Theoretical and practical measurements

UM1737

6/14

DocID025983 Rev 1

Considering the errors due to resistor inaccuracy, Equation 3 is obtained.

Equation 3

Where:

Vth = Rs x I x (Rf/Rg)

∆R/R is the resistance tolerance. In the current application example, the tolerance of Rg and

Rf is 0.1 % and that of Rshunt is 1 %.

If the accuracy

Equation 4

Equation 3 can be simplified as Equation 5

Equation 5

Figure 3 shows the theoretical behavior of the above defined application.

Vout_th, Vout_min, and Vout_max are referred to the left Y-axis.

Vout_min and Vout_max take into consideration the Voh and Vol errors due to input

currents, Vio, and resistor inaccuracy. In practice, the measured values should be between

the Vout_min and Vout_max curves.

Figure 3. Theoretical output voltage and error vs. Is/Imax

Vout

Vth

1 Δ

Rs

Rs

-----------

Rf

Rf Rg

+

-------------------- Δ

Rf1

Rf1

--------------

ΔRg1

Rg1

---------------

–

⎝⎠

⎛⎞

Rg

Rf Rg

+

-------------------- Δ

Rf2

Rf2

--------------

ΔRg2

Rg2

---------------

–

⎝⎠

⎛⎞

++

+

⎝⎠

⎛⎞

×

Rf

Iio Vio 1

Rf

Rg

--------

+

⎝⎠

⎛⎞

–

×

+

=

Vth Δ

Rs

Rs

-----------

Rf

Rf Rg

+

-------------------- Δ

Rf1

Rf1

--------------

ΔRg1

Rg1

---------------

+

⎝⎠

⎛⎞

Rg

Rf Rg

+

-------------------- Δ

Rf2

Rf2

--------------

ΔRg2

Rg2

---------------

+

⎝⎠

⎛⎞

++

⎝⎠

⎛⎞

Vth 2

+

()

=

Vout

Vth 1 2

++

() Rf Iio Vio 1

Rf

Rg

--------

+

⎝⎠

⎛⎞

–

×

+

=

100m

1

10

100

1E-3

0.01

0.1

1

10

1

10

100

1000

10000

Isense/Isense_max (%)

Vout_max

Maximum theoretical error on Vout

Vout_min

Vout_th