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Amplifying a Hall sensor output.

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stobbz

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Hello all,

I'm having trouble amplifying the output signal of an SS495A Hall sensor using an inverting op-amp. Measuring the output of the sensor with an excitation coil gives a reading of around 500 mV, without any amplification. Also, moving the coil around the sensor causes the output to vary from 15 mV up to 500 mV. There is also a 3 V DC offset which I would expect.

I implemented a simple amplifier using a TL081CN op-amp. When I connect the output of the Hall sensor to the inverting input of the amplifier and measured the amplifier output voltage, it is around 15 mV (sine wave). Moving the coil around causes only a tiny change in output amplitude. Is there any obvious reason as to why that might be?

Attached is a VERY rough schematic of what's going on. Signal generator connected to a power amplifier, which provides the excitation for the coil (the inductor in the diagram). This coil is placed in close proximity to the sensor, the output of which I'm trying to amplify.

Any help appreciated!
 

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I think it is the DC offset from the sensor that has the op amp against the rail. If you don't need dc coupling from the sensor you could just add a cap in series with the sensor output.
 
First, the data sheet says that the output of the sensor can be from (Vss+0.2V) to (Vcc-0.2V), where your Vss=0V and Vcc=6V, so the signal can range from 0.2V to 5.8V. If you want to amplify the sensor output with a gain of ~10, then you will have to bias the amplifier so that it amplifies a small subrange of sensor's full range.

In the sim below, I show how to use a pot to select which part of the sensor's output range gets amplified. Note that the pot is powered from the same voltage as the sensor. The horizontal axis of the plot is the sensor output. The vertical axis is the opamp output. The various traces correspond to different values of k (pot wiper position). Pick the pot position so that the sensor output is centered near zero volts out of the opamp.
 

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Thanks for the response guys, really helpful. I added in the blocking capacitor at the sensor output to remove the DC offset and that was fine, a 500 mV signal as before without any offset.

However, when I make the connection to the amplifier and measure both sensor output and the amplifier output, the amplifier output appears to be smaller than the sensor output. An offset is also reintroduced, presumably by the op-amp?

Could this 'new' offset be the cause of this problem? The green trace is the sensor output, the purple trace is the 'amplified' signal.
 

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Can you post a schematic of how your opamp is configured? There is something seriously wrong, because you show an inverting opamp stage, but your scope trace shows that the output is in phase with the input (non-inverting).

Oh, I went an looked at the schematic on your first post. The 1K input resistor is way too low. Since it is connected between the sensor and a virtual ground at the inverting input of the opamp, it is loading the sensor.

Reconfigure it so that the sensor output goes to the non-inverting input of the opamp as I showed in my previous post. That was based on not having a coupling capacitor, so if you now want one, we will have to provide a dc path for the opamp's input bias current.

You could also change the input resistor to ~50K, and the feedback resistor to ~500K in the inverting configuration you showed.

At what level do you want the output of the opamp to sit in the Hall's nominal resting state?

Here is my favorite non-inverting, ac-coupled gain amplifier. If you use a modern single-supply rail-to-rail opamp, you can power it with only the 6V sensor supply; no +-15V supplies required.
 

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Last edited:
When I was connecting up the amplifier, I was following the gain of 10 inverting amplifier circuit shown in the data sheet, screenshot of which is below.

As for the output of the op amp in the Hall's nominal state, the idea was to add a further compensation stage so that a reading of 0 volts would be given when there is no magnetic field present.

I'm very grateful for the detailed replies.
 

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