AD7492 datasheet(16/24 Pages) AD

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AD7492

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CONVST

BUSY

DBx

tWAKEUP

tCONVERT

Figure 21. Mode 2 Operation

VDRIVE

The VDRIVE pin is used as the voltage supply to the digital output

drivers and the digital input circuitry. It is a separate supply

from AVDD and DVDD. The purpose of using a separate supply

for the digital input/output interface is that the user can vary

the output high voltage, VOH, and the logic input levels, VINH

and VINL, from the VDD supply to the AD7492. For example, if

AVDD and DVDD are 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

AD7492 to the top bracket of high performance 12-bit ADCs.

Of course, the ADC can have its VDRIVE and DVDD pins

connected together and be powered from a 3 V or 5 V supply.

The trigger levels are VDRIVE × 0.7 and VDRIVE × 0.3 for the digital

inputs. The pins that are powered from VDRIVE are DB11 to DB0,

CS, RD, CONVST, and BUSY.

PS/FS PIN

As previously mentioned, the PS/FS pin is used to control the

type of power-down mode that the AD7492 can enter into if

operated in Mode 2. This pin can be hardwired either high or

low, or even controlled by another device. It is important to

note that toggling the PS/FS pin while in power-down mode

does not switch the part between partial sleep and full sleep

modes. To switch from one sleep mode to another, the AD7492

has to be powered up and the polarity of the PS/FS pin changed.

It can then be powered down to the required sleep mode.

POWER-UP

It is recommended that the user performs a dummy conversion

after power-up, as 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

2. VDD

3. VDRIVE

4. Digital Inputs

5. VIN

Power vs. Throughput

The two modes of operation for the AD7492 produces different

power vs. throughput performances, Mode 1 and Mode 2; see

the Operating Modes section of the data sheet for more detailed

descriptions of these modes. Mode 2 is the sleep mode

(partial/full) of the part and it achieves the optimum power

performance.

Mode 1

Figure 22 shows the AD7492 conversion sequence in Mode 1

using a throughput rate of 500 kSPS. At 5 V supply, the current

consumption for the part when converting is 3 mA and the

quiescent current is 1.8 mA. The conversion time of 880 ns

contributes 6.6 mW to the overall power dissipation in the

following way:

(880 ns/2 μs) × (5 × 3 mA) = 6.6 mW

The contribution to the total power dissipated by the remaining

1.12 μs of the cycle is 5.04 mW

(1.12 μs/2 μs) × (5 × 1.8 mA) = 5.04 mW

Thus the power dissipated during each cycle is

6.6 mW + 5.04 mW = 11.64 mW

CONVST

BUSY

880ns

tQUIESCENT

tCONVERT

1.12µs

2µs

Figure 22. Mode 1 Power Dissipation

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