# Sensor output resistance

I want to design a circuit using a Hall sensor (reference is SS49E, see datasheet).

I want to size the output resistance of the sensor. The block diagram of the sensor is:

The output current of the sensor is 1.5mA maximum. If I use a 5V supply, does it mean the minimum output resistance value is 5V / 1.5mA = 3.33kOhms ?

What happens if you probe the voltage between the Output and V- when there is no load ? Thank you.

• I suggest you use a voltage divider. Feb 10, 2022 at 8:40

From the data sheet you linked: -

Output voltage span is 1.05 to (Vs - 1.05) Vdc

So, if your power supply (Vs) is 5 volts, the maximum output voltage is 3.95 volts.

From the data sheet regarding minimum (that means guaranteed) output current: -

Output current (Vs > 3.0 V) 1.0 mA

So, realistically the load resistor should not be lower than 3.95 volts / 1 mA = 3.95 kΩ. Here's the only mention of load resistance in the data sheet you linked: -

On that basis alone I would strongly consider 10 kΩ as the load resistor.

• I didn't notice this load value! I will use 10kΩ. Feb 10, 2022 at 8:58
• With a load of 10kOhms (between the Output and V-), the output is stuck at 4.5V (5V supplied), even with a magnet around. Any clue? Feb 10, 2022 at 21:13
• What does the value change to when the magnet is removed. Maybe the magnet is too strong for the sensor? Feb 11, 2022 at 0:57
• It doesn't change, that's the problem. There is no saturation, it just stays at 4.5V even with no magnet around. Feb 11, 2022 at 8:00
• Have you got a replacement sensor? Are you testing the sensor away from any iron that might have become magnetized without you realizing it? Feb 11, 2022 at 8:18

The datasheet says output current might be 1 mA minimum and 1.5mA typical and 1.5mA maximum it can provide still within specs. It would be bad idea to always load it with maximum possible allowable load, or even typical. Your design specs should at most use 1mA rating, and preferably much less. So 5k resistance or larger.

The typical curves are measured with 10k load, which might be a reasonable value.

If you probe the output voltage without load, you measure the unloaded output voltage. It might be slightly higher than loaded voltage. And it might affect the signal bandwidth as the output stage is an emitter follower with constant current sink.