PXD20 Datasheet, PDF(92/130 Page) Freescale Semiconductor, Inc

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

PXD20 Datasheet, PDF (92/130 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

◁

Electrical characteristics

A real filter can typically be obtained by using a series resistance with a capacitor on the input pin (simple RC filter). The RC

filtering may be limited according to the value of source 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 kï is obtained (REQ

= 1 / (fc Ã 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 5:

ï·

-R----S-----+----R-----F-----+----R-----L-----+-----R----S----W-------+-----R----A-----D--

REQ

ï¼

1--

LSB

Eqn. 5

Equation 5 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.

EXTERNAL CIRCUIT

INTERNAL CIRCUIT SCHEME

Source

Filter

Current Limiter

VDD

Channel

Selection

Sampling

RSW1

RAD

CP1

CP2

RS Source Impedance

RF Filter Resistance

CF Filter Capacitance

RL Current Limiter Resistance

RSW1 Channel Selection Switch Impedance

RAD Sampling Switch Impedance

CP Pin Capacitance (two contributions, CP1 and CP2)

CS Sampling Capacitance

Figure 14. Input equivalent circuit (precise channels)

PXD20 Microcontroller Data Sheet, Rev. 2

PreliminaryâSubject to Change Without Notice

Freescale Semiconductor

▷