EVAL-AD7795EB datasheet(30/36 Pages) AD

30 / 36 page

AD7794/AD7795

Rev. D | Page 30 of 36

Single Conversion Mode

In single conversion mode, the AD7794/AD7795 are placed in

shutdown mode between conversions. When a single

conversion is initiated by setting MD2 to 0, MD1 to 0, and MD0

to 1 in the mode register, the AD7794/AD7795 power up,

perform a single conversion, and then return to shutdown

mode. The on-chip oscillator requires 1 ms to power up. A

conversion requires a time period of 2 × tADC. DOUT/RDY goes

low to indicate the completion of a conversion. When the data-

word has been read from the data register, DOUT/RDY goes

high. If CS is low, DOUT/RDY remains high until another

conversion is initiated and completed. The data register can be

read several times, if required, even when DOUT/RDY has

gone high.

Continuous Conversion Mode

This is the default power-up mode. The AD7794/AD7795

continuously convert with the RDY pin in the status register

going low each time a conversion is complete. If CS is low, the

DOUT/RDY line also goes low when a conversion is complete.

To read a conversion, the user writes to the communications

pin as soon as SCLK pulses are applied to the ADC. DOUT/RDY

returns high when the conversion is read. The user can read this

ensure that the data register is not being accessed at the completion

of the next conversion, or else the new conversion word is lost.

Continuous Read

Rather than write to the communications register each time a

conversion is complete to access the data, the AD7794/AD7795

can be configured so that the conversions are placed on the

DOUT/RDY line automatically. By writing 01011100 to the

communications register, the user need only apply the

appropriate number of SCLK cycles to the ADC. The 24-bit

word is automatically placed on the DOUT/RDY line when a

conversion is complete. The ADC should be configured for

continuous conversion mode.

When DOUT/RDY goes low to indicate the end of a conversion,

sufficient SCLK cycles must be applied to the ADC, and the

data conversion is placed on the DOUT/RDY line. When the

conversion is read, DOUT/RDY returns high until the next

conversion is available.

In this mode, the data can be read only once. Also, the user must

ensure that the data-word is read before the next conversion is

complete. If the user has not read the conversion before the

completion of the next conversion, or if insufficient serial clocks are

applied to the AD7794/AD7795 to read the word, the serial

output register is reset when the next conversion is complete.

The new conversion is then placed in the output serial register.

To exit the continuous read mode, the instruction 01011000

must be written to the communications register while the RDY

pin is low. While in the continuous read mode, the ADC

monitors activity on the DIN line so that it can receive the

instruction to exit the continuous read mode. Additionally, a

reset occurs if 32 consecutive 1s are seen on DIN. Therefore,

DIN should be held low in continuous read mode until an

instruction is to be written to the device.

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