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STR720 - ELECTRICAL CHARACTERISTICS

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28.11 S-D ADC characteristics

The static electrical characteristics of A/D converter are reported in Table 105. In the

subsequent sections, parameters related to input circuit and A/D converter performance are

listed.

VDD(all) = 1.80 V ± 10%, VDD3(all) = 3.30 V ± 10%, TA = -40 / +85 °C unless otherwise specified.

As long as CLK pad is used to provide system clock, either directly or thorugh PLL, a consumption will be observed

on AVDD pad, due to CLK pad reference current generators. Only when STR720 is put in SLOW mode (32 kHz) or

STOP mode (no clock) CLK pad reference current generators can be switched off and the minimum AVDD consump-

tion can be achieved.

VAIN may exceed AGND or AVDD up to the absolute maximum ratings. However, the conversion result in these cases

will be meaningless. Data based on design guidelines, not tested in production.

28.11.1 ADC analog input pins

To improve the accuracy of the A/D converter, the first recommendation is to properly connect

decoupling capacitors on AVDD pin, so to reduce as much as possible the amount of noise

introduced through the analog supply voltage pin. Subsequently it is definitively necessary for

analog input pins to have low AC impedance. Connecting 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 attenuate noise present

on the input pin; moreover, it sources charge during the sampling phase, when the analog

signal source is a high-impedance source.

On each analog input pin an anti-aliasing filter is required so to avoid that components beyond

the sampling frequency are folded back into the actual signal bandwidth. Due to the structure

of Sigma-Delta ADC, these components cannot be removed by software post-processing of

the acquired samples, so it is of fundamental importance to remove them before feeding the

signal to the analog input pin. An anti-aliasing filter can be obtained in general by using a

series resistance with a capacitor on the input pin (simple RC Filter), though other more

complex solutions with better band rejection characteristics can be used. 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.

Table 105. ADC static characteristics

Symbol

Parameter

Test Conditions(1)

Value

Unit

Min.

Typ

Max

AVDD

Analog supply voltage for ADC With reference to AVSS pin

3.0

3.30

3.6

AIDD

AVDD operating mode current

1.8

AIDD_O

AVDD off current

ADC off but CLK pad active(2)

AIDD_S

AVDD standby/stop current

ADC off and CLK pad not

used(2)

µA

VAIN

Analog input voltage

See note (3)

2.5

IALEAK

Analog input leakage current

200

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