Hello everyone,
I'm struggling with the best way to create a current sense circuit. At first, I decided to use an instrumentation amplifier for this. I have used two sense resistors, one per leg of an H-Bridge. These are run differentially into an instrumentation amplifier and produce an output that is indicitive of direction and magnitude of current. During slow-decay of the bridge, I will switch the instrumentation amplifier's gain from 10 to 5 to compensate for the 'double' current problem.
I went ahead and built this and wasn't impressed with the noise present in the circuit. I made a post about how to lay it out a bit better for the final design, but I have accepted that a lot of it had to do with the circuit design.
I want to use an opamp, like in the schematic attached, to filter out as much noise as possible. I am having a lot of trouble selecting the values for the schematic, even though there are suggestions. My sense resistor is 0.1 Ohms. Does 1K count as much greater than 0.1 Ohms, or should I go a bit lower? Also, if I am switching at 50KHz, what should I set my cutoff frequency to?
I am using an inductive load of roughly 50mH and 0.8 Ohms. I want the dynamic response to be as quick as possible.
Would connecting this value load to the bridge and monitoring the frequency content shed more light into this?
The plan is to use two of these circuits on each leg of the h-bridge, then feed them directly into an instrumentation amplifier. Another reason I wanted to do this, instead of putting an input filter on the instrumentation amplifier, is because I wanted to run a kelvin ground from the 'ground' side of the sense resistor. I can use this differentially to help cancel any noise buildup.
Any suggestions??
Steve
My other thread about layout and noise can be found here. My original 'noisy' schematic and layout are shown.
http://forum.allaboutcircuits.com/showthread.php?t=9011
I'm struggling with the best way to create a current sense circuit. At first, I decided to use an instrumentation amplifier for this. I have used two sense resistors, one per leg of an H-Bridge. These are run differentially into an instrumentation amplifier and produce an output that is indicitive of direction and magnitude of current. During slow-decay of the bridge, I will switch the instrumentation amplifier's gain from 10 to 5 to compensate for the 'double' current problem.
I went ahead and built this and wasn't impressed with the noise present in the circuit. I made a post about how to lay it out a bit better for the final design, but I have accepted that a lot of it had to do with the circuit design.
I want to use an opamp, like in the schematic attached, to filter out as much noise as possible. I am having a lot of trouble selecting the values for the schematic, even though there are suggestions. My sense resistor is 0.1 Ohms. Does 1K count as much greater than 0.1 Ohms, or should I go a bit lower? Also, if I am switching at 50KHz, what should I set my cutoff frequency to?
I am using an inductive load of roughly 50mH and 0.8 Ohms. I want the dynamic response to be as quick as possible.
Would connecting this value load to the bridge and monitoring the frequency content shed more light into this?
The plan is to use two of these circuits on each leg of the h-bridge, then feed them directly into an instrumentation amplifier. Another reason I wanted to do this, instead of putting an input filter on the instrumentation amplifier, is because I wanted to run a kelvin ground from the 'ground' side of the sense resistor. I can use this differentially to help cancel any noise buildup.
Any suggestions??
Steve
My other thread about layout and noise can be found here. My original 'noisy' schematic and layout are shown.
http://forum.allaboutcircuits.com/showthread.php?t=9011