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Q&A Should I connect signal reference and chassis ground in a battery powered application?

It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected. If the circuit is not intended to be truly isolated...

posted 4y ago by Olin Lathrop‭  ·  edited 4y ago by Olin Lathrop‭

Answer
#6: Post edited by user avatar Olin Lathrop‭ · 2020-07-31T13:03:00Z (over 4 years ago)
  • It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected.
  • If the circuit is not intended to be truly isolated, then signal and chassis ground need to be connected at some point. The trick is to think about all the ground currents, and make sure that things like power return currents or ground offset currents don't flow along the signal ground wire.
  • Generally, this means the signal and chassis grounds should be connected <i>in exactly one place</i>. That lets the signal follow whatever common mode voltage is on the chassis ground, but does not allow chassis ground currents to flow thru the signal ground wire.
  • The best place for this single connection is usually right where the signal ground wire enters the ground-connected box. Any other ground connections between the circuit and the chassis must then be avoided.
  • Sometimes it is more convenient to let the signal wires feed thru a small hole in the chassis to connection points on the PCB. In that case, connect the PCB ground to the chassis ground close to where the signal ground is connected to the PCB. I sometimes do this with a mounting screw hole that is also a PCB pad, with the holes for all the other mounting screws insulated.
  • Think of the chassis as shunting unwanted ground currents around the circuit. You have to assume there are currents flowing between any two points of the chassis. By connecting the circuit to the chassis in exactly one spot, these chassis currents can't flow thru the circuit. If you connect the circuit to the chassis at two points, then the currents that chassis was meant to shunt might flow thru your circuit, causing ground offsets.
  • <blockquote>
  • what happens when the two load cells are placed on the rail (see diagram) and the chassis ground is bound to be at earth potential?</blockquote>
  • That depends on what this undefined "rail" thing is, and then possibly whether the sensors are electrically connected to it.
  • If the sensors are not close to the box, then you should be using differential signals.
  • If they are really far, then you should consider isolating the immediate analog input circuitry, with data shipped to the rest of the circuit digitally over opto-couplers or something. That would require a small isolated power supply, enough to power the analog circuitry and microcontroller to manage the conversions and ship out the digital values. I've done that a few times using a small POE transformer in flyback mode.
  • <blockquote>"rail" is a single railroad track</blockquote>
  • I re-read your whole question, and things make more sense now. You did mention railroad track before, but I had forgotten that by the time the diagram was added.
  • In general, you have too many ground connections, and therefore need to consider ground loops carefully.
  • One advantage you have is that your circuit is isolated due to being battery powered. However, that no longer applies when you connect the USB charging input to something that is ground-referenced. If normal operation is always without a USB connection, then you get to consider this isolation as part of the design.
  • I would tie all the "external" grounds together everywhere, but make sure it never carries any signal. These external grounds would be the rail, the two cable shields, and the box your circuit is in. This becomes a shield, connected to earth by the rail. This should be connected to your circuit ground in exactly one place. I'd probably make one of the mounting screws carry the ground connection.
  • The two data wires from each sensor carry a single differential signal, and should be received that way by your analog front end. Each would have a little filtering immediately where they connect to the PCB, then go into an instrumentation amp. The point of the filtering is to squash frequencies, like radio pickup, that the diff amp can't handle.
  • The tricky question is what to do with the two 0 V lines coming from the sensors. This depends on whether the sensors are electrically connected to the rail.
  • Ideally the sensor is not electrically connected to the rail, in which case you don't really have a "too many grounds" problem. If the senors are connected to their external mounting plates, maybe you can put an insulating pad between the mounting plates and the rail. Or, get a different model of sensor that is insulated. Note that the cable shields should still be connected to the rail.
  • If the 0 V lines and the cable shields are unavoidably connected at the sensors, then things get more tricky. You need to tie the 0 V lines to your analog circuit ground because that carries the return current of the 5 V lines. In this case, I would not add a deliberate connection between your circuit ground and the chassis. You basically already have two such connections, one at each sensor.
  • It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected.
  • If the circuit is not intended to be truly isolated, then signal and chassis ground need to be connected at some point. The trick is to think about all the ground currents, and make sure that things like power return currents or ground offset currents don't flow along the signal ground wire.
  • Generally, this means the signal and chassis grounds should be connected <i>in exactly one place</i>. That lets the signal follow whatever common mode voltage is on the chassis ground, but does not allow chassis ground currents to flow thru the signal ground wire.
  • The best place for this single connection is usually right where the signal ground wire enters the ground-connected box. Any other ground connections between the circuit and the chassis must then be avoided.
  • Sometimes it is more convenient to let the signal wires feed thru a small hole in the chassis to connection points on the PCB. In that case, connect the PCB ground to the chassis ground close to where the signal ground is connected to the PCB. I sometimes do this with a mounting screw hole that is also a PCB pad, with the holes for all the other mounting screws insulated.
  • Think of the chassis as shunting unwanted ground currents around the circuit. You have to assume there are currents flowing between any two points of the chassis. By connecting the circuit to the chassis in exactly one spot, these chassis currents can't flow thru the circuit. If you connect the circuit to the chassis at two points, then the currents that chassis was meant to shunt might flow thru your circuit, causing ground offsets.
  • <blockquote>
  • what happens when the two load cells are placed on the rail (see diagram) and the chassis ground is bound to be at earth potential?</blockquote>
  • That depends on what this undefined "rail" thing is, and then possibly whether the sensors are electrically connected to it.
  • If the sensors are not close to the box, then you should be using differential signals.
  • If they are really far, then you should consider isolating the immediate analog input circuitry, with data shipped to the rest of the circuit digitally over opto-couplers or something. That would require a small isolated power supply, enough to power the analog circuitry and microcontroller to manage the conversions and ship out the digital values. I've done that a few times using a small POE transformer in flyback mode.
  • <blockquote>"rail" is a single railroad track</blockquote>
  • I re-read your whole question, and things make more sense now. You did mention railroad track before, but I had forgotten that by the time the diagram was added.
  • In general, you have too many ground connections, and therefore need to consider ground loops carefully.
  • One advantage you have is that your circuit is isolated due to being battery powered. However, that no longer applies when you connect the USB charging input to something that is ground-referenced. If normal operation is always without a USB connection, then you get to consider this isolation as part of the design.
  • I would tie all the "external" grounds together everywhere, but make sure it never carries any signal. These external grounds would be the rail, the two cable shields, and the box your circuit is in. This becomes a shield, connected to earth by the rail. This should be connected to your circuit ground in exactly one place. I'd probably make one of the mounting screws carry the ground connection.
  • The two data wires from each sensor carry a single differential signal, and should be received that way by your analog front end. Each would have a little filtering immediately where they connect to the PCB, then go into an instrumentation amp. The point of the filtering is to squash frequencies, like radio pickup, that the diff amp can't handle.
  • The tricky question is what to do with the two 0 V lines coming from the sensors. This depends on whether the sensors are electrically connected to the rail.
  • Ideally the sensor is not electrically connected to the rail, in which case you don't really have a "too many grounds" problem. If the senors are connected to their external mounting plates, maybe you can put an insulating pad between the mounting plates and the rail. Or, get a different model of sensor that is insulated. Note that the cable shields should still be connected to the rail.
  • If the 0 V lines and the cable shields are unavoidably connected at the sensors, then things get more tricky. You need to tie the 0 V lines to your analog circuit ground because that carries the return current of the 5 V lines. In this case, I would not add a deliberate connection between your circuit ground and the chassis. You basically already have two such connections, one at each sensor.
  • <h2>Added</h2>
  • From your latest description, it sounds like you have done all the reasonable things. I don't know what else I can say from here.
#5: Post edited by user avatar Olin Lathrop‭ · 2020-07-30T13:09:02Z (over 4 years ago)
  • It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected.
  • If the circuit is not intended to be truly isolated, then signal and chassis ground need to be connected at some point. The trick is to think about all the ground currents, and make sure that things like power return currents or ground offset currents don't flow along the signal ground wire.
  • Generally, this means the signal and chassis grounds should be connected <i>in exactly one place</i>. That lets the signal follow whatever common mode voltage is on the chassis ground, but does not allow chassis ground currents to flow thru the signal ground wire.
  • The best place for this single connection is usually right where the signal ground wire enters the ground-connected box. Any other ground connections between the circuit and the chassis must then be avoided.
  • Sometimes it is more convenient to let the signal wires feed thru a small hole in the chassis to connection points on the PCB. In that case, connect the PCB ground to the chassis ground close to where the signal ground is connected to the PCB. I sometimes do this with a mounting screw hole that is also a PCB pad, with the holes for all the other mounting screws insulated.
  • Think of the chassis as shunting unwanted ground currents around the circuit. You have to assume there are currents flowing between any two points of the chassis. By connecting the circuit to the chassis in exactly one spot, these chassis currents can't flow thru the circuit. If you connect the circuit to the chassis at two points, then the currents that chassis was meant to shunt might flow thru your circuit, causing ground offsets.
  • <blockquote>
  • what happens when the two load cells are placed on the rail (see diagram) and the chassis ground is bound to be at earth potential?</blockquote>
  • That depends on what this undefined "rail" thing is, and then possibly whether the sensors are electrically connected to it.
  • If the sensors are not close to the box, then you should be using differential signals.
  • If they are really far, then you should consider isolating the immediate analog input circuitry, with data shipped to the rest of the circuit digitally over opto-couplers or something. That would require a small isolated power supply, enough to power the analog circuitry and microcontroller to manage the conversions and ship out the digital values. I've done that a few times using a small POE transformer in flyback mode.
  • It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected.
  • If the circuit is not intended to be truly isolated, then signal and chassis ground need to be connected at some point. The trick is to think about all the ground currents, and make sure that things like power return currents or ground offset currents don't flow along the signal ground wire.
  • Generally, this means the signal and chassis grounds should be connected <i>in exactly one place</i>. That lets the signal follow whatever common mode voltage is on the chassis ground, but does not allow chassis ground currents to flow thru the signal ground wire.
  • The best place for this single connection is usually right where the signal ground wire enters the ground-connected box. Any other ground connections between the circuit and the chassis must then be avoided.
  • Sometimes it is more convenient to let the signal wires feed thru a small hole in the chassis to connection points on the PCB. In that case, connect the PCB ground to the chassis ground close to where the signal ground is connected to the PCB. I sometimes do this with a mounting screw hole that is also a PCB pad, with the holes for all the other mounting screws insulated.
  • Think of the chassis as shunting unwanted ground currents around the circuit. You have to assume there are currents flowing between any two points of the chassis. By connecting the circuit to the chassis in exactly one spot, these chassis currents can't flow thru the circuit. If you connect the circuit to the chassis at two points, then the currents that chassis was meant to shunt might flow thru your circuit, causing ground offsets.
  • <blockquote>
  • what happens when the two load cells are placed on the rail (see diagram) and the chassis ground is bound to be at earth potential?</blockquote>
  • That depends on what this undefined "rail" thing is, and then possibly whether the sensors are electrically connected to it.
  • If the sensors are not close to the box, then you should be using differential signals.
  • If they are really far, then you should consider isolating the immediate analog input circuitry, with data shipped to the rest of the circuit digitally over opto-couplers or something. That would require a small isolated power supply, enough to power the analog circuitry and microcontroller to manage the conversions and ship out the digital values. I've done that a few times using a small POE transformer in flyback mode.
  • <blockquote>"rail" is a single railroad track</blockquote>
  • I re-read your whole question, and things make more sense now. You did mention railroad track before, but I had forgotten that by the time the diagram was added.
  • In general, you have too many ground connections, and therefore need to consider ground loops carefully.
  • One advantage you have is that your circuit is isolated due to being battery powered. However, that no longer applies when you connect the USB charging input to something that is ground-referenced. If normal operation is always without a USB connection, then you get to consider this isolation as part of the design.
  • I would tie all the "external" grounds together everywhere, but make sure it never carries any signal. These external grounds would be the rail, the two cable shields, and the box your circuit is in. This becomes a shield, connected to earth by the rail. This should be connected to your circuit ground in exactly one place. I'd probably make one of the mounting screws carry the ground connection.
  • The two data wires from each sensor carry a single differential signal, and should be received that way by your analog front end. Each would have a little filtering immediately where they connect to the PCB, then go into an instrumentation amp. The point of the filtering is to squash frequencies, like radio pickup, that the diff amp can't handle.
  • The tricky question is what to do with the two 0 V lines coming from the sensors. This depends on whether the sensors are electrically connected to the rail.
  • Ideally the sensor is not electrically connected to the rail, in which case you don't really have a "too many grounds" problem. If the senors are connected to their external mounting plates, maybe you can put an insulating pad between the mounting plates and the rail. Or, get a different model of sensor that is insulated. Note that the cable shields should still be connected to the rail.
  • If the 0 V lines and the cable shields are unavoidably connected at the sensors, then things get more tricky. You need to tie the 0 V lines to your analog circuit ground because that carries the return current of the 5 V lines. In this case, I would not add a deliberate connection between your circuit ground and the chassis. You basically already have two such connections, one at each sensor.
#4: Post edited by user avatar Olin Lathrop‭ · 2020-07-28T18:44:40Z (over 4 years ago)
  • It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected.
  • If the circuit is not intended to be truly isolated, then signal and chassis ground need to be connected at some point. The trick is to think about all the ground currents, and make sure that things like power return currents or ground offset currents don't flow along the signal ground wire.
  • Generally, this means the signal and chassis grounds should be connected <i>in exactly one place</i>. That lets the signal follow whatever common mode voltage is on the chassis ground, but does not allow chassis ground currents to flow thru the signal ground wire.
  • The best place for this single connection is usually right where the signal ground wire enters the ground-connected box. Any other ground connections between the circuit and the chassis must then be avoided.
  • Sometimes it is more convenient to let the signal wires feed thru a small hole in the chassis to connection points on the PCB. In that case, connect the PCB ground to the chassis ground close to where the signal ground is connected to the PCB. I sometimes do this with a mounting screw hole that is also a PCB pad, with the holes for all the other mounting screws insulated.
  • Think of the chassis as shunting unwanted ground currents around the circuit. You have to assume there are currents flowing between any two points of the chassis. By connecting the circuit to the chassis in exactly one spot, these chassis currents can't flow thru the circuit. If you connect the circuit to the chassis at two points, then the currents that chassis was meant to shunt might flow thru your circuit, causing ground offsets.
  • <blockquote>
  • what happens when the two load cells are placed on the rail (see diagram) and the chassis ground is bound to be at earth potential?</blockquote>
  • That depends on what this undefined "rail" thing is, and the possibly whether the sensors are electrically connected to it.
  • It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected.
  • If the circuit is not intended to be truly isolated, then signal and chassis ground need to be connected at some point. The trick is to think about all the ground currents, and make sure that things like power return currents or ground offset currents don't flow along the signal ground wire.
  • Generally, this means the signal and chassis grounds should be connected <i>in exactly one place</i>. That lets the signal follow whatever common mode voltage is on the chassis ground, but does not allow chassis ground currents to flow thru the signal ground wire.
  • The best place for this single connection is usually right where the signal ground wire enters the ground-connected box. Any other ground connections between the circuit and the chassis must then be avoided.
  • Sometimes it is more convenient to let the signal wires feed thru a small hole in the chassis to connection points on the PCB. In that case, connect the PCB ground to the chassis ground close to where the signal ground is connected to the PCB. I sometimes do this with a mounting screw hole that is also a PCB pad, with the holes for all the other mounting screws insulated.
  • Think of the chassis as shunting unwanted ground currents around the circuit. You have to assume there are currents flowing between any two points of the chassis. By connecting the circuit to the chassis in exactly one spot, these chassis currents can't flow thru the circuit. If you connect the circuit to the chassis at two points, then the currents that chassis was meant to shunt might flow thru your circuit, causing ground offsets.
  • <blockquote>
  • what happens when the two load cells are placed on the rail (see diagram) and the chassis ground is bound to be at earth potential?</blockquote>
  • That depends on what this undefined "rail" thing is, and then possibly whether the sensors are electrically connected to it.
  • If the sensors are not close to the box, then you should be using differential signals.
  • If they are really far, then you should consider isolating the immediate analog input circuitry, with data shipped to the rest of the circuit digitally over opto-couplers or something. That would require a small isolated power supply, enough to power the analog circuitry and microcontroller to manage the conversions and ship out the digital values. I've done that a few times using a small POE transformer in flyback mode.
#3: Post edited by user avatar Olin Lathrop‭ · 2020-07-28T18:35:17Z (over 4 years ago)
  • It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected.
  • If the circuit is not intended to be truly isolated, then signal and chassis ground need to be connected at some point. The trick is to think about all the ground currents, and make sure that things like power return currents or ground offset currents don't flow along the signal ground wire.
  • Generally, this means the signal and chassis grounds should be connected <i>in exactly one place</i>. That lets the signal follow whatever common mode volate is on the chassis ground, but does not allow chassis ground currents to flow thru the signal ground wire.
  • The best place for this single connection is usually right where the signal ground wire enters the ground-connected box. Any other ground connections between the circuit and the chassis must then be avoided.
  • Sometimes it is more convenient to let the signal wires feed thru a small hole in the chassis to connection points on the PCB. In that case, connect the PCB ground to the chassis ground close to where the signal ground is connected to the PCB. I sometimes do this with a mounting screw hole that is also a PCB pad, with the holes for all the other mounting screws insulated.
  • Think of the chassis as shunting unwanted ground currents around the circuit. You have to assume there are currents flowing between any two points of the chassis. By connecting the circuit to the chassis in exactly one spot, these chassis currents can't flow thru the circuit. If you connect the circuit to the chassis at two points, then the currents that chassis was meant to shunt might flow thru your circuit, causing ground offsets.
  • <blockquote>
  • what happens when the two load cells are placed on the rail (see diagram) and the chassis ground is bound to be at earth potential?</blockquote>
  • That depends on what this undefined "rail" thing is, and the possibly whether the sensors are electrically connected to it.
  • It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected.
  • If the circuit is not intended to be truly isolated, then signal and chassis ground need to be connected at some point. The trick is to think about all the ground currents, and make sure that things like power return currents or ground offset currents don't flow along the signal ground wire.
  • Generally, this means the signal and chassis grounds should be connected <i>in exactly one place</i>. That lets the signal follow whatever common mode voltage is on the chassis ground, but does not allow chassis ground currents to flow thru the signal ground wire.
  • The best place for this single connection is usually right where the signal ground wire enters the ground-connected box. Any other ground connections between the circuit and the chassis must then be avoided.
  • Sometimes it is more convenient to let the signal wires feed thru a small hole in the chassis to connection points on the PCB. In that case, connect the PCB ground to the chassis ground close to where the signal ground is connected to the PCB. I sometimes do this with a mounting screw hole that is also a PCB pad, with the holes for all the other mounting screws insulated.
  • Think of the chassis as shunting unwanted ground currents around the circuit. You have to assume there are currents flowing between any two points of the chassis. By connecting the circuit to the chassis in exactly one spot, these chassis currents can't flow thru the circuit. If you connect the circuit to the chassis at two points, then the currents that chassis was meant to shunt might flow thru your circuit, causing ground offsets.
  • <blockquote>
  • what happens when the two load cells are placed on the rail (see diagram) and the chassis ground is bound to be at earth potential?</blockquote>
  • That depends on what this undefined "rail" thing is, and the possibly whether the sensors are electrically connected to it.
#2: Post edited by user avatar Olin Lathrop‭ · 2020-07-28T18:31:59Z (over 4 years ago)
  • It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected.
  • If the circuit is not intended to be truly isolated, then signal and chassis ground need to be connected at some point. The trick is to think about all the ground currents, and make sure that things like power return currents or ground offset currents don't flow along the signal ground wire.
  • Generally, this means the signal and chassis grounds should be connected <i>in exactly one place</i>. That lets the signal follow whatever common mode volate is on the chassis ground, but does not allow chassis ground currents to flow thru the signal ground wire.
  • The best place for this single connection is usually right where the signal ground wire enters the ground-connected box. Any other ground connections between the circuit and the chassis must then be avoided.
  • Sometimes it is more convenient to let the signal wires feed thru a small hole in the chassis to connection points on the PCB. In that case, connect the PCB ground to the chassis ground close to where the signal ground is connected to the PCB. I sometimes do this with a mounting screw hole that is also a PCB pad, with the holes for all the other mounting screws insulated.
  • Think of the chassis as shunting unwanted ground currents around the circuit. You have to assume there are currents flowing between any two points of the chassis. By connecting the circuit to the chassis in exactly one spot, these chassis currents can't flow thru the circuit. If you connect the circuit to the chassis at two points, then the currents that chassis was meant to shunt might flow thru your circuit, causing ground offsets.
  • It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected.
  • If the circuit is not intended to be truly isolated, then signal and chassis ground need to be connected at some point. The trick is to think about all the ground currents, and make sure that things like power return currents or ground offset currents don't flow along the signal ground wire.
  • Generally, this means the signal and chassis grounds should be connected <i>in exactly one place</i>. That lets the signal follow whatever common mode volate is on the chassis ground, but does not allow chassis ground currents to flow thru the signal ground wire.
  • The best place for this single connection is usually right where the signal ground wire enters the ground-connected box. Any other ground connections between the circuit and the chassis must then be avoided.
  • Sometimes it is more convenient to let the signal wires feed thru a small hole in the chassis to connection points on the PCB. In that case, connect the PCB ground to the chassis ground close to where the signal ground is connected to the PCB. I sometimes do this with a mounting screw hole that is also a PCB pad, with the holes for all the other mounting screws insulated.
  • Think of the chassis as shunting unwanted ground currents around the circuit. You have to assume there are currents flowing between any two points of the chassis. By connecting the circuit to the chassis in exactly one spot, these chassis currents can't flow thru the circuit. If you connect the circuit to the chassis at two points, then the currents that chassis was meant to shunt might flow thru your circuit, causing ground offsets.
  • <blockquote>
  • what happens when the two load cells are placed on the rail (see diagram) and the chassis ground is bound to be at earth potential?</blockquote>
  • That depends on what this undefined "rail" thing is, and the possibly whether the sensors are electrically connected to it.
#1: Initial revision by user avatar Olin Lathrop‭ · 2020-07-27T16:56:57Z (over 4 years ago) 
It's not totally clear from just the verbal description, but it seems the real question is when/if signal and chassis ground should be connected.

If the circuit is not intended to be truly isolated, then signal and chassis ground need to be connected at some point.  The trick is to think about all the ground currents, and make sure that things like power return currents or ground offset currents don't flow along the signal ground wire.

Generally, this means the signal and chassis grounds should be connected <i>in exactly one place</i>.  That lets the signal follow whatever common mode volate is on the chassis ground, but does not allow chassis ground currents to flow thru the signal ground wire.

The best place for this single connection is usually right where the signal ground wire enters the ground-connected box.  Any other ground connections between the circuit and the chassis must then be avoided.

Sometimes it is more convenient to let the signal wires feed thru a small hole in the chassis to connection points on the PCB.  In that case, connect the PCB ground to the chassis ground close to where the signal ground is connected to the PCB.  I sometimes do this with a mounting screw hole that is also a PCB pad, with the holes for all the other mounting screws insulated.

Think of the chassis as shunting unwanted ground currents around the circuit.  You have to assume there are currents flowing between any two points of the chassis.  By connecting the circuit to the chassis in exactly one spot, these chassis currents can't flow thru the circuit.  If you connect the circuit to the chassis at two points, then the currents that chassis was meant to shunt might flow thru your circuit, causing ground offsets.