I am using a circuit represented by the attached to (1) amplify signals from a hall sensor and (b) send the output to an LM331 to convert the voltage to frequency.
This is working fine for my needs but I am curious as to why I can not get the LM358 to adjust to 0 volts when there is no magnetic field on the hall sensor. The sensor's "quiescent" voltage is Vs/2 (in this case 5V/2) when there is no magnetic field detected. I thought that by adjusting the pot so that the voltage into the non-inverting side matched the inverting side , I would get 0 volts output from the LM358.
The feedback resistor(s) are on a switch so that I can choose 10K (no amplification) or 100K (10X amplification).
The original configuration of this circuit used the first Op Amp circuit in the LM358 dual package to perform the zeroing function (pot connected to non-inverting side, configured as a voltage follower). However, this produced the same results as the circuit presented here, so I stopped using this approach.
I have experimented with the circuit to see what I can learn.
When the feedback resistor is 10K (no amplification), the lowest output I can achieve from the LM358 is 0.63 volts. I have to adjust the pot so that the output climbs slightly higher than this to ensure that the circuit responds to the smallest of changes in the hall sensor output. By adjusting the pot to about 10K, I get an output of 0.7 volts and I can use the circuit.
When the feedback resistor is 100K (10X amplification), the lowest output I can achieve from the LM358 is 0.09 volts. Again, I have to adjust the pot so that the output climbs slightly higher than this to use the circuit. By adjusting the pot to about 18K, I get an output of approx 0.2 volts and I can use the circuit.
As I said, this is not a major problem and I can use the circuit as is to meet my needs. However, given that the effective working range of the LM331 is 0 to Vs-2 (in this case 5V-2), losing 0.6V of "range" is a little annoying. Also, given that this is the first time I have used an Op Amp (but hopefully not the last) I would like to understand this a little better.
As always, thanks in advance for help, thoughts, suggestions.
DamoRC
This is working fine for my needs but I am curious as to why I can not get the LM358 to adjust to 0 volts when there is no magnetic field on the hall sensor. The sensor's "quiescent" voltage is Vs/2 (in this case 5V/2) when there is no magnetic field detected. I thought that by adjusting the pot so that the voltage into the non-inverting side matched the inverting side , I would get 0 volts output from the LM358.
The feedback resistor(s) are on a switch so that I can choose 10K (no amplification) or 100K (10X amplification).
The original configuration of this circuit used the first Op Amp circuit in the LM358 dual package to perform the zeroing function (pot connected to non-inverting side, configured as a voltage follower). However, this produced the same results as the circuit presented here, so I stopped using this approach.
I have experimented with the circuit to see what I can learn.
When the feedback resistor is 10K (no amplification), the lowest output I can achieve from the LM358 is 0.63 volts. I have to adjust the pot so that the output climbs slightly higher than this to ensure that the circuit responds to the smallest of changes in the hall sensor output. By adjusting the pot to about 10K, I get an output of 0.7 volts and I can use the circuit.
When the feedback resistor is 100K (10X amplification), the lowest output I can achieve from the LM358 is 0.09 volts. Again, I have to adjust the pot so that the output climbs slightly higher than this to use the circuit. By adjusting the pot to about 18K, I get an output of approx 0.2 volts and I can use the circuit.
As I said, this is not a major problem and I can use the circuit as is to meet my needs. However, given that the effective working range of the LM331 is 0 to Vs-2 (in this case 5V-2), losing 0.6V of "range" is a little annoying. Also, given that this is the first time I have used an Op Amp (but hopefully not the last) I would like to understand this a little better.
As always, thanks in advance for help, thoughts, suggestions.
DamoRC