MAX9924–MAX9927
Variable Reluctance Sensor Interfaces with
Differential Input and Adaptive Peak Threshold
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Detailed Description
The MAX9924–MAX9927 interface with variable reluc-
tance (VR) or magnetic coil sensors. These devices
produce accurate pulses aligned with flywheel gear-
teeth even when the pickup signal is small and in the
presence of large amounts of system noise. They inter-
face with new-generation differential VR sensors as well
as legacy single-ended VR sensors.
The MAX9924/MAX9925 integrate a precision op amp,
a precision comparator, an adaptive peak threshold
block, a zero-crossing detection circuit, and precision
matched resistors (MAX9924). The MAX9926 and
MAX9927 are dual versions of the MAX9924 and
MAX9925, respectively. The MAX9926 also provides a
rotational output that is useful for quadrature-connected
VR sensors used in certain high-performance engines.
The input op amp in the MAX9925/MAX9927 are typical-
ly configured as a differential amplifier by using four
external resistors (the MAX9924/MAX9926 integrate
precision-matched resistors to give superior CMRR per-
formance). This input differential amplifier rejects input
common-mode noise and converts the input differential
signal from a VR sensor into a single-ended signal. The
internal comparator produces output pulses by compar-
ing the output of the input differential amplifier with a
threshold voltage that is set depending on the mode
that the device is in (see the
Mode Selection
section).
Mode Selection
The MAX9924/MAX9926 provide four modes of opera-
tion: Mode A1, Mode A2, Mode B, and Mode C as deter-
mined by voltages applied to inputs ZERO_EN and
INT_THRS (see Tables 1, 2, and 3). In Modes A1 and
A2, the internal adaptive peak threshold and the zero-
crossing features are enabled. In Mode A2, an internally
generated reference voltage is used to bias the differen-
tial amplifier and all internal circuitry instead of an exter-
nal voltage connected to the BIAS input—this helps
reduce external components and design variables lead-
ing to a more robust application. In Mode B, the adap-
tive peak threshold functionality is disabled, but
zero-crossing functionality is enabled. In this mode, an
external threshold voltage is applied at EXT allowing
application-specific adaptive algorithms to be imple-
mented in firmware. In Mode C, both the adaptive peak
threshold and zero-crossing features are disabled and
the device acts as a high-performance differential ampli-
fier connected to a precision comparator (add external
hysteresis to the comparator for glitch-free operation).
Table 1. MAX9924/MAX9926 Operating Modes
