LQFP176 datasheet(98/133 Pages) STMICROELECTRONICS

98 / 133 page

Electrical characteristics

SPC560B54x/6x

98/133

DocID15131 Rev 9

impedance of the transducer or circuit supplying the analog signal to be measured. The filter

at the input pins must be designed taking into account the dynamic characteristics of the

input signal (bandwidth) and the equivalent input impedance of the ADC itself.

In fact a current sink contributor is represented by the charge sharing effects with the

sampling capacitance: being CS and Cp2 substantially two switched capacitances, with a

frequency equal to the conversion rate of the ADC, it can be seen as a resistive path to

ground. For instance, assuming a conversion rate of 1 MHz, with CS+Cp2 equal to 3 pF, a

resistance of 330 k

 is obtained (R

EQ = 1 / (fc × (CS+Cp2)), where fc represents the

conversion rate at the considered channel). To minimize the error induced by the voltage

partitioning between this resistance (sampled voltage on CS+Cp2) and the sum of RS + RF,

the external circuit must be designed to respect the Equation 4:

Equation 4

Equation 4 generates a constraint for external network design, in particular on a resistive

path.

Figure 17. Input equivalent circuit (precise channels)

---------------------



---LSB



RSW1

CP2

VDD

Sampling

Source

Filter

Current Limiter

EXTERNAL CIRCUIT

INTERNAL CIRCUIT SCHEME

RS Source Impedance

RF Filter Resistance

CF Filter Capacitance

RL Current Limiter Resistance

RSW1 Channel Selection Switch Impedance

RADSampling Switch Impedance

CP Pin Capacitance (two contributions, CP1 and CP2)

CS Sampling Capacitance

CP1

RAD

Channel

Selection

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