SPC56EC64L8 datasheet(83/118 Pages) STMICROELECTRONICS

83 / 118 page

SPC564Bxx - SPC56ECxx

Electrical Characteristics

Doc ID 17478 Rev 4

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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: CS being substantially a switched capacitance, 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 equal to 3 pF, a resistance of 330

Ω is obtained (REQ = 1 / (fc × CS), 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) and the sum of RS + RF + RL + RSW + RAD, 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 resistive path.

Internal switch resistances (RSW and RAD) can be neglected with respect to external

resistances.

Figure 16. Input equivalent circuit (precise channels)

RS RF RL RSW RAD

REQ

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

•

---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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