AD7811YN datasheet(14/19 Pages) AD

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AD7811/AD7812

–14–

REV. B

DIN

DOUT

CONVST

VDD

6040 HEX

NOT VALID

4040 HEX

VALID DATA

4040 HEX

POWER-UP

1.5 s

CONVERT

2.3 s

CONVERT

2.3 s

Figure 15. Read/Write Sequence for AD7812

OPERATING MODES

The mode of operation of the AD7811 and AD7812 is selected

when the (logic) state of the

CONVST is checked at the end of

a conversion. If the

CONVST signal is logic high at the end

of a conversion, the part does not power down and is operat-

ing in Mode 1. If, however, the

CONVST signal is brought

logic low before the end of a conversion, the AD7811 and AD7812

will power down at the end of the conversion. This is Mode 2

operation.

Mode 1 Operation (High Speed Sampling)

When the AD7811 and AD7812 are operated in Mode 1 they

are not powered down between conversions. This mode of opera-

tion allows high throughput rates to be achieved. The timing

diagram in Figure 16 shows how this optimum throughput rate

is achieved by bringing the

CONVST signal high before the end

of the conversion.

The sampling circuitry leaves its tracking mode and goes into

hold on the falling edge of

CONVST. A conversion is also initi-

ated at this time. The conversion takes 2.3

µs to complete. At

this point, the result of the current conversion is latched into the

serial shift register and the state of the

CONVST signal checked.

The

CONVST signal should be logic high at the end of the

conversion to prevent the part from powering down. The serial

port on the AD7811 and AD7812 is enabled on the rising edge

of the first SCLK after the rising edge of the RFS signal—see

Serial Interface section. As explained earlier, this rising edge

should occur before the end of the conversion process if the part

is not to be powered down. A serial read can take place at any

stage after the rising edge of

CONVST. If a serial read is initi-

ated before the end of the current conversion process (i.e., at

time “A”), the result of the previous conversion is shifted out on

the DOUT pin. It is possible to allow the serial read to extend

beyond the end of a conversion. In this case the new data will

not be latched into the output shift register until the read

has finished. The dynamic performance of the AD7811 and

AD7812 typically degrades by up to 3 dBs while reading during

a conversion. If the user waits until the end of the conversion

process, i.e., 2.3

µs after the falling edge of CONVST (Point

“B”) before initiating a read, the current conversion result is

shifted out. The serial read must finish at least 100 ns prior to

the next falling edge of

CONVST to allow the part to accurately

acquire the input signal.

Mode 2 Operation (Automatic Power-Down)

When used in this mode of operation the part automatically

powers down at the end of a conversion. This is achieved by

leaving the

CONVST signal low until the end of the conversion.

Because it takes approximately 1.5

µs for the part to power-up

after it has been powered down, this mode of operation is intended

to be used in applications where slower throughput rates are

required, i.e., in the order of 250 kSPS and improved power

performance is required—see Power vs. Throughput section.

There are two power-down modes the AD7811/AD7812 can

CURRENT CONVERSION

RESULT

CONVST

DOUT

SCLK

Figure 16. Mode 1 Operation Timing Diagram

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