AD7877 datasheet(19/44 Pages) AD

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AD7877

Rev. A | Page 19 of 44

AUXILIARY INPUTS

The AD7877 has three auxiliary analog inputs, AUX1 to AUX3.

These channels have a full-scale input range from 0 V to VREF.

The ADC channel addresses for AUX1 to AUX3 are 0011b,

0100b, and 0101b, and the results are stored in Registers 10011b,

10100b, and 10101b. These pins can also be reconfigured as

general-purpose logic inputs/outputs, as described in the GPIO

Configuration section.

LIMIT COMPARISON

The AUX1 measurement, the two battery measurements, and

the TEMP1 measurement can all be compared with high and

low limits, and an out-of-limit result made to generate an alarm

output at the ALERT pin. The limits are stored in registers with

addresses from 00100b to 01011b. After a measurement from

any one of the four channels is converted, it is compared with

the corresponding high and low limits. An out-of-limit result

sets one of the status bits in the alert status/enable register. For

details on these and other registers, see the Register Maps and

Detailed Register Descriptions sections. For details on writing

and reading data, see the Serial Interface section.

As mentioned previously, the temperature comparison is made

using the result of the TEMP1 measurement, which is the diode

forward voltage. Because the temperature coefficient of the

diode is known but the actual forward voltage can have a wide

tolerance, it is not possible to program the high and low limit

registers with predetermined values.

Instead, it is necessary to calibrate the temperature measure-

ment, calculate the TEMP1 readings at the high and low limit

temperatures, and then program those values into the limit

registers, as follows:

Calculate

LSB per degree = −2.1 mV/(VREF/4096).

Save the calibration reading

DCAL at calibration temperature

TCAL.

Subtract

TCAL from limit temperatures THIGH and TLOW to get

the difference in degrees between the limit temperatures

and the calibration temperature.

Multiply this value by

LSB per degree to get the value in

LSBs.

Add these values to the digital value at the calibration

temperature to get the digital high and low limit values.

Example:

The internal 2.5 V reference is used.

THIGH = +65°C and TLOW = −10°C.

LSB per degree = −2.1 × 10−3/(2.5/4096) = −3.44.

DCAL = 983 decimal at 25°C.

DHIGH = (65 − 25) × −3.44 + 983 = 845.

DLOW = (−10 − 25) × −3.44 + 983 = 1103.

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