SPC56EL54X datasheet(120/165 Pages) STMICROELECTRONICS

120 / 165 page

Electrical characteristics

SPC56ELx, SPC564Lx

120/165

DocID15457 Rev 12

Figure 9. ADC characteristics and error definitions

3.16.1

Input Impedance and ADC Accuracy

To preserve the accuracy of the A/D converter, it is necessary that analog input pins have

low AC impedance. Placing a capacitor with good high frequency characteristics at the input

pin of the device can be effective: the capacitor should be as large as possible, ideally

infinite. This capacitor contributes to attenuating the noise present on the input pin; further, it

sources charge during the sampling phase, when the analog signal source is a high-

impedance source.

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 and Cp2 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 Cp2 + CS equal to 7.5 pF, a

resistance of 133 k

 is obtained (REQ = 1 / (fS*(Cp2+CS)), where fS 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, the

external circuit must be designed to respect the Equation 4:

(2)

(1)

(3)

(4)

(5)

Offset Error OSE

Gain Error GE

1 LSB (ideal)

Vin(A) (LSBideal)

(1) Example of an actual transfer curve

(2) The ideal transfer curve

(3) Differential non-linearity error (DNL)

(4) Integral non-linearity error (INL)

(5) Center of a step of the actual transfer

curve

code out

4095

4094

4093

4092

4091

4090

4089 4090 4091 4092 4093 4094 4095

1 LSB ideal =(VrefH-VrefL)/ 4096 =

3.3V/ 4096 = 0.806 mV

Total Unadjusted Error

TUE = +/- 6 LSB = +/- 4.84mV

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