TSC2012 datasheet(33/54 Pages) STMICROELECTRONICS

33 / 54 page

Due to a small mismatch of the internal resistance the error, on the output common mode voltage, can be

described as equation 8:

Vocm= Vref1+Vref2

2 . 1+εAcc

(8)

Where

εAccistheaccuracyreferredtotheoutputwithatypicalvalueof0.1%.

•

Noise

The Section 4.1 expresses the noise referred to the input of the TSC2011. This device shows a 1/f noise until 10

kHz frequency. Above this limit the white noise density is 29 nV/

Hz,untilthebandwidthoftheTSC2011.

The noise can be then expressed as two terms, the former related to the 1/f noise and the latter due to the white

noise. If we consider that there is no additional filter on the TSC2011 and it is only bandwidth limited, it can be

considered that over the 750 kHz, there is an attenuation of the noise with a first order filtering. So the equivalent

noise bandwidth is

750kHz.π2.

The RMS value of the output noise is the integration of the spectral noise over the bandwidth of interest and can

be expressed as equation 9:

enRMS= ∫0.110000 29.10−9f

10.103

2df+∫0.1750000.π229.10−92df*Gain

(9)

•

Total error

The maximum total error expected on the output of the device can be described as the sum of the different source

described just above. The total output accuracy can be written as equation 10.

Vouterr=Gain*Rsense*Iload εgain+εRsense +Gain.Vio+Gain. Vicm−12V

CMR +

Vocm εAcc +noise

(10)

Iload is described in Figure 63. Input current and the output noise is described by the equation 9.

Note that the input bias currents are not taken into account in this section, as they are already integrated in the

Vsense. The Figure 63. Input current below depicts the current flowing from the source to the load when the input

common mode voltage is higher than the supply voltage.

Figure 63. Input current

From a calculation approach, when Vicm voltage is beyond Vcc, Iload must be considered as the sum of Isource and

Input bias current (Iib). Note that the input bias current on the pin – is largely lower and can be neglected.

The Figure 63. Input current also expresses that the TSC2011 cannot measure the current in the same order as

input bias current (several hundreds of µA).

TSC2010, TSC2011, TSC2012

Error calculation

DS13057 - Rev 4

page 33/54

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