6.12

Filtering input stage

When a current sensing works in a high frequency noisy environment, it is mandatory to correctly filter the input

of the TSC21x device, avoiding any parasitic offset due to the high frequency spikes. Both common mode and

differential mode paths must be filtered. The best scenario is to match the capacitance between them to avoid

adding differential error on the input of the current sensing.

Otherwise, it is important to combine a common mode filter and a differential mode filter to compensate the

mismatch of the capacitance. (Refer to Application note AN4304).

Figure 47. Input filtering

TSC21x

Vcc

+

-

Vref

10nF

100nF

Vicm

Rshunt

Cdiff

Ccm

Ccm

Rs

Rs

The differential voltage error due to the mismatch of the Ccm capacitance can be written:

Vdiff≅Vin. Ccm2−Ccm1

Rs.ω3. 2.Cdiff.Ccm1+Ccm12

(9)

where ω3 is the input signal frequency.

It is still possible to improve the filtering by increasing the differential capacitance value. The drawback is that the

response time increases.

The TSC21x current sensing family have some trimmed input resistance; any external resistance added in series

produces mismatches leading to both gain and CMR errors, typically calculated as follows (Rin is the specified

amplifier input resistance):

Gain error% =100−100. RinRin+Rs

(10)

CMR dB =20log.Rs. Rserror% .2

Rin

(11)

The internal resistance of the TSC210 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 is a

direct impact on the whole CMR as the TSC210 has a minimum CMR of 105 dB.

The fact that Rs is quite big adds 2%. to the gain error. Therefore, care must be taken 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 ensures that the high original accuracy of the TSC21x 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.

TSC210, TSC211, TSC212, TSC213, TSC214, TSC215

Filtering input stage

DS13237 - Rev 6

page 24/38