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[AmpeROSE] Calibration intervals

One of the task done this week is the choice of the calibration intervals that will be used for the automatic calibration of the AmpeROSE. The first thing you need to know is that we are going to use a shunt ammeter, so this work is done to exploit this type of ammeter.

First, we fixed the voltage range that will be measured by the ADC. Since our Voltage reference will be 3 V, the maximum input is 3V.  Then we decided to have a minimum input voltage of 10 mV because it becomes hard to convert a voltage around 1 mV precisely.

With these values and the maximum allowed burden voltage of 50 mV, we can calculate the calibration intervals. First, we deduce the amplification needed from the maximum burden voltage and maximum input of the ADC, we obtain an amplification of 60. Then, we calculate the minimum burden voltage which is 166.67 µV. These values define the intervals which have the following form [Rshunt*50e-3; Rshunt*166.67e-6] (or [x; x*300] where x is the minimum current measured).

With all that we got the following intervals (with an ADC of 24 bits):

Current min (A)

Rshunt(Ω)

Current max (A)

Resolution (A)

Error max

Error min

Interval 1

1.00E-09

166666.6667

0.0000003

1.79E-14

0.0017881393%

0.0000059605%

Interval 2

1.00E-07

1666.666667

0.00003

1.79E-12

0.0017881393%

0.0000059605%

Interval 3

1.00E-05

16.66666667

0.003

1.79E-10

0.0017881393%

0.0000059605%

Interval 4

1.00E-03

0.166666667

0.3

1.79E-08

0.0017881393%

0.0000059605%

 

But here we have a problem which is that we cannot have a resistor of 166.67 kΩ or 166.67 mΩ. Moreover, we cannot have any value we want but only multiples of the normalized values. The closest normalized value for us is 165 which will allow us to have resistor of 165 kΩ in the first interval for example. With the normalized values we obtain slightly different values which are the following ones:

 

Current min (A)

Rshunt(Ω)

Current max (A)

Resolution (A)

Error max

Error min

Interval 1

1,0101E-09

165000

3,0303E-07

1,81E-14

0,0017881393%

0,0000059605%

Interval 2

1,0101E-07

1650

3,0303E-05

1,81E-12

0,0017881393%

0,0000059605%

Interval 3

1,0101E-05

16,5

0,003030303

1,81E-10

0,0017881393%

0,0000059605%

Interval 4

0,001010101

0,165

0,303030303

1,81E-08

0,0017881393%

0,0000059605%

 

These values are good for us because we still are able to measure up to 300 mA and 10 nA.

1 comment to [AmpeROSE] Calibration intervals

  • Phh

    Some quick drawing might be useful to better understand Rshunt/burden voltage

    What’s minimum burden voltage? That’s the minimum quantum of burden voltage measurement?

    What’s the error related to resistor imprecision?

    What’s the error related to the burden voltage if we speak in Watt? (Since you don’t measure the input voltage)

    If the input of a chip/board goes down by maximum burden voltage, how is power consumption affected?

    Related to the above, how has maximum burden voltage been computed?

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