AN4366

Influence of the external resistances

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Influence of the external resistances

The values of Rs1 and Rs2 should be equal to balance the contribution on both amplifier

inputs. The TSC1031 has several trimmed input resistances. Any external resistances

added in series change the value of the original gain, K1 = 25 (see equation 5).

Where Rin is the specified amplifier input resistance.

Assuming that Rs = 100 Ω, the gain, K1, becomes:

Therefore, to keep the gain as close as possible to 25, ensure that the input series

resistors, Rs1 and Rs2, are small compared to Rin. Using a resistor that is less than 10 Ω

is strongly recommended.

To balance the contribution of Rs1 and Rs2 in the current sense amplifier gain, an output

resistor Rs3 should be connected between pin A1 and the Gnd of the TSC1031. The value

of Rs3 should be chosen according to equation 6:

To keep the gain constant (i.e. K1 = 25), let Rs3 = 250 Ω for Rs1 = 10 Ω.

To avoid an error on the gain or output offset voltage it is extremely important to:

•

keep the value of the external resistances, Rs1 and Rs2, as low as possible

•

match resistances

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

resistances must also be taken into consideration regarding the current measurement

error. All calculations of the total error due to external resistances are detailed in the

application note "AN4369 Adjustable gain with a current sensing".

Rg

125kΩ

RgRin Rs3Rs1