MAX9927AEE/V+ Datasheet MAX9925AUB+, MAX9926UAEE/V+, MAX9926UAEE+T | P13-P15

Differential Amplifier

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

output precision amplifier with CMOS input bias cur-

rents, low offset voltage (V

) and drift. A novel input

architecture eliminates crossover distortion at the oper-

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

structures. These features enable reliable small-signal

detection 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

resistors produce extremely high DC and AC CMRR

that is important to maintain noise immunity. The

matched ppm-drift of the resistors guarantees perfor-

mance 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 V

/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 sta-

ble external BIAS reference.

The minimum threshold, adaptive peak threshold, zero-

crossing threshold signals are all referenced to this

voltage. 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 volt-

age scheme provides robust noise immunity to the input

VR signal, preventing false triggers from occurring due

to broken 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

arming level for the zero-crossing circuit of the com-

parator (see the

Zero Crossing

section).

If the input signal voltage remains lower than the adap-

tive peak threshold for more than 85ms, an internal

watchdog timer drops the threshold level to a default

minimum threshold (V

MIN_THRESH

). This ensures pulse

recognition recovers even in the presence of intermit-

tent sensor connection.

The internal adaptive peak threshold can be disabled

and directly fed from the EXT input. This mode of opera-

tion is called Mode B, and allows implementations of cus-

tom threshold algorithms in firmware. This EXT voltage is

typically generated by filtering a PWM-modulated output

from an onboard microcontroller (µC). An external opera-

tional amplifier can also be used to construct an active

lowpass filter to filter the PWM-modulated EXT signal.

MAX9924–MAX9927

Variable Reluctance Sensor Interfaces with

Differential Input and Adaptive Peak Threshold

______________________________________________________________________________________ 13

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

Figure 1. Adaptive Peak Threshold Operation

MAX9924–MAX9927

Zero Crossing

The zero-crossing signal provides true timing informa-

tion for engine-control applications. The zero-voltage

level in the VR sensor signal corresponds to the center

of the gear-tooth and is the most reliable marker for

position/angle-sensing applications. Since the output of

the differential amplifier is level-shifted to the BIAS volt-

age, the zero of the input VR signal is simply BIAS. The

comparator output state controls the status of the input

switch that changes the voltage at its noninverting input

from the adaptive/external threshold level to the BIAS

level. The difference in these two voltages then effec-

tively acts as hysteresis for the comparator, thus pro-

viding noise immunity.

Comparator

The internal comparator is a fast open-drain output

comparator with low input offset voltage and drift. The

comparator precision affects the ability of the signal

chain to resolve small VR sensor signals. An open-drain

output allows the comparator to easily interface to a

variety of µC I/O voltages.

When operating the MAX9924/MAX9925/MAX9926 in

Mode C, external hysteresis can be provided by adding

external resistors (see Figures 5 and 8). The high and

low hysteresis thresholds in Mode C can be calculated

using the following equations,

and

Rotational Direction Output

(MAX9926 Only)

For quadrature-connected VR sensors, the open-drain

output DIRN indicates the rotational direction of inputs

IN1 and IN2 based on the output state of COUT1 and

COUT2. DIRN goes high when COUT1 is leading

COUT2, and low when COUT1 is following COUT2.

Applications Information

Bypassing and Layout Considerations

Good power-supply decoupling with high-quality

bypass capacitors is always important for precision

analog circuits. The use of an internal charge pump for

the front-end amplifier makes this more important.

Bypass capacitors create a low-impedance path to

ground for noise present on the power supply.

The minimum impedance of a capacitor is limited to the

effective series resistance (ESR) at the self-resonance

frequency, where the effective series inductance (ESL)

cancels out the capacitance. The ESL of the capacitor

dominates past the self-resonance frequency resulting

in a rise in impedance at high frequencies.

Bypass the power supply of the MAX9924–MAX9927

with multiple capacitor values in parallel to ground. The

use of multiple values ensures that there will be multiple

self-resonance frequencies in the bypass network, low-

ering the combined impedance over frequency. It is

recommended to use low-ESR and low-ESL ceramic

surface-mount capacitors in a parallel combination of

10nF, 0.1µF and 1µF, with the 10nF placed closest

between the V

and GND pins. The connection

between these capacitor terminals and the power-sup-

ply pins of the part (both V

and GND) should be

through wide traces (preferably planes), and without

vias in the high-frequency current path.

TL BIAS

⎛

⎝

⎜

⎞

⎠

⎟

RV V

RR R

PULLUP BIAS

PULLUP

BIAS

−

⎛

⎝

⎜

⎞

⎠

⎟

()

Variable Reluctance Sensor Interfaces with

Differential Input and Adaptive Peak Threshold

14 ______________________________________________________________________________________

MAX9924–MAX9927

Variable Reluctance Sensor Interfaces with

Differential Input and Adaptive Peak Threshold

______________________________________________________________________________________ 15

Application Circuits

IN+

IN-

BIAS

ZERO_EN INT_THRS GND

COUT

EXT

TPU

μC

PULLUP

1nF

10kΩ

SENSOR

+5V

1kΩ1kΩ

10μF || 0.1μF

MAX9924

MAX9926

Figure 2. MAX9924/MAX9926 Operating Mode A1

IN+

IN-

BIAS

ZERO_EN INT_THRS GND

COUT

EXT

TPU

μC

PULLUP

1nF

10kΩ

SENSOR

+5V

MAX9924

MAX9926

Figure 3. MAX9924/MAX9926 Operating Mode A2

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P23

MAX9927AEE/V+

Mfr. #:

Buy MAX9927AEE/V+

Manufacturer:

Maxim Integrated

Description:

Sensor Interface Variable-Reluctance Sensor Interface

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MAX9927AEE/V+

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