Data Sheet ADA4571
DIAGNOSTICS
Radius Calculation
The V
SIN
and V
COS
outputs can be used to calculate a radius
value. These outputs have a fixed 90° phase relationship and
therefore the calculated radius value remains in a predictable,
predetermined range that varies with the temperature of the
device independent of the current magnetic field direction. This
radius, V
RAD
, can be used to validate the V
SIN
and V
COS
readings
in the ECU. When the calculated radius is no longer within the
acceptable bounds, a fault may occur in the system. To calculate
radius, use the following formula:
It is important to perform offset calibration before calculating
the radius.
Figure 32 shows the allowable radius values when GC mode is
enabled and Figure 33 shows the allowable radius values when
GC mode is disabled. The maximum and minimum V
RAD
values
are calculated based on the allowable amplitude range for V
SIN
and V
COS
, over the entire operating temperature of the device as
specified in the Specifications section. This range is represented
by the shaded region in Figure 32 and Figure 33.
Typical V
RAD
values for −40°C, +25°C, +125°C, and +150°C are
indicated as well.
Figure 32. GC On Radius Values
Figure 33. GC Off Radius Values
Monitoring of the VTEMP pin can allow an even tighter range
for radius length at the known temperature. See the
Specifications section and the Typical Performance
Characteristics section for exact values and output amplitude
specifications at each temperature.
Broken Bond Wire Detection
The ADA4571 includes circuitry to detect broken bond wire
conditions between the AMR sensor and the instrumentation
amplifier. The detection circuitry consists of current sources
and window comparators placed on the signal connections
between the AMR sensor and the ASIC. The purpose of the
current sources is to pull the signal node outside of the normal
operating region in the event of an open bond wire between the
AMR sensor and the ASIC. The purpose of the window
comparators is to detect when the signal from the AMR sensor
is outside of the normal operating region. When the comparators
detect that the signal nodes are outside the normal operating
region, the circuit pulls the VSIN and/or VCOS node to ground
to indicate the fault to the host controller.
In addition to the active circuitry, there are applications
recommendations, such as the utilization of pull-up and pull-
down resistors, which detect broken bond wires by pulling
nodes outside of the defined operating regions. A broken bond
wire at VTEMP, VCOS, and VSIN interrupts the corresponding
outputs. To ensure that the output enters into a known state if
there is a broken bond wire on these pins, connect a 200 kΩ
pull-down resistor at these pins. Pulling these nodes outside of
the normal operating region signals a fault to the host
controller.
