Tolerance calculations induced by adding external resistors.

AN4439

10/16

DocID025908 Rev 1

3

Tolerance calculations induced by adding external

resistors.

The L99ASC03 internal resistors have a large tolerance associated with them. The

advantage is that they will track with a very high degree of accuracy. As a result resistor

value ratios are maintained regardless of their absolute value variations. This is an

advantage as long as external resistors are not involved in the equation. Unfortunately to

add offset to our system we added two external resistors.

Some advantages are that these resistors are added to the ground side of the op-amp

system. This means that the effect is simply a DC offset. Variations in the internal resistors

with respect to the external resistors will then only affect a simple DC offset and not

adversely affect gain. As a result, the DC offset can be calibrated any time there is no

current in the ground leg of the inverter (i.e. whenever the motor is not being driven or

during freewheeling).

Using 1% resistors in Equation 13 above and inserting the worst case min and max values

for the internal resistors the variation is calculated.

Worst case high:

•

•

•

Worst case low:

•

•

•

As the gain increases the RIN value drops. This reduces the offset error due to resistor

tolerances.

Figure 6. Transfer function tolerance at a gain of 20

0

0.05

0.1

0.15

0.2

0.25

0

1

2

3

4

5

typ

min

max

Tolerance at AV=20

CSI+ (V)

GAPG2002140834MS