Current source

AN4366

10/14

DocID025330 Rev 1

5

Current source

The current necessary to compensate the Vos should be high enough not to be impacted

by the gain error and low enough to allow reasonable values of Rs1 and Rs2 to be chosen.

Note that the current is generated by the current source and that gain error may appear in

temperature due to the external resistors.

To achieve good accuracy and to maintain the initial gain, the external resistances are set

as follow:

•

Rs

1 = Rs2 = 10 Ω

•

Rs3 = 250 Ω which maintains K1 = 25

•

K2 = 4 by setting SEL = Vcc+

In this configuration the total gain is: AV = K1.K2 = 100.

If we consider the worst situation, i.e. when the TSC1031 has a positive Vos offset of

+1.1 mv, in this case the output of the TSC1031 could become saturated. To avoid this,

add a negative offset compensation of 2 mV. Then, we can deduce that the current which

must be drawn by the current source is:

The current delivered by the current source is mainly fixed thanks to the reference voltage

TL431 (2.5V) and the resistance Rc4. So,

A possible op-amp to drive the NPN transistor is the automotive grade LMV821A (order

code LMV821AIYLT).

Rs2 2mV10Ω

200μA