Hall sensor

gjoo

May 22, 2013
66
I don't understand why we need to connect together u4b and u4a op amps. In addition why do we use a resistor r24. I thought that u4a was setting the reference voltage at the microcontroller, so what would the need be for u4a to set a reference voltage at resistor r24?

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Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,716
Look up "differential amplifier", that's what you have here.

gjoo

May 22, 2013
66
The bottom op amp is a voltage follower that sets the bottom of r24 at roughly 2.048v, but why is it necessary to connect r24 to the top op amp? Why are they connected at all, since the bottom op amps sets the reference voltage at the microcontroller?

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,716
I admit being a bit too fast. Its not a differential amplifier.
U4A creates a buffered 2.048 V. Therefore VHALL- = 2.048 V
U4B works as a non-inverting summing amplifier, roughly VHALL+ = 1/2 × (2.048 V + VHALL_SIG_IN) (roughly because R24 is different from the resistance of the network in the path of HALL_SIG_IN).

U5 is an ADC with differential input so ADC = VIN+ - VIN- = 1/2 × (2.048 V + VHALL_SIG_IN) + VVR2 - 2.048 V
My guess is that VR1 and VR2 shall be trimmed such that the constant terms in the above equation cancel and the ADC measures VHALL_SIG_IN only. The setting of VR1 and VR2 will depend on the output characteristics of the hall sensor.

gjoo

May 22, 2013
66
But is r24 necessary and why use a summing amplifier that add the reference voltage to u4b, when we already applied the reference voltage to the a/d converter with u4a?

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,716
I don't know. You'd have to ask the designer of the circuit. Maybe it's got to do with the output signal of the hall sensor?

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