AD7472ARZ datasheet(13/20 Pages) AD

13 / 20 page

REV. B

AD7470/AD7472

–13–

OPERATING MODES

The AD7470 and AD7472 have two possible modes of opera-

tion, depending on the state of the

CONVST pulse at the end of

a conversion, Mode 1 and Mode 2. There is a continuous clock

on the CLKIN pin.

Mode 1 (High Speed Sampling)

In this mode of operation, the

CONVST pulse is brought high

before the end of conversion i.e., before BUSY goes low (see

Figure 10). If the

CONVST pin is brought from high to low

while BUSY is high, the conversion is restarted. When operat-

ing in this mode, a new conversion should not be initiated until

the acquisition time has elapsed after BUSY goes low. This

acquisition time allows the track-and-hold circuit to accurately

acquire the input signal. As mentioned earlier, a read should

not be done during a conversion. This mode facilitates the

fastest throughput times for the AD7470/AD7472.

Mode 2 (Sleep Mode)

Figure 13 shows AD7470/AD7472 in Mode 2 operation where

the ADC goes into sleep mode after conversion. The

CONVST

line is brought low to initiate a conversion and remains low until

after the end of conversion. If

CONVST goes high and low

again while BUSY is high, the conversion is restarted. Once the

BUSY line goes from a high to a low, the

CONVST line has its

status checked and, if low, the part enters sleep mode.

The device wakes up again on the rising edge of the

CONVST

signal. There is a wake-up time of typically 1

µs after the rising

edge of

CONVST before the BUSY line can go high to indicate

start of conversion. BUSY will only go high once

CONVST goes

low. The

CONVST line can go from a high to a low during this

wake-up time, but the conversion will still not be initiated until

after the 1

µs wake-up time. Superior power performance can be

achieved in this mode of operation by waking up the AD7470

and AD7472 only to carry out a conversion.

Burst Mode

Burst mode on the AD7470/AD7472 is a subsection of Mode 1

and Mode 2; the clock is noncontinuous. Figure 12 shows how

the ADC works in burst mode for Mode 2. The clock needs to

be switched on only during conversion, a minimum of 12 clock

cycles for the AD7470 and 14 clock cycles for the AD7472.

Because the clock is off during nonconverting intervals, system

power is saved. The BUSY signal can be used to gate the CLKIN

pulses. The ADC does not begin the conversion process until

CONVST

BUSY

DBx

tWAKEUP

tCONVERT

Figure 13. Mode 2 Operation

the first CLKIN rising edge after BUSY goes high. The clock

needs to start less than two clock cycles away from the

CONVST active edge, otherwise INL deteriorates. For

example, if the clock frequency is 28 MHz, the clock must start

within 71.4 ns of

CONVST going low. In Figure 12, the A/D

converter section is put into sleep mode once conversion is

completed. On the rising edge of

CONVST, it is woken up

again. The user must be wary of the wake-up time because it

will reduce the sampling rate of the ADC.

VDRIVE

The VDRIVE pin is used as the voltage supply to the output driv-

ers and is a separate supply from AVDD and DVDD. The purpose

of using a separate supply for the output drivers is that the user

can vary the output high voltage, VOH, from the VDD supply to

the AD7470/AD7472. For example, if AVDD and DVDD is using

a 5 V supply, the VDRIVE pin can be powered from a 3 V supply.

The ADC has better dynamic performance at 5 V than at 3 V,

so operating the part at 5 V, while still being able to interface to

3 V parts, pushes the AD7470/AD7472 to the top bracket of

high performance 10-bit/12-bit ADCs. Of course, the ADC can

have its VDRIVE and DVDD pins connected together and be pow-

ered from a 3 V or 5 V supply.

All outputs are powered from VDRIVE. These are all the data out

pins and the BUSY pin. The

CONVST, CS, RD, and CLKIN

signals are related to the DVDD voltage.

POWER-UP

It is recommended that the user perform a dummy conversion

after power-up, because the first conversion result could be

incorrect. This also ensures that the part is in the correct mode

of operation. The recommended power-up sequence is as

follows:

1. GND

4. Digital Inputs

2. VDD

5. REF IN

3. VDRIVE

6. VIN

Power vs. Throughput

The two modes of operation for the AD7470 and AD7472 will

produce different power versus throughput performances,

Mode 1 and Mode 2; see Operating Modes section of the data

sheet for more detailed descriptions of these modes. Mode 2 is

the sleep mode of the part and it achieves the optimum power

performance.

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