ADBMS2950BCCSZ datasheet(94/97 Pages) AD

94 / 97 page

ADBMS2950B

Data Sheet

Rev. 0 | Page 94 of 97

t [s]

I1CNT-MIN

I1CNT-NOM

I1CNT-MAX

CntACC-MIN

CntACC-NOM

CntACC-MAX

264

1.848

1663

1847

2032

207

230

254

265

1.855

1669

1854

2040

208

231

255

266

1.862

1675

1861

209

232

256

267

1.869

1682

1868

210

233

256

268

1.876

1688

1875

211

234

257

...

291

2.037

1833

2036

192

229

254

280

292

2.044

1839

2043

200

229

255

281

293

2.051

1845

208

230

256

282

294

2.058

1852

215

231

257

282

295

2.065

1858

223

232

258

283

...

324

2.268

2041

219

446

255

283

311

325

2.275

2047

226

454

255

284

312

326

2.282

233

462

256

285

313

327

2.289

240

469

257

286

314

328

2.296

247

477

258

286

315

Measuring the ADC Conversion Time

The oscillator of the ADBMS2950B has a part-to-part variation

and changes over temperature. The specified limit is ±10%. To

allow precise coulomb counting, the ADC conversion time can

be measured using the I1CNTPHA readings and a reference

timer in the host controller.

The conversion time must be measured at a rate that is still fast

enough to track the oscillator drift over temperature but not too

fast to have sufficient resolution and to avoid errors due to

timing uncertainties. For example, variations in the time at

which the reference timer is captured and when SPI

transactions happen. Reasonable update periods are in the

range of 100 ms to 1000 ms.

Taking the data from Table 89 and 252 ms as an example

period, the conversion time is calculated as follows:

tCONV = t ÷ I1CNT × ACCN

The resulting conversion time is shown in Table 90 for

hypothetical ICs running at minimum (tCONV,MAX), nominal

(tCONV,NOM), and maximum (tCONV,MIN) oscillator frequency.

Table 90. Calculating tCONV from I1CNT

t [s]

I1CNTMIN

I1CNTNOM

I1CNTMAX

tCONV,MAX [ms]

tCONV,NOM [ms]

tCONV,MIN [ms]

0.238

213

237

261

8.94

8.03

7.30

0.245

220

244

268

8.91

8.03

7.31

0.252

226

251

276

8.92

8.03

7.30

0.259

232

258

284

8.93

8.03

7.30

0.266

238

265

292

8.94

8.03

7.29

For maximum resolution and highest precision, it is

recommended to take I1PHA into account and do the

calculation by using the 13-bit I1CNTPHA counter:

tCONV = t ÷ I1CNTPHA × ACCN × 4

Because the conversion time measurement must be executed

continuously, and the I1CNTPHA counter roll-over after ~2

seconds, the calculation must be done using the differences of

I1CNTPHA readings between the beginning and the end of the

measurement window. When a roll-over is detected (new

I1CNTPHA reading smaller than previous reading), the host

must correct the difference by adding 213 = 8192.

Calculating Charge

Once continuous sample data is read, and the conversion time is

measured, the charge (Q) per sample can be calculated as

follows:

Q = tCONV × IxACC ÷ ACCN × IxADCLSB

Typically, the coulomb counting is done using integers. In this

case, the ADC conversion word can be scaled with the deviation

from the nominal ACCN × 1 ms conversion time. Similar

scaling is also done to correct for the shunt’s nominal resistance

value (gcShuntNominal) and to correct the temperature dependent

drift of the shunt (gcShuntTC).

Q = gcShuntNominal × gcShuntTC(TSHUNT) × tCONV × IxACC ÷ ACCN

The charge (Q) can now be accumulated to the coulomb

counter (CC) as follows:

CC = CC + Q

The LSB size of this integer based coulomb counter is:

1 μV ÷ (ACCN × RSNS,NOM) × (ACCN × 1 ms) = 1 μV × 1 ms ÷

RSNS,NOM

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