Full Version: Current measurement circuit
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MPU_Beginner
Apr 12 2011, 9:20 AM
Hey,
I need to design a circuit to measure the current consumption of my uP using a current shunt monitor. I have already chosen the current shunt monitor which offers a very large common mode range. The current i want to measure is 1uA ... 100 mA. Then the analog values have to be forwarded to an ADC etc...
Any proposals?

coolmirza143
Apr 12 2011, 3:46 PM

Attachment

ExperimenterUK
Apr 12 2011, 5:37 PM

Hey,
I need to design a circuit to measure the current consumption of my uP using a current shunt monitor. I have already chosen the current shunt monitor which offers a very large common mode range. The current i want to measure is 1uA ... 100 mA. Then the analog values have to be forwarded to an ADC etc...
Any proposals?

MPU_Beginner

1uA ... 100 mA is a very wide range, too wide for a simple ADC.
What current shunt monitor are you using ?

rumman89
Apr 12 2011, 5:56 PM
I also used the same circuit of a transimpedance apmlfr ,
i got a very stable output on op-amp
I m facing problems in ADC

when I interfaced op-amp with ADC ,i am getting horribe fluctuations .The
ADC value jumps over a big range .even when the input which is output from opamp is stable.

MPU_Beginner
Apr 13 2011, 8:35 AM
@rumman89: how does your circuit look like? Which ADC and aamplfr do u use?

I thought about using TINA213 from Texas Instruments... The problem is, using a single shunt resistor will never be sufficient to cover the whole current range...
I'm using a 12 bit ADC with 2,5V ref voltage...

how can I decide for the shunt resistor value/values?!

thx
rumman89
Apr 13 2011, 11:32 AM
here is the circuit ...
I used simple op-amp 741 and 1 one mega ohm resistr
actually it depends on the range of current you want to measure ,,
just remember the formula ...
V(output voltg.)=R*I
where 'R' is the feedback res. u use
and 'I' is the input current
Attachment
MPU_Beginner
Apr 14 2011, 8:47 AM
sorry but I did not really get it, how u connect your circuit to the uP.
Please see my circuit attached, is that what you mean?
Thanks!

rumman89
Apr 14 2011, 3:48 PM
ok ,,u said your currnet range is from 1uA ... ,,ryt ?? ,,

then try a resistor of 2 to 3 mega ohms ,,you should get an output from
2 to 3 vlts according to the formula ,,

but make sure that current flows from right to left ,,(opposite to the direction shown in your diagram) to get a +ve output vltg ,as the op-amp is used in inverting mode.
rumman89
Apr 14 2011, 3:51 PM
One important thing after getting the gain is,, "stabilibty" of the circuit,
a feedback capacitor must be connected in parallel with the feedback resistor to gain stability ,,even I am workin on it to find the value of the capacitor.
I will tell it later ,,first you try to get the required output vltg ...
MPU_Beginner
Apr 14 2011, 3:53 PM
Do u think that the way shown in my diagramm is correct?! To which pin of the uP will u connect the Amp?
I thought I will connect it to the supply voltage pin, BUT the way shown in my diagramm the input voltage of the uP is different from the voltage given by the source (3V)!
rumman89
Apr 14 2011, 4:02 PM
connect that pin of to uP to op-amp whose current you want to measure ,,

connect it to the non-inverting terminal as shown in the diagram
rumman89
Apr 14 2011, 4:08 PM
sorry I meant inverting terminal ,,
just make sure that the current you want to measure flows throw op-amp ,,via inverting terminal ...as in your diagram ,,butt in opposite direction
MPU_Beginner
Apr 14 2011, 4:10 PM
please exuse my ignorence. Look at this diagramm, I think this the way to measure the current flowing through a resistor which s supplied by 3V. Is this true?

Now I want to achieve 2 things: connect the power-supply-pin of the uP with the voltage source, and then measure its current. In the diagramm shown here, the resistor has 2 pins, one goes to power supply and one to -ive input of the Amp... so how about the uP? which pin goes to the Amp
MPU_Beginner
Apr 14 2011, 4:13 PM
sorry I think the image has not been attached
rumman89
Apr 14 2011, 4:19 PM
heyy ,,first let me be clear ,,you want to measure current ...ryt ??

then you dont need a resistr at input side ,, just a feedback resistr is enough .
rumman89
Apr 14 2011, 4:22 PM
and I dont know about single power supply op amp ,,
SI used dual power supply op-amp
with +5 and -5V powr sply
MPU_Beginner
Apr 14 2011, 4:23 PM
Yes rumman, I want to measure the current and the diagramm above shows the circuit, used to measure the current through 'Last' resistor. Yes we don't need it, and it does not belong the measurement circuit. its gonna be replaced by the uP.

Now which pin exactly do I have to connect to the -tive pin if the amp?
MPU_Beginner
Apr 14 2011, 4:25 PM

and I dont know about single power supply op amp ,,
SI used dual power supply op-amp
with +5 and -5V powr sply

rumman89

Ok forget about the op amp first, the thing I don't understand is which pin of the uP do I have to connect to the IN- of the amp!

Thank you for your patience :-)
rumman89
Apr 14 2011, 4:33 PM
see its like this,, you need to connect that pin from which current to be measured should flow throw the op-amp ,,as the op-amp have infinite input res ideally all this currenl wil flow throw the feedback path ..
MPU_Beginner
Apr 15 2011, 12:44 PM
Sorry Rumman, I did not really figured out what do you mean, it would be really awesome if you can post a diagram and mention exactly how does your circuit looks like... I was talking to an anlog designer and he told me this never would be possible!
majoka
Apr 15 2011, 1:23 PM
here is an opamp circuits that can measure current through a load
at output voltage will be equal to the amount of current

Attachment
MPU_Beginner
Apr 15 2011, 1:50 PM
Hi Majoka, which current are u measuring in each circuit? (current through which resistor?) Is it possible to use this circuits to measure the current of a uP?
majoka
Apr 15 2011, 1:52 PM
@ MPU_Beginner
microprocessor has too low currents
this circuit has less efficiency at low currents
for that u has to use maxim current measuring chips
MPU_Beginner
Apr 15 2011, 1:55 PM
Actually I wanted to build my own circuit using a transimpedance amplifier, as suggested above in this thread, which output voltgae is qual to the feedback resistor multiplied by the current to be measured.
Is there any way to apply that to current measurement for uP?
Thx
rumman89
Apr 15 2011, 4:24 PM

Sorry Rumman, I did not really figured out what do you mean, it would be really awesome if you can post a diagram and mention exactly how does your circuit looks like...

MPU_Beginner

hey bro here is my circuit ,

rumman89
Apr 15 2011, 4:30 PM
I used ,,
Rf = 1 mega ohms
powr supply=+/- 5 volts

and Cf ,, I am still working on to find its exact value for stability

one more change in above circuit that I connected cathode to inverting terminal of op-amp and anoode is grounded ,,so that current flows from right to left in feedback resistor branch and I get a positive output vltg ,,