Gaussmeter circuit

[gjoo]

Hi,
This is part of a gaussmeter circuit. Can someone explain exactly what the resistors R19 to R24 are there for? Also, why are the two op amps places in parallel and connected that way. I believe they are voltage followers, but I don't understand why you need both of them. I

 

[Bluejets]

As far as I can see they are doing 2 different operations.
4B is watching the hall output while 4A is comparing the hall output to 1/2 of the reference voltage.
What is the hall effect type?
Think you may need to provide more info.

 

[gjoo]

As far as I can see they are doing 2 different operations.
4B is watching the hall output while 4A is comparing the hall output to 1/2 of the reference voltage.
What is the hall effect type?
Think you may need to provide more info.

It's an A1324 hall effect sensor from allegro systems

 

[gjoo]

It's an A1324 hall effect sensor from allegro systems

Also, I am a little concerned confused about the resistors R19 to R24

 

[DBingaman]

U4A is just providing a 2.024V Reference to the ADC negative input and to the bottom side of R24. This is not measuring the HAL in any way. It is a unity gain following with input of 1/2 * 4.096V.

U4B is just a unity gain follower with input of:

1.505V + 0.26 * (0 to 4.096V) from Hall sensor or range of 1.505V to 2.57V which is provided to the positive input of the ADC. I have not accounted here for the parallel resistance of what appears to be a fine tune gain control consisting of the 100K pot and two 51K resistors.

In short:
The ADC negative input is being held at 2.024V while the positive input has a range of 1.505 to 2.57V so the ADC output can swing from -0.519V to +0.526V or about 1V swing. The actual swing is a little better than this due to the stuff in parallel with the 10K and C17 is just providing some low pass filtering with cutoff better than RC (10K*100e-9) = 0.1ms Time constant or 10000 rad/s or 1.6Khz cutoff frequency.

To your questions direct:
R24 is providing a bias point for a voltage divider with the input being 2.024V the tap point of the voltage divider is going to the positive input of U4B which measures the HALL Sensor signal. While the hall sensor (it appears) provides a 0 to 4.096V range at most.

R19 is the other side of the divider. You have to use superposition to get the voltage range of U4B from that.

Hope that helps?

 

[DBingaman]

U4A is just providing a 2.024V Reference to the ADC negative input and to the bottom side of R24. This is not measuring the HAL in any way. It is a unity gain following with input of 1/2 * 4.096V.

U4B is just a unity gain follower with input of:

1.505V + 0.26 * (0 to 4.096V) from Hall sensor or range of 1.505V to 2.57V which is provided to the positive input of the ADC. I have not accounted here for the parallel resistance of what appears to be a fine tune gain control consisting of the 100K pot and two 51K resistors.

In short:
The ADC negative input is being held at 2.024V while the positive input has a range of 1.505 to 2.57V so the ADC output can swing from -0.519V to +5.26V or about 1V swing. The actual swing is a little better than this due to the stuff in parallel with the 10K and C17 is just providing some low pass filtering with cutoff better than RC (10K*100e-9) = 0.1ms Time constant or 10000 rad/s or 1.6Khz cutoff frequency.

To your questions direct:
R24 is providing a bias point for a voltage divider with the input being 2.024V the tap point of the voltage divider is going to the positive input of U4B which measures the HALL Sensor signal. While the hall sensor (it appears) provides a 0 to 4.096V range at most.

R19 is the other side of the divider. You have to use superposition to get the voltage range of U4B from that.

Hope that helps?

Correction slight mistake 1/2 of 4.096 is 2.048 NOT 2.024 my fault!

As far as I can see they are doing 2 different operations.
4B is watching the hall output while 4A is comparing the hall output to 1/2 of the reference voltage.
What is the hall effect type?
Think you may need to provide more info.

If you look carefully 4B is actually a unity gain buffer that has a 2.048V reference input that it is then feeding to the bottom of R24 and the negative input of the ADC.

U4A is just providing a 2.024V Reference to the ADC negative input and to the bottom side of R24. This is not measuring the HAL in any way. It is a unity gain following with input of 1/2 * 4.096V.

U4B is just a unity gain follower with input of:

1.505V + 0.26 * (0 to 4.096V) from Hall sensor or range of 1.505V to 2.57V which is provided to the positive input of the ADC. I have not accounted here for the parallel resistance of what appears to be a fine tune gain control consisting of the 100K pot and two 51K resistors.

In short:
The ADC negative input is being held at 2.024V while the positive input has a range of 1.505 to 2.57V so the ADC output can swing from -0.519V to +5.26V or about 1V swing. The actual swing is a little better than this due to the stuff in parallel with the 10K and C17 is just providing some low pass filtering with cutoff better than RC (10K*100e-9) = 0.1ms Time constant or 10000 rad/s or 1.6Khz cutoff frequency.

To your questions direct:
R24 is providing a bias point for a voltage divider with the input being 2.024V the tap point of the voltage divider is going to the positive input of U4B which measures the HALL Sensor signal. While the hall sensor (it appears) provides a 0 to 4.096V range at most.

R19 is the other side of the divider. You have to use superposition to get the voltage range of U4B from that.

Hope that helps?

In my opinion the circuit could be improved, it is not taking advantage of the entire dynamic range of the ADC.

In my opinion the circuit could be improved, it is not taking advantage of the entire dynamic range of the ADC.

Also, VR2 is allowing you to modify the positive output of U4B from 0 to 5V. But when in the center position is having hardly any effect on the circuit being that the output of U4B has 100 ohms impedance, the VR2 in center makes 50K so it has no effect until it is practically at the end of the potentiometer range.

Hi,
This is part of a gaussmeter circuit. Can someone explain exactly what the resistors R19 to R24 are there for? Also, why are the two op amps places in parallel and connected that way. I believe they are voltage followers, but I don't understand why you need both of them. I

The outputs of the two Op Amps are not really in parallel. Each one having 100 ohms and a 100nF capacitor, the cutoff frequency being 15.9Khz. So no real effect. the one that is in the center C21 is being used to filter VR2. C21 other side could probably have just been routed to ground without much change in circuit behavior.

 

[gjoo]

I appreciate the detailed response. I think you cleared up most of my confusion. I will use this information and review the circuit. Thanks for the help.

 

[DBingaman]

I appreciate the detailed response. I think you cleared up most of my confusion. I will use this information and review the circuit. Thanks for the help.

Sure thing! I love doing circuit analysis. Much success on your project!

 

[Bluejets]

U4A is just providing a 2.024V Reference to the ADC negative input and to the bottom side of R24. This is not measuring the HAL in any way.

Appears to me the negative input to 4A is fed from the hall output.

 

[DBingaman]

Appears to me the negative input to 4A is fed from the hall output.

On a quick look I also thought that at first, until I saw the output is sourcing R24 and holding it at the + input of 2.048V

 

[DBingaman]

Appears to me the negative input to 4A is fed from the hall output.

Easier to see if redrawn:

 

[gjoo]

What purpose does feeding the negative input from hall output?

 

[gjoo]

Also, why do you need to adjust the out voltage of the hall sensor?

 

[DBingaman]

What purpose does feeding the negative input from hall output?

That is not what is happening HALL- is really nothing more than a 2.048V reference to the ADC and provides a reference input to R24 that in combination with the R19 and HALL SIG in provides a biased input of the HALL sensor to U4B positive input. HALL- should really in my opinion be called HALL Reference Voltage.

Also, why do you need to adjust the out voltage of the hall sensor?

VR1 allows you to fine tune the gain, that is how much of the HALL voltage makes it to U4B positive input.
VR2 allows you to fine tune the DC bias of the ADC positive input.

The reason has to do with component tolerances and difference I would guess in the HALL sensor. Each one is unique, so the author of the circuit allowed for some fine tuning. Though I agree with you that could have been done in the software after being converted into digital by the ADC.

VR1 allows you to fine tune the gain, that is how much of the HALL voltage makes it to U4B positive input.
VR2 allows you to fine tune the DC bias of the ADC positive input.

VR2 does not do a good job because it is a 100K pot being used in conjunction with a 100 ohm output impedance from the U4B. Until the pot gets fairly close to it's limits, it will have little or no effect on the circuit.

 

[gjoo]

Probably a dumb question, but is the reason for limiting the output voltage of the hall sensor is that the adc can only handle upto a certain amount of voltage?

 

[DBingaman]

C21 along with filtering any noise on power line from VR2 also allows any high frequency common noise to pass to both sides of the ADC + and - input. Thus allowing the ADC to ignore that noise as the ADC only looks at the difference between + and -.

 

[gjoo]

C21 along with filtering any noise on power line from VR2 also allows any high frequency common noise to pass to both sides of the ADC + and - input. Thus allowing the ADC to ignore that noise as the ADC only looks at the difference between + and -.

 

[DBingaman]

Probably a dumb question, but is the reason for limiting the output voltage of the hall sensor is that the adc can only handle upto a certain amount of voltage?

Can you give me the part number of that ADC? It is a little blurry on the schematic, I cannot make it out. It looks like ADS8318?

 

[gjoo]

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?

 

[DBingaman]

Probably a dumb question, but is the reason for limiting the output voltage of the hall sensor is that the adc can only handle upto a certain amount of voltage?

I think I got the part number correct (can't be to many ADC's out there with differential inputs). According to the datasheet the input can + and - inputs to the ADC can go from 0V all the way up to the reference voltage. But it only looks at the difference when performing an ADC conversion.