Q
I would like to use the AD22151 in an application where the magnetic field
strenght changes from 260 Gauss to 470 Gauss.Can the AD22151 be configured to
have an output span of 0.5V to 4.5V for the above magnetic field span.
A
The AD22151 is one of those sensor components which not only incorporates the
sensor itself, but also throws in some blocks of uncommitted circuitry "for
free" which might be useful in integrating the sensor in a real system.
On the AD22151, the functional blocks are
1. Complete Hall effect magnetic transducer, with on-chip demodulation,
producing a linear output of typ 0.4mV/G @25C.
2. A non-inverting amplifier, whose gain can be set using external resistors,
to set the desired scaling factor.
3. An on chip reference at Vcc/2, to allow the offset of the voltage output to
be adjusted, when combined with the NI amplifier.
4. A temperature sensor, which senses the actual die temperature, and therefore
allows a accurate compensation for the "gain drift with temperature" associated
with the Hall effect sensor.
In your case, the linear transfer function you want is approximately
Vout = 0.01905 x B - 4.643
This means that you need to change the gain from 0.0004V/Gauss to
0.01905V/Gauss (Non Inv. amplifier must have gain of 47.6)
You also need to introduce an offset at the inverting input, which you can
achieve with the configuration on figure 2 (pg1) of the datasheet. A bit of op
amp analysis will yield the precise values. You will have to make sure that
none of the voltage limits of the amplifier are broken. Also, since the part is
quite loosely specified (mostly typ specs, not guaranteed mins and maxes) you
will have to put in place some facility for trimming the resistor values on
each part in production. If this is in the context of a uP based system then
you might consider using some of our digital potentionmeters with EEPROM
memory, to implement a production trim.
The gain of the amplifier has to be quite high to achieve the 0.5V to 4.5V
directly from the AD22151. It might make the design easier if you had a more
moderate gain in the AD22151 and used a rail to rail precision op amp to
introduce the required offset and gain down the line. The temperature
compensation may need to be quite elaborate if your accuracy requirements are
high. Moving the high gain to an external op amp would simplify the temperature
compensation.