Gaussmeter circuit

[gjoo]

So, that reason for resistors R19 to R22 and the resistance is a to keep it under that reference voltage?

 

[DBingaman]

My mistake, I meant that the parallel resistors on the output of the hall sensor, are they there to limit how much voltage is fed to the adc?

They are there to control the over all gain...
First lets have VR1 wiper all the way to the left, then total resistance looking back is:
10K||((100K||51K) + 51K) = 10K || (34K + 51K) = 10K || 85K = 8.9K

Now with VR1 wiper all the way to the right:
10K||51K = 8.36K

...

 

[gjoo]

I am trying to wrap my head around this, I am not very knowledgeable about op amps and hall sensors , so I appreciate your patience. But, when you say to control the gain , what is the main reason that we want to control the gain?

 

[DBingaman]

Now using superposition VR1 wiper all the way left:
U4B + input = 2.048V (3.6/(8.9+3.6)) + HALL_SIG_IN (8.9/(8.9+3.6)) = 0.712 * HALL_SIG_IN + 0.59V

...

Now using superposition VR1 wiper all the way left:
U4B + input = 2.048V (3.6/(8.9+3.6)) + HALL_SIG_IN (8.9/(8.9+3.6)) = 0.712 * HALL_SIG_IN + 0.59V

...

And again with VR1 wiper all the way right:
U4B+ input = 2.048 (3.6/(8.36+3.6)) + HALL_SIG_IN (8.36/(3.6+8.36)) = 0.699 * HALL_SIG_IN + 0.62V

So the wiper changes the gain by only a small amount range of 0.699 to 0.712 but it has the side effect of changing the bias from 0.59 to 0.62 Volts

 

[gjoo]

By changing the bias, we keep the op amp from saturating?

 

[DBingaman]

I am trying to wrap my head around this, I am not very knowledgeable about op amps and hall sensors , so I appreciate your patience. But, when you say to control the gain , what is the main reason that we want to control the gain?

The author of the circuit apparently wanted to fine tune the gain and offset to allow for difference in HALL Sensors.

By changing the bias, we keep the op amp from saturating?

Yes, we keep it close to 1/2 the power supply voltage.

 

[gjoo]

Just one last question. I'm still confused about the need for r24 and why did that pick that size for it?

 

[DBingaman]

I am trying to wrap my head around this, I am not very knowledgeable about op amps and hall sensors , so I appreciate your patience. But, when you say to control the gain , what is the main reason that we want to control the gain?

Don't feel to bad. I know nothing at all about HALL sensors, though I am an expert at Op Amps.

 

[DBingaman]

Just one last question. I'm still confused about the need for r24 and why did that pick that size for it?

It provides the positive DC bias of 0.59 to 0.62V to keep the HALL sensor signal well within the range of the Op Amp. I have not looked at this op amp's datasheet but I have a feeling it is not a rail to rail input op-amp, so you can only get within about 1/2V of ground and still have the op amp work. The author (my guess) choose that value so the input signal to U4B+ would not be anything less than around a 1/2 a volt to stay within the op-amps working range.

 

[gjoo]

I asked you a bunch of questions and I thank you for your very helpful responses. You cleaned up a lot for me. All your help and knowledge on op amps is so greatly appreciated. Thank you.

 

[DBingaman]

Just one last question. I'm still confused about the need for r24 and why did that pick that size for it?

By the formulas I derived in the from y=mx+b :
U4B+ = 0.712 * HALL_SIG_IN + 0.59V = (0.59V to 3.5V)
U4B+ = 0.699 * HALL_SIG_IN + 0.62V = (0.62V to 3.48V)

The offset it 'b' and it comes about because of R24, without that the range would go down to zero which is not acceptable for the op-amp.

Augg! Those formulas are a little messed up. It is getting late here and I must be tired. do you want me to recalculate them for you or are you familiar with superposition?
Never mind, I fixed them. Could not live with myself otherwise see the final post for the correct formulas.

 

[DBingaman]

I asked you a bunch of questions and I thank you for your very helpful responses. You cleaned up a lot for me. All your help and knowledge on op amps is so greatly appreciated. Thank you.

You are welcome. Glad to be helpful

 

[DBingaman]

Just one last question. I'm still confused about the need for r24 and why did that pick that size for it?

I feel bad about messing up the formulas here is the correct values:

U4B + input = 2.048V (8.9/(8.9+3.6)) + HALL_SIG_IN (3.6/(8.9+3.6)) = 0.288 * HALL_SIG_IN + 1.46
U4B+ input = 2.048 (8.36/(8.36+3.6)) + HALL_SIG_IN (3.6/(3.6+8.36)) = 0.300 * HALL_SIG_IN + 1.43V

And thus the ranges: 1.46V to 2.64V
and 1.43V to 2.66V

As you can see these values are right in the middle of the Op amp rail of 5V. So the designer choose R24 very well. Though he/she still did not take more advantage of the ADC's dynamic range.

Sorry about that.

 

[gjoo]

I feel bad about messing up the formulas here is the correct values:

U4B + input = 2.048V (8.9/(8.9+3.6)) + HALL_SIG_IN (3.6/(8.9+3.6)) = 0.288 * HALL_SIG_IN + 1.46
U4B+ input = 2.048 (8.36/(8.36+3.6)) + HALL_SIG_IN (3.6/(3.6+8.36)) = 0.300 * HALL_SIG_IN + 1.43V

And thus the ranges: 1.46V to 2.64V
and 1.43V to 2.66V

As you can see these values are right in the middle of the Op amp rail of 5V. So the designer choose R24 very well. Though he/she still did not take more advantage of the ADC's dynamic range.

Sorry about that.

 

[gjoo]

Thanks for the added info.