Is there a way to share this current sense resistor?

[rmacintosh]

Attached a high level diagram of the circuit.

The green circled section is an already existing, tested and working battery charger product of mine.
The additional circuitry is being added to provide discharge and battery capacity capabilities to the charger.

I am not sure how to implement the current sense resistor. I initially decided I would design to share the current sense resistor and went ahead with that idea, I even found the LT1999 bi-directional current sense amplifier from LT to do the job of current sensing on a resistor in both directions. Then I realized that my feedback loop for controlling the FET as a load would need to connect to the node between the sense resistor and the battery -ve. Thus, with the opamp being referenced to ground the feedback loop would be feeding a slightly -ve voltage to the inverting input of the op-amp.

Is this going to work, or is there another way I am not seeing, to share the current sense resistor?
I could always just add another sense resistor, but I was trying to be slick and try to share the one sense resistor.

[german77]

If you do not want to add more components. You could measure the current directly on the fuse. But if you look in the schematics the figure 8 and 9. Have the V-in almost at ground.  So I see why not the current sense resistor could be shared. The op-amp should work fine.

But based on your schematic it will not work as it is. You have to use the lt1999 on the sense resistor to put the signal at 2.5v + current. Then use the output on the other opamp with the DAC. to actuate the FET.

[David Hess]

The operational amplifier can be configured to accept the negative voltage from the current sense resistor in inverting mode.  This is commonly done in switching regulators which include both positive and negative feedback inputs.

[rmacintosh]

OK, if I correctly understand what you are suggesting David Hess this is what I ended up with.

Seems to simulate OK. This solution would be great as the opamp I have is a dual package and I am just using the second as a buffer so as to not leave an unused floating opamp.

[Zero999]

OK, if I correctly understand what you are suggesting David Hess this is what I ended up with.

Seems to simulate OK. This solution would be great as the opamp I have is a dual package and I am just using the second as a buffer so as to not leave an unused floating opamp.

It appears you've got far too much gain, so it will oscillate. Doing a DC simulation will not show you whether it will be stable or not.

[rmacintosh]

I don’t have extensive experience running all variations of simulations but I wasn’t able to simulate the circuit instability creating oscillations.

I know that a common way to fix oscillations in this circuit is an RC network in the negative feedback loop of the opamp driving the fet. Component values chosen around the oscillation frequency.

I might have to build it to observe the oscillations and try to fix from there

[rmacintosh]

So i switched over to LTSpice, that seemed to simulate and show the oscillations

Not sure what the difference was between LT and circuitlab. I normally use LTSpice but i was trying out circuitlab 

Added the RC network in the second opamp feedback loop and oscillations gone

[Zero999]

Yes, the additional RC circuit cuts the gain at high frequencies. Note the much slower step response.

The other simulator program would have quite likely shown the oscillation, given the correct models. You need to use the time or frequency domain simulations to see them. LTSpice also does a DC simulation too: just select the DC operating point, rather than transient, when running the simulation.

[David Hess]

OK, if I correctly understand what you are suggesting David Hess this is what I ended up with.

That is the idea.  You got it.