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參數(shù)資料
| 型號： | MAX9927AEE/V+ |
| 廠商： | Maxim Integrated Products |
| 文件頁數(shù)： | 5/23頁 |
| 文件大小： | 0K |
| 描述： | IC VR SENSOR INTERFACE 16QSOP |
| 標(biāo)準(zhǔn)包裝： | 100 |
| 系列： | * |
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 (VOS) 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 VCC/2 signal for the BIAS
input that is used to reference all internal electronics in
the device. BIAS should be bypassed with a 0.1F and
10F 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 (VMIN_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 (C). 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
V1
40ms
60ms
COUT
VR
SIGNAL
ADAPTIVE
THRESHOLD
SET BY V1
ADAPTIVE
THRESHOLD
SET BY V2
MIN
THRESHOLD
80ms
100ms
120ms
140ms
160ms
85ms
V1
1
3
V2
1/3 V2
180ms
200ms
Figure 1. Adaptive Peak Threshold Operation
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