domain. With the advancement of thistechnology, the restriction of input signal variation has alsochanged. For many previous ICs, there was a restriction on
the total input magnetic magnitude change that could occur
during a given power-on instance. ICs with this updated
specification are capable of continuously tracking a signal
where the magnetic magnitude varies throughout the fulloperational range of the IC. There is a restriction on the rateof change of the magnetic magnitude from one input period
to the next to maintain uninterrupted output.
The datasheet representation of this limitation has changed andmay not be immediately obvious how it is to be interpreted.
This application note is to be used to aid the understanding of these datasheet line items. This specification change
can be seen in the A19302 and A19571 datasheets and willcontinue to be used in many parts that use this technology
moving forward.

Background
With the invention of SolidSpeed Digital Architecture parts,
a signal can be tracked throughout its entire dynamic range
using a fixed gain. This fixed gain removes the need for automatic gain control. Instead of comparing the current signal’s
magnitude to the maximum and minimum since power-on,
parts compare the current signal magnitude to a previously
stored tracked magnitude.
This creates a requirement that the current period’s peak-topeak value must be within a certain amount to the previous
or several previous peak-to-peak values. This is represented
in datasheets using the Allowable Signal Variation line items
and is shown in the Typical Datasheet Representation sectionof this document.

TCYCLE
SignalPositive Peak Tracker
Negative Peak Tracker

Figure 1: Peak Tracking Example

These line items are typically accompanied by two figures
showing the common cases of signal variation. A signal
period-to-period variation where the peak-to-peak valuechanges over a single period and a repeated period-to-periodvariation where the peak-to-peak value changes continuously
over multiple periods. One can see that these are similar casesthat are further enforced by both using the same accompanied
equation and limitations.

How to Interpret
This section will discuss how to interpret these datasheet line
items and how to know what cases are within datasheet limits.
Looking at the line items separately, it can be understood
what each means.
Operating Magnetic Input Signal Window is a rolling windowthat sets the limit for how many periods the IC has “memory”
of in the past. Or if looking at a waveform, it is how many
periods in the future that must be compared to the current
signal magnitude.

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Operating Magnetic Input Signal Variation is the symmetricallimit that the signal can increase or decrease in peak-to-peakamplitude without the inclusion of offset within this specifiedwindow. This value is represented in terms of a ratio of thecurrent peak-to-peak to any of the previous period’s peak-topeak within the specified windows.
Using the equation shown in Figure 2 and Figure 3:
Equation 1:

starting from period n and using a window size of TWINDOW.
The signal peak-to-peak variation, ΔBDIFF(pk-pk), of periodn+m with respect to period n must be within datasheet limitsof ∆min and ∆max.
How the magnitude changes within this window does not matter so long as ΔBDIFF(pk-pk) remains in specification. That is tosay that the signal can have a single period-to-period variationof ∆min or ∆max and no other changes within this window;
or the signal can have a repeated period-to-period variationsuch that Bn+m has a ratio of ∆min or ∆max; or there can be a
mixture of the two such that ΔBDIFF(pk-pk) is not violated within
any window of size TWINDOW.

Typical Datasheet Representation
Characteristic

SymbolNotes

Min.

Typ.

Max.

Unit

Operating Magnetic Input
Signal Variation∆BDIFF(pk-pk)

Bounded amplitude ratio within TWINDOW [1]. No missed
output transitions or flat line condition. ossible incorrect
direction information. See Figure 2 and Figure 3.

∆min–

∆max–

Operating Magnetic Input
Signal Window

TWINDOW

Rolling window in which ∆BDIFF(pk-pk) cannot exceed
bounded ratio. See Figure 2 and Figure 3.

Wmin––TCYCLE[1] Symmetrical signal variation is defined as the largest amplitude ratio from B

while BDIFF remains in the operating magnetic range.

n to Bn + TWINDOW. Signal variation may occur continuously

Bn
Bn+m
Applied B DIFF

TWINDOW

Figure 2: Single period-to-period variation
Bn
Bn+m
Applied B DIFF

TWINDOW

Figure 3: Repeated period-to-period variation

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ExamplesCase 2It may be useful to further understand this datasheet specification
by looking at several hypothetical scenarios for a hypotheticalpart. For the following examples, a part with a ∆min of 0.4,
no ∆max requirement, and a Wmin of 3 TCYCLE is used, and asecond part with the same ∆min and ∆max as the previous
part but a Wmin of 4 will be considered.

This case will look at a simple repeated period-to-periodvariation. This case the peak-to-peak of a signal will reduce by
25% for each peak-to-peak with respect to the previous peakto-peak. Starting with period 1 to period 9, the peak-to-peakvalues are [200, 150, 112, 84, 63, 47, 36, 27, 20] respectively.

Case 1This case will look at a simple single period-to-period variation. There are two events that change the peak-to-peak by
50% relative to the previous peak. Periods [1,2,3] have apeak-to-peak value of 200 units, [4,5,6] have 100 units, and[7,8,9] have 50 units.
100

15021007512345067893456725890

Figure 4Using equation 1, it can be determined if this signal remains
within datasheet specifications. Starting with n equal to 1 andm equal to 1, the following is obtained:
Equation 2:

Figure 5

The same process presented in case 1 will be used for the firstsensor, with a Wmin of 3. Starting with n equal to 1 and m equal
to 1, 2, and 3, it can be seen that the largest ratio change will
be for m equal to 3, where B1 is equal to 100 and B4 is equalto 42. This will result in a ratio change of 0.42, which remains
within datasheet specifications. Repeating this math on B2and B5, the resultant reduction is 0.42 again. In fact, with aconstant 25% reduction for all cases, this signal will remain indatasheet specification.
For the second sensor, with Wmin of 4, the same process canbe taken for B1 and B5, and it can be seen that a reduction
ratio of 0.32 occurs. This input signal would be outside ofdatasheet specification.

This gives a ratio of 1, which is greater than ∆min. This process
can be repeated for m equal to 2 and 3, which give a ratioof 1 and 0.5 respectively. These are both greater than ∆min.
The process is then repeated for n equal to 2 and m equal to1, 2, and 3, which give ratios of 1, 0.5, and 0.5—still within
datasheet specifications. In fact, this waveform will continue
to remain in specification for all cases.
Changing Wmin to 4, the steps can be followed again. For nequal to 1 and m equal to 1, 2, 3, and 4, the ratios are 1, 1,0.5, 0.5 respectively. Following these steps, one can see withn equal to 3 and m equal to 1, 2, 3, and 4, the ratios are 0.5,0.5, 0.5, and 0.25. Since 0.25 is less than ∆min, then thiswaveform would be out of datasheet specifications.

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Revision History
NumberDate

Description

Responsibility–June 7, 2021

Initial release

S. Veilleux

Copyright 2021, Allegro MicroSystems.

The information contained in this document does not constitute any representation, warranty, assurance, guaranty, or inducement by Allegro to the customer with respect to the subject matter of this document. The information being provided does notguarantee that a process based on this information will be reliable, or that Allegro has explored all of the possible failure modes.
It is the customer’s responsibility to do sufficient qualification testing of the final product to ensure that it is reliable and meetsall design requirements.
Copies of this document are considered uncontrolled documents.

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