Influence of external serial resistances on

accuracy

AN4304

16/20

DocID024731 Rev 1

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Influence of external serial resistances on accuracy

The TSC current sensing family has some trimmed input resistance. Any external

resistance, added in series, produces mismatches leading to both gain and CMR errors.

Such errors are typically calculated as follows (where Rin is the specified amplifier input

resistance):

The i

nternal resistance of the TSC103 is 5 kΩ. Assuming that an external resistor of 100 Ω,

with a tolerance of 1 %, is used for filtering, the common mode rejection ratio due to these

external components is 68 dB. There will be a direct impact on the whole CMR as the

TSC103 has a minimum CMR of 90 dB.

The fact that Rs is quite big adds to the gain error of 2 %. This really impacts the

performance of the device whose typical accuracy provides a maximum total error of 3 %.

Therefore, care must be used when introducing input filters.

The only way to control this additional gain error is to ensure that the input series resistor,

Rs, is small compared to Rin. Using a filter resistor that is less than 10 Ω is strongly

recommended. This will ensure that the high original accuracy of the TSC103 is

maintained.

Other parameters such as process variation or the temperature coefficient of the

resistances must also be taken into consideration as possible error factors of current

measurement. The calculation of total error due to external resistances is detailed in the

application note AN4369 (Adjustable gain with a current sensing).