MAX9924 datasheet(13/23 Pages) AD

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Differential Amplifier

The input operational amplifier is a rail-to-rail input and

output precision amplifier with CMOS input bias currents,

low offset voltage (VOS) and drift. A novel input archi-

tecture eliminates crossover distortion at the operational

amplifier inputs normally found in rail-to-rail input struc-

tures. These features enable reliable small-signal detec-

tion for VR sensors.

The MAX9924/MAX9926 include on-chip precision-

matched low-ppm resistors configured as a differential

amplifier. High-quality matching and layout of these resis-

tors produce extremely high DC and AC CMRR that is

important to maintain noise immunity. The matched ppm-

drift of the resistors guarantees performance across the

entire -40°C to +125°C automotive temperature range.

Bias Reference

In Modes A1, B, and C, a well-decoupled external

resistor-divider generates a VCC/2 signal for the BIAS

input that is used to reference all internal electronics in the

device. BIAS should be bypassed with a 0.1μF and 10μF

capacitor in parallel with the lower half of the resistor-

divider forming a lowpass filter to provide a stable external

BIAS reference.

The minimum threshold, adaptive peak threshold, zero-

crossing threshold signals are all referenced to this volt-

age. An input buffer eliminates loading of resistor-dividers

due to differential amplifier operation. Connect BIAS to

ground when operating in Mode A2. An internal (2.5V

typical) reference is used in Mode A2, eliminating external

components.

Adaptive Peak Threshold

Modes A1 and A2 in the MAX9924–MAX9927 use an

internal adaptive peak threshold voltage to trigger the

output comparator. This adaptive peak threshold voltage

scheme provides robust noise immunity to the input VR

signal, preventing false triggers from occurring due to bro-

ken tooth or off-centered gear-tooth wheel. See Figure 1.

The sensor signal at the output of the differential gain

stage is used to generate a cycle-by-cycle adaptive

peak threshold voltage. This threshold voltage is 1/3 of

the peak of the previous cycle of the input VR signal.

As the sensor signal peak voltage rises, the adaptive

peak threshold voltage also increases by the same ratio.

Conversely, decreasing peak voltage levels of the input

VR signal causes the adaptive peak threshold voltage

used to trigger the next cycle also to decrease to a new

lower level. This threshold voltage then provides an arm-

ing level for the zero-crossing circuit of the comparator

(see the Zero Crossing section).

If the input signal voltage remains lower than the adaptive

peak threshold for more than 85ms, an internal watchdog

timer drops the threshold level to a default minimum

threshold (VMIN_THRESH). This ensures pulse recogni-

tion recovers even in the presence of intermittent sensor

connection.

The internal adaptive peak threshold can be disabled and

directly fed from the EXT input. This mode of operation

is called Mode B, and allows implementations of custom

threshold algorithms in firmware. This EXT voltage is typi-

cally generated by filtering a PWM-modulated output from

an onboard microcontroller (μC). An external operational

amplifier can also be used to construct an active lowpass

filter to filter the PWM-modulated EXT signal.

Figure 1. Adaptive Peak Threshold Operation

20ms

40ms

60ms

COUT

SIGNAL

ADAPTIVE

THRESHOLD

SET BY V1

ADAPTIVE

THRESHOLD

SET BY V2

MIN

THRESHOLD

80ms

100ms

120ms

140ms

160ms

85ms

1/3 V2

180ms

200ms

www.maximintegrated.com

Maxim Integrated │ 13

MAX9924–MAX9927

Variable Reluctance Sensor Interfaces with

Differential Input and Adaptive Peak Threshold

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