SPC560B64L5 datasheet(101/134 Pages) STMICROELECTRONICS

101 / 134 page

SPC560B54/6x

Electrical characteristics

Doc ID 15131 Rev 6

101/134

Figure 18.

Input equivalent circuit (extended channels)

A second aspect involving the capacitance network shall be considered. Assuming the three

capacitances CF, CP1 and CP2 are initially charged at the source voltage VA (refer to the

equivalent circuit reported in

Figure 17): A charge sharing phenomenon is installed when

the sampling phase is started (A/D switch close).

Figure 19.

Transient behavior during sampling phase

In particular two different transient periods can be distinguished:

A first and quick charge transfer from the internal capacitance CP1 and CP2 to the

sampling capacitance CS occurs (CS is supposed initially completely discharged):

considering a worst case (since the time constant in reality would be faster) in which

RSW1

CP3

VDD

Sampling

Source

Filter

Current Limiter

EXTERNAL CIRCUIT

INTERNAL CIRCUIT SCHEME

RS Source Impedance

RF Filter Resistance

CF Filter Capacitance

RL Current Limiter Resistance

RSW Channel Selection Switch Impedance (two contributions RSW1 and RSW2)

RADSampling Switch Impedance

CP Pin Capacitance (three contributions, CP1, CP2 and CP3)

CS Sampling Capacitance

CP1

RAD

Channel

Selection

CP2

Extended

RSW2

Switch

VA1

VA2

VCS

Voltage Transient on CS

ΔV < 0.5 LSB

τ1 < (RSW + RAD) CS << tS

τ2 = RL (CS + CP1 + CP2)

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