Electronics > Beginners
High side current sensing circuit gives wrong voltage.
Jay_Diddy_B:
Hi,
Let me use LTspice to illustrate why the original posters circuit doesn't work very well in practice.
Simplified Model
If I build a model for the shunt part of the circuit and the voltage dividers that bring the shunt signals into the common mode range of the amplifier I get:
And the circuit has the desired output 1V per Amp of load current.
Component Tolerances
As described by other contributors, the accuracy of the voltage dividers is very important in this circuit. If the two dividers are not the same a voltage will be appear between the two outputs.
In LTspice we can use Monte Carlo analysis to implement component tolerance. The format is:
{mc (nominal value, tolerance)}
{mc(10K,0.01)} this is a 10K 1% resistor.
The distribution is rectangular. That is equal probability of getting any value in the tolerance window.
.step param X 0 9 1
This causes the simulation to run 10 times, from 0 to 9 step 1
The results are:
If I increase the number of runs to 100 I get:
This is like building the circuit 100 times with 1% resistors, randomly selected.
IC manufacturers can do circuits like this, because the can trim and match the resistors.
Check LT1999
Link: https://www.analog.com/en/products/lt1999.html
I have attached the LTspice models.
Regards,
Jay_Diddy_B
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