I have a square wave output from a hall effect sensor that is DC-shifted by about 4V. If I understand the image below correctly, the output of the hall sensor is a square wave between 4V and 7.5V, and that the output is DC-shifted by 4V. Is this interpretation correct? If yes, how do I remove this DC shift? The square wave is as shown below. Would a simple cap at the output work? I need to interface the signal to an MCU and it needs to be between 0-3.3V. I simulated the design shown here but it does not seem to work in my case.

 

hi ep,
I read the datasheet the same as you.
For a 0v to 1.2V output you need a OPA level shifter, with Gain to give 0V to 3.3V
Which type of OPA do you have on the bench.?
E

 

I have an LM339, MCP6004, an LMV324, and LT1215. Only the LT1215 is a through-hole package so that would be easy to prototype with if it works.

 

hi,
OK,
Can you measure the actual voltage that comes from your Hall device, as you can see the spread is wide according to the d/s
See image.
BTW: Note the 'tight' supply voltage required, 8V +/- 0.2%
E

 

hi,
The MCP6004 is a rail to rail OPA, but the maximum Vsupply is 5V, do you have a 5V supply.?
The LM324 is OK with a 8V supply and can Vout down to approx 25mV.
The LM339 is a comparator
The LT1215 looks OK.

Have you measured the Vout range of your Hall device.?
E

 

Try a little bit simpler solution

 

hi,
The MCP6004 is a rail to rail OPA, but the maximum Vsupply is 5V, do you have a 5V supply.?
The LM324 is OK with a 8V supply and can Vout down to approx 25mV.
The LM339 is a comparator
The LT1215 looks OK.

Have you measured the Vout range of your Hall device.?
E

Hi, yes I did. The 'low' value is 4.1V and the 'high' value is 7.5V. I noticed the tight supply voltage but I'm wondering why. I forgot to attach the other part of the spec sheet but it seems to be using Allegro's A1221 sensor which is rated up to 24V. Also, yes I do have a 5V supply.

 

Try a little bit simpler solutionView attachment 224545

Not sure how this would work. I'm guessing the Zener is there to weed out the negative? However, all voltages are positive. Also I could derive the output from the emitter of the first 2N2222 rather than have another to get the polarity right...

 

hi ep,
You said 4V to 7.5V pulse level, are you doing a frequency count or a level detection of a pulse between 4V and 7.5v.???

E

 

To try a simple method first and that is to using a CR integral circuit, the C is used to cutoff the DC voltage.

RC values will affecting the input frequency, choosing the properly RC values to pass the input frequency and stop the DC voltage, if you get sucess then you can also add the other components like bjt or op amp.

 

Try placing the series resistor RP on the GND side of the circuit.
You should get a signal from 1.2V to 3.4V. Signal will be inverted.
0.5V min - 4.0V max

Also experiment with lower Vcc and different RP values.

 

Just try.
The second transistor is not necessary if you don't care that the signal would be 180 deg. out of phase (inverted). The the output from the collector of the first stage then.

 

hi ep,
You said 4V to 7.5V pulse level, are you doing a frequency count or a level detection of a pulse between 4V and 7.5v.???

E

Level detection.

 

To try a simple method first and that is to using a CR integral circuit, the C is used to cutoff the DC voltage.

RC values will affecting the input frequency, choosing the properly RC values to pass the input frequency and stop the DC voltage, if you get sucess then you can also add the other components like bjt or op amp.

You mean with a capacitor at the output to block DC and a resistor after that to ground? Yeah I did simulate this with LTSpice but got weird results. The frequency is about 2.2Hz. If I assume the R to be 10k I get a cap value of 10u. I’ll post the result soon.

 

Try placing the series resistor RP on the GND side of the circuit.
You should get a signal from 1.2V to 3.4V. Signal will be inverted.
0.5V min - 4.0V max

Also experiment with lower Vcc and different RP values.

Ok let me try this.

 

Just try.
The second transistor is not necessary if you don't care that the signal would be 180 deg. out of phase (inverted). The the output from the collector of the first stage then.

Ok let me try.

 

hi ep,
I read your original post in that way,
ie: you wanted to detect the voltage level of a square wave at a fixed frequency.

The square wave has fixed offset of 4v to 5v and its maximum amplitude can be 6.2v to 7.5v

So you want a Vout of 0V thru 3.3V for the above, is that correct.???

E

 

I like @dupaweza ’s circuit, but if you have a continuous squarewave output, the this would also work.
The device is a comparator. if you are interefacing to 3.3V or 5V logic use an open collector (or drain) comparator such as LM339/LM393. If it’s a microcontroller, it may already have a pull-up resistor.
Choose R and C for a time constant about 10 times the slowest time between two pulses.

 

hi ep,
I read your original post in that way,
ie: you wanted to detect the voltage level of a square wave at a fixed frequency.

The square wave has fixed offset of 4v to 5v and its maximum amplitude can be 6.2v to 7.5v

So you want a Vout of 0V thru 3.3V for the above, is that correct.???

E

Yes that’s correct. The frequency is fixed at about 2.2Hz.

 

I like @dupaweza ’s circuit, but if you have a continuous squarewave output, the this would also work.
The device is a comparator. if you are interefacing to 3.3V or 5V logic use an open collector (or drain) comparator such as LM339/LM393. If it’s a microcontroller, it may already have a pull-up resistor.
Choose R and C for a time constant about 10 times the slowest time between two pulses.

Ok I do have a LM339 but in SMD form. let me try though.