AD7091 datasheet(12/20 Pages) AD

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AD7091

Data Sheet

Rev. A | Page 12 of 20

MODES OF OPERATION

The mode of operation of the AD7091 is selected by controlling

the logic level of the CONVST signal when a conversion is complete.

The two modes of operation are normal mode and power-down

mode. These modes of operation provide flexible power manage-

ment options, allowing optimization of the power dissipation to

throughput rate ratio for different application requirements.

The logic level of the CONVST pin at the end of a conversion

determines whether the AD7091 remains in normal mode or

enters power-down mode (see the Normal Mode section and the

Power-Down Mode section). Similarly, if the device is in power-

down mode, CONVST controls whether the device returns to

normal mode or remains in power-down mode.

Normal Mode

The normal mode of operation is intended to achieve the fastest

throughput rate performance. In normal mode, the AD7091

remains fully powered at all times, so power-up times are not

a concern. Figure 20 shows the general timing diagram of the

AD7091 in normal mode.

In normal mode, the conversion is initiated on the falling edge of

CONVST, as described in the Serial Interface section. To ensure

that the part remains fully powered at all times, CONVST must

return high after t7 and remain high until the conversion is com-

plete. At the end of a conversion (denoted as EOC in Figure 20),

the logic level of CONVST is tested.

To read back data stored in the conversion result register, wait until

the conversion is complete, and then pull CS low. The conversion

data is subsequently clocked out on the SDO pin (see Figure 20).

Because the output shift register is 12 bits wide, data is shifted out

of the device as a 12-bit word under the control of the serial clock

input (SCLK). After reading back the data, the user can pull

CONVST low again to start another conversion after the tQUIET

time has elapsed.

Power-Down Mode

The power-down mode of operation is intended for use in applica-

tions where slower throughput rates and lower power consumption

are required. In this mode, the ADC can be powered down after

each conversion or after a series of conversions performed at a

high throughput rate, with the ADC powered down for relatively

long durations between these bursts of several conversions. When

the AD7091 is in power-down mode, the serial interface remains

active even though all analog circuitry is powered down.

To enter power-down mode, pull CONVST low and keep it low

prior to the end of a conversion (denoted as EOC in Figure 21).

After the conversion is complete, the logic level of the CONVST

pin is tested. If the CONVST signal is logic low, the part enters

power-down mode.

The serial interface of the AD7091 is functional in power-down

mode; therefore, users can read back the conversion result after

the part enters power-down mode.

NOTES

IS DON’T CARE.

CONVST

CONVERSION DATA

t10

SDO

EOC

t12

2. EOC IS THE END OF A CONVERSION.

Figure 20. Normal Mode of Operation, Serial Interface Read Timing

CONVST

CONVERSION DATA

t10

SDO

EOC

POWER-DOWN MODE

t12

t13

NOTES

IS DON’T CARE.

2. EOC IS THE END OF A CONVERSION.

Figure 21. Entering and Exiting Power-Down Mode

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