ADBMS1818ASWAZ-R7 datasheet(27/92 Pages) AD

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Data Sheet

ADBMS1818

THEORY OF OPERATION

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Rev. B | 27 of 92

ADC OPERATION

There are three ADCs inside the ADBMS1818. The three ADCs

operate simultaneously when measuring 18 cells. Only one ADC is

used to measure the general-purpose inputs. This section uses the

term ADC to refer to one or all ADCs, depending on the operation

being performed. This section refers to ADC1, ADC2, and ADC3

when it is necessary to distinguish between the three circuits, such

as in the timing diagrams.

ADC Modes

The ADCOPT bit (CFGAR0, Bit 0) in Configuration Register Group

A and the mode selection bits, MD, Bits[1:0], in the conversion

command together provide eight modes of operation for the ADC

which correspond to different oversampling ratios (OSRs). The

accuracy and timing of these modes are summarized in Table 19. In

each mode, the ADC first measures the inputs and then performs a

calibration of each channel. The names of the modes are based on

the –3 dB bandwidth of the ADC measurement.

Mode 7 kHz (normal): In this mode, the ADC has high resolution

and low TME. This mode is considered the normal operating mode

because of the optimum combination of speed and accuracy.

Mode 27 kHz (fast): In this mode, the ADC has maximum through-

put but has some increase in TME. Therefore, this mode is also

referred to as the fast mode. The increase in speed comes from a

reduction in the OSR. This increase results in an increase in noise

and average measurement error.

Mode 26 Hz (filtered): In this mode, the ADC digital filter –3 dB

frequency is lowered to 26 Hz by increasing the OSR. This mode is

also referred to as the filtered mode due to its low –3 dB frequency.

The accuracy is similar to the 7 kHz (normal) mode with lower

noise.

Modes 14 kHz, 3 kHz, 2 kHz, 1 kHz, and 422 Hz: Modes 14 kHz, 3

kHz, 2 kHz, 1 kHz, and 422 Hz provide additional options to set the

ADC digital filter –3 dB at 13.5 kHz, 3.4 kHz, 1.7 kHz, 845 Hz, and

422 Hz, respectively. The accuracy of the 14 kHz mode is similar to

the 27 kHz (fast) mode. The accuracy of the 3 kHz, 2 kHz, 1 kHz,

and 422 Hz modes is similar to the 7 kHz (normal) mode.

The filter bandwidths and the conversion times for these modes

are provided in Table 19. If the core is in the standby state,

an additional tREFUP time is required to power up the reference

before beginning the ADC conversions. The reference can remain

powered up between ADC conversions if the REFON bit in Configu-

ration Register Group A is set to 1 so that the core is in REFUP

state after delay tREFUP. The subsequent ADC commands do not

have the tREFUP delay before beginning ADC conversions.

Table 19. ADC Filter Bandwidth and Accuracy

Mode

–3 dB Filter BW

–40 dB Filter BW

TME Specification at 3.3 V,

25°C

TME Specification at

3.3 V, –40°C, +85°C

(ADBMS1818CSWAZ, +125°C)

27 kHz (Fast Mode)

27 kHz

84 kHz

±8.5 mV

14 kHz

13.5 kHz

42 kHz

±8.5 mV

7 kHz (Normal Mode)

6.8 kHz

21 kHz

±3 mV

±4 mV

3 kHz

3.4 kHz

10.5 kHz

±3 mV

±4 mV

2 kHz

1.7 kHz

5.3 kHz

±3 mV

±4 mV

1 kHz

845 Hz

2.6 kHz

±3 mV

±4 mV

422 Hz

1.3 kHz

±3 mV

±4 mV

26 Hz (Filtered Mode)

26 Hz

82 Hz

±2.2 mV

±3.0 mV

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