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Shunt current monitor in DIP package

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Could you get some SOIC to DIP adapters instead?

As you have noted, supplies of DIP packaged ICs are dwindling very fast.
 

Maybe I could, as a last resort. But still I'll have trouble soldering a SOIC package though.

What about the "over the rail" supply? What could be the downside? The lack of voltage stability? I could use a LM7808 right after the voltage doubler.

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I managed to found some rail-to-rail & low offset DIP8 opamps available: TLV2462-IP (Texas Instruments) and MCP6021-IP (Microchip). Could anyone suggest one over the other?
 

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I managed to found some rail-to-rail & low offset DIP8 opamps available: TLV2462-IP (Texas Instruments) and MCP6021-IP (Microchip). Could anyone suggest one over the other?
Those both should work but operate on 6V and 7V maximum so the power configuration must accommodate that.

Here's the simulation with a 5V supply (referenced to the rail) for those opamps.

It shows another option, RR opamp (LMC6482/LMC6484) that's available in a DIP.
It does however, have a higher offset than the two you referenced.

Capture.PNG
 
I've already ordered some MCP6021 and I got shipping confirmation. ;)

Many thanks for the simulation. I have one more question though: usually, there's no such a resistor (R3) in a transconductance amplifier loop. Is it related to the opamp used for your simulation or do I really need to add one in my circuit?
 

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Many thanks for the simulation. I have one more question though: usually, there's no such a resistor (R3) in a transconductance amplifier loop. Is it related to the opamp used for your simulation or do I really need to add one in my circuit?
I added it to stabilize the loop and avoid the overshoot and oscillations I found in the simulation, likely due to the gate capacitance of the MOSFET affecting the loop phase.
You may not need it it the actual circuit, so you could experiment with that, if you like, but my experience has been that if the simulation has a stability problem then so will the actual circuit. :???:
The resistor value may have to be adjusted to get the desired response in the built circuit, particularly since the opamp you are using has different frequency response characteristics than the ones I simulated.
 
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Just for the reference: is there any advantage of using a P-channel MOSFET instead of a PNP BJT?

I have both of them available but, like I've just said, I'm just asking in the name of science. ;) They both work in the linear region thus power disipation should be identical (V * I). Slew rate should not be a factor either..
 

Just for the reference: is there any advantage of using a P-channel MOSFET instead of a PNP BJT?
The BJT has a base-emitter current which will cause an error in the collector current as compared to the emitter current (perhaps giving an error of 1% or so in the output current).

A MOSFET has no gate current so there is no error, as the source current is identical to the drain current.
 
Oh, I see.. that's a great advantage, indeed.

That's the way to go BUT.. (for science sake, once again!) couldn't that BJT base current be compensate by a proper calibration of R2 / Rout ratio?

After all, Ie = Ic * (1 + 1/hFE). Now, if you choose a R'out = Rout * (1 + 1/hFE), you get the correct output voltage (Rout*Ie).

Of course, this calculation could be avoided if you are using a Mosfet instead but you still need to make a calibration, right? Okay, maybe that hFE is not constant over the entire Ic range (0 - 2mA, in your simulation) but the actual error could be lower than 1/hFE. Am I missing something?!

PS: I'll follow your schematics / simulation but I just want to further learn about this topology with these small talks .. ;)
 

Oh, ..............
That's the way to go BUT.. (for science sake, once again!) couldn't that BJT base current be compensate by a proper calibration of R2 / Rout ratio?
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Yes you can, but the hFE varies with current and temperature so you can only completely compensate for it at one current and one temperature.

If you used 0.1% resistors, a low offset op amp with a MOSFET, and an accurate reference voltage for the comparator, then you could probably avoid calibration for most engineering requirements. ;-)
 
Still waiting for the parts to arrive. :(

In the mean time, I found this schematic in an appnote (ST-AN4835) document:

high-side-mon.png

I've read that document but found no answer regarding the use of that 100kohm resistor (from Vcc_H to MOSFET source). It adds some component to MOSFET drain current hence that 330ohm resistor from the opamp IN- is no longer setting the MOSFET drain current alone.

Does anyone have any logical explanation of this particular topology?
 

You are correct, it is adding a very, very small current load to the supply............but you are interested in measuring the motor current, correct? Then it will not have any effect.
 

It is not just adding a small current to the load but to the Rout (that 100Kohm resistor from the MOSFET drain to the ground).

That's introducing an output error and I really don't understand what's the big catch. Every transconductance amplifier topology I've seen didn't have that sort of MOSFET biasing circuit.

I guess it might be related to the particular opamp used in that circuit (a chopper opamp).
 

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