DRV425 / DRV421

Also ..... Using the DRV425EVM board, it specifies that changing R1 (100 ohm) will change the sensitivity..
what is the high end for R1 that can be used to achieve the HIGHEST sensitivity ?

Hi Charles,

Our DRV expert will get back to you soon.

Regards

Charles,

The DR421 needs a compensation coil in order to measure the magnetic field. This is a closed system that works by the DR421 forcing current through a compensation coil to make the magnetic field at the device close to zero.  The range limitation then come from number of windings, magnetic core gain, compensation current (250mA @ Vdd =5V).  There are other considerations discussed in the application notes.

You can also see the DRV421 System Parameter Calculator in the software section of the tools page.

 A larger Rshunt decreases the range and increases sensitivity and is described by equation 1 on the datasheet. 

This Vout is referenced to Refout. The offset needs to be taken into account which is +/-8uT.   Also note earths magnetic field varies around 50uT.

We also have a system calculator for this device also located at the bottom of the tools page in the software section.

Regards,

Javier

Thanks Javier..

when you say the Rshunt 'Decreases' Range, can you enhance this explanation..

what I need from this is to sense a metal container at a given distance of about 12-18 inches...

I also need to keep the system Current to a minimum as this is battery operated.. so changing the Shunt resistor to get maximum range and sensitivity while keeping current draw to a minimum is the goal..

I have chosen a 300ohm resistor, using the DRV425 evaluation board to get max sensitivity and range  ( IS THIS CORRECT ? )

I am also confused from the System parameter calc, that if 0.100 or 2 maximum Mag Field mT , which is considered to be more sensitive ?

Also is there any Benefit to using two chip at 90Deg to each other with the sense side UP to measure the field to be monitored.?

 

 

 

 

Charles,

I sincerely apologies for my over sight and delay.

Increasing Rshunt decreases range due to the fact that we are now increasing the gain of the circuit.  When we have 12.2mA/mT and we increase the Rshunt then we are gaining up the voltage we are measuring but we are limited by the voltage output of the amplifier by VCC voltage and the Vref voltage.

When you are sensing the metal container you need to make sure that it is ferromagnetic.  Otherwise it will not change the results.  Also a magnetic material will not alone generate a magnetic field.  The ferromagnetic material could bend the surrounding magnetic fields and redirect them.

Our part will still draw higher current 10mA.  You can limit by turning the part on and off.  Increasing the Shunt resistor will limit the current but it may not be enough.

With a 300ohm resistor and assuming 5V  you will get a max output voltage of 2.5V.  

2.5V(OutputV) / 4(gain of amplifier) = 625mV (across shunt)

625mV / 300 ohm = 2.083mA

2.083mA / 12.2mA/mT =  .1707mT  

This is close to the calculator but the calculator shows an overvoltage error and says it can measure 0.16mT.  This is due to the Resistance of the compensation coil and its variability.  

Therefore the Range is +/- 160 uT.  Please take into account earths magnetic field is about 70uT and could be at full strength or zero depending on the orientation of the DRV425 compared to the earths magnetic field.

The maximum range is 2mT so that would be less sensitive.  if you increase the Rshunt to 300ohm your full scale range is now 0.16mT and the voltage will change at a higher rate for magnetic field making it more sensitive.

There are many ways to orient two sensors but it depends on your objective.  We have a TI design that places two sensors facing 180 apart and we use analog circuitry to get the difference and eliminate common magnetic fields.  Basically we are then only looking at the gradient of the magnetic field.  See TIPD205

Regards,

Javier