The sensing and switching could easily be done in software in MUCH less than a millisecond. As I said previously,hello,
sorry for the delay i have been out for a few weeks.
the maximum time to switch between resitors shouldn't be more than 1milisec.
Why are you resisting this approach? If you know how to program your MCU, it would be much easier than a hardware design. If you don't know how to program it, then how are you going to complete the project?Why can't you do your range switching based on the A/D output? You could set up your hysteresis in software, and use an MCU output to control the range switch.
You will probably need to use a PMOS transistor to switch Rin, due to the low resistance (2 ohms).
I may be wrong about the amount of time required to do this in software. What is the conversion time of your ADC? I forgot to consider that.I didn't think that it would be fast enough through the MCU.
I will do as you say.
Concerning the conditioning circuit before the MCU, is it ok if i use a fast and low-offset opamp(ex:LTC1050) in differential mode to probe the voltage across the sensing resistor with the apropriate gain to match the full range of the MCU ADC.
At the output i'll have 2 logic signals controlling a 1:3 DEMUX (ex:ts5a3357) rooting the proper Rin (1, 2 or 3).
OK, then there should be no problem with getting the switching done in less than a millisecond.conversion time should be about 1us and acquisition time could be a bit long, about 20us in worst case
Have you looked at the schematic and the gain equation of a difference amplifier? Look at the bottom circuit, "Amplified Difference".why will i need to switch 2 of the opamp resistor?
In your old circuit, Rin was your gain changing resistor.Yes i know that we need high precision resistors to match the ratios, but i don't get the point of changing two of the resistors when i switch from one Rin to another.
is it because you wanna change the gain??
Apparently you didn't understand the scheme I proposed.In your old circuit, Rin was your gain changing resistor.
How would you change gain with a diff amp?
Yes it was, I change Rin according to the ranges in order to match accuracy.
what i need to measure with the dif amp is the voltage across Rsense.
This voltage can go from 1.8uV to 1.8V. i want toamplifie this voltage through the dif amp in order to fill the MCU's ADC range 0->3.3V.
Here is a brief schematic of what i have in mind.
View attachment 43572
That sounds right. You will want to play with some examples to find the best thresholds.ok,
I think i got it,
i just monitor the switch in resistors depending on the output Voltage of the 6102 (Vout) compared to the full scale voltage(VMax).
and the behaviour MCU would be like:
IF VOUT<3% of VMax (just to retake your example)
THEN, RIN=RIN/100 (18)
Same thing to increase RIN
with Vfinal the registered Value to current consumption!
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