ADBMS1818ASWZ-R7 datasheet(28/92 Pages) AD

28 / 92 page

Data Sheet

ADBMS1818

THEORY OF OPERATION

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Rev. B | 28 of 92

ADC Range and Resolution

The C inputs and GPIO inputs have the same range and resolution.

The ADC inside the ADBMS1818 has an approximate range from

–0.82 V to +5.73 V. Negative readings are rounded to 0 V. The

format of the data is a 16-bit unsigned integer where the LSB rep-

resents 100 μV. Therefore, a reading of 0x80E8 (33,000 decimal)

indicates a measurement of 3.3 V.

Δ-Σ ADCs have quantization noise which depends on the input

voltage, especially at low oversampling ratios, such as in fast mode.

In some of the ADC modes, the quantization noise increases as

the input voltage approaches the upper and lower limits of the ADC

range. For example, the total measurement noise vs. input voltage

in normal and filtered modes is shown in Figure 54.

The specified range of the ADC is 0 V to 5 V. In Table 20, the preci-

sion range of the ADC is arbitrarily defined as 0.5 V to 4.5 V. This

range is where the quantization noise is relatively constant even in

the lower OSR modes (see Figure 54). Table 20 summarizes the

total noise in this range for all eight ADC operating modes. Also

shown in Table 20 is the noise free resolution. For example, 14-bit

noise free resolution in normal mode implies that the top 14 bits are

noise free with a dc input, but that the 15th and 16th LSBs flicker.

Figure 54. Measurement Noise vs. Input Voltage

ADC Range vs. Voltage Reference Value

Typical ADCs have a range that is exactly twice the value of

the voltage reference, and the ADC measurement error is directly

proportional to the error in the voltage reference. The ADBMS1818

ADC is not typical.

The absolute value of VREF1 is trimmed up or down to compensate

for gain errors in the ADC. Therefore, the ADC TME specifications

are superior to the VREF1 specifications. For example, the 25°C

specification of the TME when measuring 3.300 V in 7 kHz (normal)

mode is ±3 mV, and the 25°C specification for VREF1 is 3.150 V ±

150 mV.

Measuring Cell Voltages (ADCV Command)

The ADCV command initiates the measurement of the battery cell

inputs, Pin C0 through Pin C18. This command has options to

select the number of channels to measure and the ADC mode. See

the Commands section for the ADCV command format.

Figure 55 shows the timing of the ADCV command that measures

all 18 cells. After the receipt of the ADCV command to measure

all 18 cells, ADC1 sequentially measures the bottom 6 cells. ADC2

measures the middle 6 cells and ADC3 measures the top 6 cells.

After the cell measurements complete, each channel is calibrated to

remove any offset errors.

Table 21 shows the conversion times for the ADCV command

measuring all 18 cells. The total conversion time is given by t6C

which indicates the end of the calibration step.

Figure 56 shows the timing of the ADCV command that measures

only 3 cells.

Table 20. ADC Range and Resolution

Mode

Full Range1

Specified Range

Precision Range2

LSB

Format

Maximum Noise

Noise Free

Resolution3

27 kHz (Fast)

–0.8192 V to

+5.7344 V

0 V to 5 V

0.5 V to 4.5 V

100 μV

Unsigned 16 bits

±4 mV p-p

10 bits

14 kHz

–0.8192 V to

+5.7344 V

0 V to 5 V

0.5 V to 4.5 V

100 μV

Unsigned 16 bits

±1 mV p-p

12 bits

7 kHz (Normal)

–0.8192 V to

+5.7344 V

0 V to 5 V

0.5 V to 4.5 V

100 μV

Unsigned 16 bits

±250 μV p-p

14 bits

3 kHz

–0.8192 V to

+5.7344 V

0 V to 5 V

0.5 V to 4.5 V

100 μV

Unsigned 16 bits

±150 μV p-p

14 bits

2 kHz

–0.8192 V to

+5.7344 V

0 V to 5 V

0.5 V to 4.5 V

100 μV

Unsigned 16 bits

±100 μV p-p

15 bits

1 kHz

–0.8192 V to

+5.7344 V

0 V to 5 V

0.5 V to 4.5 V

100 μV

Unsigned 16 bits

±100 μV p-p

15 bits

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