ADBMS1818CSWAZ-R7 datasheet(38/92 Pages) AD

38 / 92 page

Data Sheet

ADBMS1818

THEORY OF OPERATION

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Rev. B | 38 of 92

Accuracy Check

Measuring an independent voltage reference is the optimal

means to verify the accuracy of a data acquisition system. The

ADBMS1818 contains a 2nd reference for this purpose. The ADAX

command initiates the measurement of the 2nd reference. The

results are placed in Auxiliary Register Group B. The range of

the result depends on the ADC1 measurement accuracy and the

accuracy of the 2nd reference, including thermal hysteresis and

long term drift. Readings outside the 2.992 V to 3.012 V range

indicate the system is out of its specified tolerance. ADC2 is verified

by comparing it to ADC1 using the ADOL command. ADC3 is

verified by comparing it to ADC2 using the ADOL command.

Mux Decoder Check

The diagnostic command DIAGN ensures the proper operation

of each multiplexer channel. The command cycles through all

channels and sets the MUXFAIL bit to 1 in Status Register Group

B if any channel decoder fails. The MUXFAIL bit is set to 0 if the

channel decoder passes the test. The MUXFAIL bit is also set to 1

on a power-on reset (POR) or after a CLRSTAT command.

The DIAGN command takes approximately 400 μs to complete if

the core is in the REFUP state and about 4.5 ms to complete if the

core is in the standby state. The polling methods described in the

Polling Methods section can be used to determine the completion of

the DIAGN command.

ADC Clear Commands

ADBMS1818 has 3 clear ADC commands: CLRCELL, CLRAUX,

and CLRSTAT. These commands clear the registers that store all

ADC conversion results.

The CLRCELL command clears Cell Voltage Register Groups A, B,

C, D, E, and F. All bytes in these registers are set to 0xFF by the

CLRCELL command.

The CLRAUX command clears Auxiliary Register Groups A, B, C,

and D. All bytes in these registers, except the last four registers of

Group D, are set to 0xFF by the CLRAUX command.

The CLRSTAT command clears Status Register Groups A and

B, except for the REV and RSVD bits in Status Register Group

B. A read back of the REV bits returns the revision code of the

part. RSVD bits always read back 0s. All overvoltage (OV) and

undervoltage (UV) flags, MUXFAIL bit, and THSD bit in Status

by the CLRSTAT command. The THSD bit is set to 0 after the

RDSTATB command. The registers storing SC, ITMP, VA, and VD

are all set to 0xFF by the CLRSTAT command.

Open Wire Check (ADOW Command)

The ADOW command is used to check for any open wires between

the ADCs of the ADBMS1818 and the external cells. This command

performs ADC conversions on the C pin inputs identically to the

ADCV command, except for two internal current sources sink or

source current into the two C pins while they are being measured.

The pull-up (PUP) bit of the ADOW command determines whether

the current sources are sinking or sourcing 100 μA.

The following simple algorithm can be used to check for an open

wire on any of the 19 C pins:

1. Run the 18-cell command ADOW with PUP = 1 at least twice.

Read the cell voltages for cells 1 through 18 once at the end

and store them in array CELLPU(n).

2. Run the 18-cell command ADOW with PUP = 0 at least twice.

Read the cell voltages for cells 1 through 18 once at the end

and store them in array CELLPD(n).

3. Take the difference between the pull-up and pull-down meas-

urements made in Step 1 and Step 2 for cells 2 to 18: CELL∆(n)

= CELLPU(n) – CELLPD(n).

4. For all values of n from 1 to 17: If CELL∆(n+1) < –400 mV,

then C(n) is open. If CELLPU(1) = 0.0000, then C(0) is open. If

CELLPD(18) = 0.0000, then C(18) is open.

The above algorithm detects open wires using normal mode con-

versions with as much as 10 nF of capacitance remaining on the

ADBMS1818 side of the open wire. However, if more external

capacitance is on the open C pin, then the length of time that

the open wire conversions run in Step 1 and Step 2 must be

increased to give the 100 μA current sources time to create a large

enough difference for the algorithm to detect an open connection.

This action can be accomplished by running more than two ADOW

commands in Step 1 and Step 2, or by using filtered mode conver-

sions instead of normal mode conversions. Refer to Table 31 to

determine how many conversions are necessary.

Table 31. Number of ADOW Commands Required

External C Pin

Capacitance

Number of ADOW Commands Required in Step 1

and Step 2

Normal Mode

Filtered Mode

≤10 nF

100 nF

1 μF

100

1 + ROUNDUP (C/10 nF) 2

Auxiliary Open Wire Check (AXOW Command)

The AXOW command is used to check for any open wires between

the GPIO pins of the ADBMS1818 and the external circuit. This

command performs ADC conversions on the GPIO pin inputs identi-

cally to the ADAX command, except internal current sources sink or

source current into each GPIO pin while it is being measured. The

pull-up (PUP) bit of the AXOW command determines whether the

current sources are sinking or sourcing 100 μA.

ADBMS1818CSWAZ-R7 数据表(PDF) 38 Page - Analog Devices

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