AD7291 datasheet(13/28 Pages) AD

13 / 28 page

AD7291

Rev. 0 | Page 13 of 28

ADC TRANSFER FUNCTION

The output coding of the AD7291 is straight binary for the

analog input channel conversion results and twos complement

for the temperature conversion result. The designed code tran-

sitions occur at successive LSB values (that is, 1 LSB, 2 LSBs,

and so forth). The LSB size is VREF/4096 for the AD7291. The

ideal transfer characteristic for the AD7291 for straight binary

coding is shown in Figure 21.

111...111

111...110

111...000

011...111

000...010

000...001

000...000

1LSB = VREF/4096

ANALOG INPUT

NOTES

1. VREF IS 2.5V.

+VREF – 1LSB

1LSB

Figure 21. Straight Binary Transfer Characteristic

TEMPERATURE SENSOR OPERATION

The AD7291 contains one local temperature sensor. The on-chip,

band gap temperature sensor measures the temperature of the

AD7291 die.

The temperature sensor module on the AD7291 is based on the

three current principle (see Figure 22), where three currents are

passed through a diode and the forward voltage drop is

measured, allowing the temperature to be calculated free of

errors caused by series resistance.

I4 × I

INTERNAL

SENSE

TRANSISTOR

BIAS

DIODE

VDD

8 × I

IBIAS

VOUT+

VOUT–

TO ADC

Figure 22. Top Level Structure of Internal Temperature Sensor

Each input integrates, in turn, over a period of several hundred

microseconds. This takes place continuously in the background,

leaving the user free to perform conversions on the other

channels. When integration is complete, a signal passes to the

control logic to initiate a conversion automatically.

If the ADC is in command mode and performing a voltage

conversion, the AD7291 waits for it to complete and then

initiates a temperature sensor conversion. If the ADC is not

performing voltage conversions, temperature conversions occur

at 5 ms intervals.

In autocycle mode, the conversion is inserted into an

appropriate place in the current sequence. If the ADC is idle,

the conversion takes place immediately. The TSENSE conversion

result register stores the result of the last conversion on the

temperature channel; this can be read at any time.

Theoretically, the temperature measuring circuit can measure

temperatures from −512°C to +511°C with a resolution of

0.25°C. However, temperatures outside TA (the specified tem-

perature range for the AD7291) are outside the guaranteed

operating temperature range of the device. The temperature

sensor is enabled by setting the TSENSE bit in the command

TEMPERATURE SENSOR AVERAGING

The AD7291 incorporates a temperature sensor averaging

feature to enhance the accuracy of the temperature measure-

ments. The temperature averaging feature is performed

continuously in the background provided the TSENSE bit

in the command register is enabled. The temperature is

measured each time a TSENSE conversion is performed and a

moving average method is used to determine the result in the

TSENSE average result register. The average result is given by the

following equation:

() ()

Result

Current

Result

Average

AVG

SENSE

The average result is then available in the TSENSE average result

The first TSENSE conversion result given by the AD7291 after the

temperature sensor has been selected in the command register

(Bit D7) is the actual first TSENSE conversion result, and this

result remains valid until the next TSENSE conversion is

completed and the result register is updated.

AD7291 датащи(PDF) 13 Page - Analog Devices

AD7291 датащи(HTML) 13 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

датащи скачать

ссылки URL