DocID025983 Rev 1

5/14

UM1737

Theoretical and practical measurements

14

4.

Choose the appropriate configuration gain

Vout = (Rf/Rg) x Rshunt x I

To avoid saturation: Vout ≤ Voh => Rf < (Voh x Rg)/(Rshunt x Imax)

In the current application configuration, Rg = 100 Ω and Voh = 4.970 V (TSZ121 at 25 °C,

Vcc = 5 V).

Therefore, Rf max = (4.970 x 100)/(0.01 x 1) = 49.7 kΩ

Consequently, Rf must be lower than 49.7 kΩ to avoid saturation of the TSZ121 at

maximum currents. It is recommended to choose the highest possible Rf to benefit from the

output voltage capability of the amplifier. Selecting Rf in the E24 series leads to an Rf of

47 kΩ.

To minimize the offset caused by the input currents, the feedback resistors must be

minimized. The higher the Rf, the higher the error due to Iio (see Section 4.1: Theoretical

measurements). An Rg of 100 Ω must be considered (the lower Rg, the lower Rf) but, Rf

should not be so low that the output saturation voltages cannot be increased.

Note:

If the accuracy obtained is not sufficient, go back to step 2 and increase the Rshunt value.

4

Theoretical and practical measurements

4.1

Theoretical measurements

Cf can be ignored for the DC analysis.

Equation 1 can be calculated using Figure 2 as a reference for the components.

Equation 1

Equation 1 can be simplified as Equation 2 assuming that Rf2 = Rf1 = Rf and Rg2 = Rg1 =

Rg.

Equation 2

Thanks to the good matching of resistors Rf and Rg on the inputs, Iib has no affect on Vout.

In Equation 2, the only error remaining is due to Vio and Iio. The Iio represents the input

offset current (Iio = Iibp - Iibn) and the Vio represents the input offset voltage. To obtain a

precision current sensing solution, the Vio should be as low as possible. For the TSZ121,

Vio is equal to 5 µV max which corresponds to a very high accuracy.

Vout

Rs

I

×

1

Rg2

Rg2 Rf2

+

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

–

⎝⎠

⎛⎞

×

1

Rf1

Rg1

-----------

+

⎝⎠

⎛⎞

×

Iibp

Rg2

Rf2

×

Rg2 Rf2

+

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

⎝⎠

⎛⎞

1

Rf1

Rg1

-----------

+

⎝⎠

⎛⎞

×

×

Iibn

Rf1 Vio

1

Rf1

Rg1

-----------

+

⎝⎠

⎛⎞

×

–

×

–

+

=

Vout

Rs

I

Rf

Rg

--------

×

×

Vio

1

Rf

Rg

--------

+

⎝⎠

⎛⎞

×

–

Rf

Iio

×

+

=