A1360 Datasheet, PDF (24/25 Pages) Allegro MicroSystems – Low-Noise Programmable Linear Hall Effect Sensor ICs with

A1360, A1361, Low-Noise Programmable Linear Hall Effect Sensor ICs with
and A1362
Adjustable Bandwidth (50 kHz Maximum) and Analog Output
Improving Sensing System Accuracy Using the FILTER Pin
In low-frequency sensing applications, it is often advantageous
to add a simple RC filter to the output of the sensor IC. Such a
low-pass filter improves the signal-to-noise ratio, and therefore
the resolution, of the device output signal. However, the addition
of an RC filter to the output of a sensor IC can result in undesir-
able device output attenuation — even for DC signals.
Signal attenuation, ∆V_ATT , is a result of the resistive divider
effect between the resistance of the external filter, R_EXT (see fig-
ure 10), and the input impedance and resistance of the customer
interface circuit, R_INTFC. The transfer function of this resistive
divider is given by:
∆V_ATT
=
V_OUT
⎜⎜⎝⎛_REX^R_T^I₊^NT_R^F_I^C_NTFC
⎞
⎟⎠
.
(16)
Even if R_EXT and R_INTFC are designed to match, the two individ-
ual resistance values will most likely drift by different amounts
V+
VCC
over temperature. Therefore, signal attenuation will vary as a
function of temperature. Note that the input impedance, R_INTFC ,
of commonly available analog-to-digital converters (ADC) can
be as low as 10 kΩ.
The A136x contains an internal resistor with buffer amplifier that
can be connected via the FILTER pin to the PCB. With this cir-
cuit architecture, users can implement a simple RC filter via the
addition of a capacitor, C_FILTER (see figure 11) from the FILTER
pin to ground. The buffer amplifier inside of the A136x (located
after the internal resistor and FILTER pin connection) eliminates
the attenuation caused by the resistive divider effect described in
equation 16. Therefore, the A136x device is ideal for use in high-
accuracy applications that require a large signal-to-noise ratio
and cannot afford the signal attenuation associated with the use
of an external RC low-pass filter.
Standard Hall Effect Device
Figure 10. When a low pass filter is constructed externally
to a standard Hall effect device, a resistive divider may
exist between the filter resistor, R_EXT, and the application
load resistance, R_INTFC. This resistive divider (shaded
area) will cause excessive attenuation, as given by the
transfer function shown in equation 16.
C_BYPASS
0.01 μF
Amp
Out
Gain
Offset
GND
Trim
Control
VOUT
Low Pass Filter
Resistive Divider
R_EXT
C_EXT
R_INTFC
Application
Interface
Circuit
Figure 11. The FILTER pin provided
on the A136x device allows separate
control of SNR, avoiding the attenua-
tion effects from the standard resistor
divider solution, shown in figure 10.
V+
VCC
Ratiometric
Hall Drive
C_BYPASS
0.01 μF
Sens TC Trim
(Factory
Programmed)
Program/Lock
Programming
Logic
Sensitivity Trim
8 Bits
(Customer
Programmed)
VOUT(Q)
8 Fine Bits
1 Coarse Bit
(Customer
Programmed)
+
Signal Recovery
–
GND
Allegro A136x
FILTER
C_FILTER
Buffer Amplifier
and Resistor
+
–
VOUT
Application
Interface
Circuit
R_INTFC
Allegro MicroSystems, Inc.
24
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com