AD7280ABSTZ datasheet(22/48 Pages) AD

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AD7280A

Rev. 0 | Page 22 of 48

CONVERSION WINDOW

As described in the Converting Cell Voltages and Auxiliary

ADC Inputs section, the AD7280A converts the selected cell

voltage and auxiliary ADC inputs in a defined sequence (see

Figure 31). As described in the Circuit Information section,

the AD7280A consists primarily of a high voltage input multi-

plexer, a low voltage input multiplexer, and a SAR ADC. The six

cell voltage channels are presented to the ADC in turn by the high

voltage multiplexer. Control is then handed to the low voltage

multiplexer that allows the six auxiliary ADC channels to be

converted. Following completion of all selected conversions,

control is handed back to the high voltage multiplexer, and the

AD7280A is ready to receive the next valid convert start command.

The conversion window of the AD7280A includes the actual con-

version time for the selected channels (see Table 10), as well as

the additional time required to return control to the high

voltage multiplexer and configure it to start acquiring the cell

voltage between VIN6 and VIN5. The conversion window

defines the minimum time that should be allowed between

successive convert start commands.

The conversion window for the AD7280A can be calculated

using the following equation:

Conversion Window = Total Conversion Time + 80 μs

where Total Conversion Time can be calculated for either a

single device or for a chain of devices, as described in the

Converting Cell Voltages and Auxiliary ADC Inputs section.

SELF-TEST CONVERSION

A self-test conversion can be initiated on the AD7280A, which

allows the operation of the ADC and reference buffer to be

verified. The self-test conversion is completed on the internal

1.2 V band gap reference voltage, and the voltage range for the

conversion is 0 V to 5 V. The self-test conversion can be initi-

ated on either a single AD7280A or on all AD7280As in the

daisy chain simultaneously.

The conversion results can be read back though the read protocols

defined in the Serial Interface section. The self-test conversion

result typically varies between Code 970 and Code 990.

The self-test conversion can also be used to verify the operation

of the alert outputs, as described in the Alert Output section.

CONNECTION OF FEWER THAN SIX VOLTAGE CELLS

The AD7280A provides six input channels for battery cell voltage

measurement. The AD7280A can also be used in applications

that require fewer than six voltage measurements. In these appli-

cations, care should be taken to ensure that the sum of the

individual cell voltages still exceeds the minimum VDD supply

voltage. For this reason, the recommended minimum number of

battery cells connected to each AD7280A is 4. Care should also

be taken to ensure that the voltage on the VIN6 input is always

greater than or equal to the voltage on the VDD supply pin. For

example, in an application with five battery cells connected to

the AD7280A, the cell voltage on Cell 5 should be applied across

VIN6 and VIN5, and the VIN4 and VIN5 inputs should be shorted

together. Figure 35 shows an example of the battery connections

to the AD7280A in a four-cell battery monitoring application.

VIN6

VIN5

VIN4

VIN3

VIN2

VIN1

VIN0

AD7280A

10kΩ

100nF

10kΩ

100nF

10kΩ

100nF

Figure 35. Typical Connections for a Four-Cell Application

Regardless of how many cell voltage measurements are required

in the user application, the AD7280A acquires and converts the

voltages on all six cell voltage input channels. The conversion data

on all six voltage channels is supplied to the DSP/microprocessor

using the SPI/daisy-chain interfaces. Users should ignore the

conversion data that is not required in their application.

It is also possible to read back a single cell voltage conversion

result from each device in the daisy chain. This can be done by

programming the read register on each device to read back the

required conversion result (see Example 4 in the Examples of

Interfacing with the AD7280A section). However, as previously

described, all six cell voltage channels are converted. When

using the device in this mode, the overall conversion sample

rate should be limited by the conversion window required for

the number of channels selected by Bits[D15:D14] of the

control register.

When using the alert function, the user should program the alert

trigger an alert output (see the Alert Output section).

AD7280ABSTZ Datasheet(PDF) 22 Page - Analog Devices

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