AD7280 datasheet(17/33 Pages) AD

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Preliminary Technical Data

AD7280

Rev. PrD | Page 17 of 33

SELF TEST CONVERSION

A self-test conversion may be initiated on the AD7280 which

allows the operation of the ADC to be verified. The self-test

conversion is completed on the internal 1.2V bandgap reference

voltage. The self-test conversion may be initiated on either a

single AD7280 or on all AD7280s in the battery stack

simultaneously. The conversion results may be read back

though the read protocols defined in the Register map section.

The self-test conversion may also be used to verify the

operation of the ALERT outputs as described in the ALERT

Output section.

CONVERSION AVERAGING

The AD7280 includes an option where the ADC conversions

completed on each cell input may be repeated with an averaged

conversion result being stored in the individual register. The

averaged conversion result may then be read back through the

SPI interface in the same manner as a standard conversion

result. The AD7280 may be programmed, through bits D10 and

D9 of the CONTROL register, to complete 1, 2, 4 or 8

conversions. The default on power up is a single conversion.

CONVERSION OF LESS THEN 6 VOLTAGE CELLS

The AD7280 provides 6 input channels for Battery Cell voltage

measurement. The AD7280 may also be used in applications

which require less then 6 voltage measurements. In these

applications care should be taken to ensure that the sum of the

individual cell voltages will still exceed the minimum VDD

supply voltage. For this reason it is recommended that the

minimum number of battery cells connected to each AD7280 is

4. Care should also be taken to ensure that the voltage on the

Vin6 inputs is always greater than or equal to the voltage on the

VDD supply pin. This design requirement is in place to allow the

use of a diode on the VDD supply pin of the AD7280 which

provides protection in the event of an open circuit in the battery

stack. Even if a protection diode is not being used in the

application the Vin6 input voltage must be greater than or equal

to the VDD supply voltage. An example of the battery

connections to the AD7280 in a 4 cell battery monitoring

application is shown in Figure 17.

Regardless of how many cell measurements are required in the

user application the AD7280 will acquire and convert the

voltages on all 6 voltage input channels. The conversion data on

all 6 channels will be supplied to the DSP/uP using the SPI

/Daisy Chain interfaces. The user should then ignore the

conversion data which is not required in their application. If

using the Alert function the user should program the Alert

incorrectly trigger an Alert output. Please refer to ALERT

Output section.

AD7280

Vin0

Vin1

Vin2

Vin3

Vin4

Vin5

Vin6

100nF

10k

Ω

10k

Ω

10k

Ω

10k

Ω

Figure 17. Typical connections for a 4 cell application

CELL TEMPERATURE INPUTS

The AD7280 provides 6 single ended analog inputs, VT1 to

VT6, to the ADC which may be used to convert the voltage

output of a thermistor temperature measurement circuit. In the

event that no temperature measurements are required, or that

individual cell temperature measurements are not required the

VT inputs may be used to convert any other 0 V to 5 V input

signal.

The AD7280 may be programmed to complete conversions on

all 6 temperature channels, on 3 temperature channels (VT1,

VT3 & VT5) or on none of the temperature input channels. The

number of conversions is programmed through bits D15 and

D14 of the CONTROL register. The number of conversions

results supplied by the AD7280 for read back by the DSP/uP is

programmed through bits D13 and D12 of the CONTROL

used but only one or two temperature inputs are required the

AD7280 should first be programmed to complete and readback

only 3 temperature conversions, by setting bits D15 and D13 of

the CONTROL register to 0, and bits D14 and D12 to 1. VT

Channels VT5 and VT3 may be removed from the Alert

detection by writing to bits D1 and D0 of the ALERT register.

Please refer to ALERT Output section.

Thermistor Termination Input

In the event that thermistors circuits are being used to measure

each individual cell temperature the Thermistors Termination

pin, VTTERM, may be used to the terminate the thermistor inputs

for each cell temperature measurement. This reduces the

termination resistor requirement from 6 resistors to 1. Bit D3

in the CONTROL register should be set to 1 when using the

VTTERM input.

It should be noted that, due to settling time requirements, the

thermistor termination resistor option should only be used

when the acquisition time of the AD7280 is set to its highest

value, that is 1.6µs. The acquisition time is configured by setting

bits D6 and D5 of the CONTROL register as outlined in Table

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