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AD7484BSTZ

3 MSPS, 14-Bit SAR ADC

Analog Devices 

AD7484

Data must not be read from the AD7484 while a conversion is

taking place. For this reason, if operating the AD7484 at

throughput speeds greater than 2.5 MSPS, it is necessary to tie

both the CS pin and RD pin on the AD7484 low and use a

buffer on the data lines. This situation may also arise in the case

where a read operation cannot be completed in the time after

the end of one conversion and the start of the quiet period

before the next conversion.

The maximum slew rate at the input of the ADC must be

limited to 500 V/µs while BUSY is low to avoid corrupting the

ongoing conversion. In any multiplexed application where the

channel is switched during conversion, this is to happen as soon

as possible after the BUSY falling edge.

Reading Data from the AD7484

Data is read from the part via a 15-bit parallel data bus with the

standard CS and RD signals. The CS and RD signals are internally

gated to enable the conversion result onto the data bus. The data

lines D0 to D14 leave their high impedance state when both CS

and RD are logic low. Therefore, CS can be permanently tied

logic low if required, and the RD signal used to access the

conversion result. Figure 29 shows a timing specification called

tQUIET. This is the amount of time that must be left after any data

bus activity before the next conversion is initiated.

Writing to the AD7484

The AD7484 features a user accessible offset register. This allows

the bottom of the transfer function to be shifted by ±200 mV. This

feature is explained in more detail in the Offset/Overrange section.

To write to the offset register, a 15-bit word is written to the

AD7484 with the 12 LSBs containing the offset value in twos

complement format. The 3 MSBs must be set to 0. The offset

value must be within the range −1310 to +1310, corresponding

to an offset from −200 mV to +200 mV. The value written to the

offset register is stored and used until power is removed from the

device, or the device is reset. The value stored may be updated at

any time between conversions by another write to the device.

Table 9 shows some examples of offset register values and their

effective offset voltage. Figure 30 shows a timing diagram for

writing to the AD7484.

Table 9. Offset Register Examples

Code

(Decimal)

D14 to D12

D11 to D0 (Twos

Complement)

−1310

000

1010 1110 0010

−512

000

1110 0000 0000

+256

000

0001 0000 0000

+1310

000

0101 0001 1110

Offset

(mV)

−200

−78.12

+39.06

+200

Driving the CONVST Pin

To achieve the specified performance from the AD7484, the

CONVST pin must be driven from a low jitter source. Because

the falling edge on the CONVST pin determines the sampling

instant, any jitter that may exist on this edge appears as noise

when the analog input signal contains high frequency components.

The relationship between the analog input frequency (fIN), timing

jitter (tj), and resulting SNR is given by

( ) SNRJITTER (dB) = 10 log

1

2π × f IN × t j

2

For example, if the desired SNR due to jitter is 100 dB with a

maximum full-scale analog input frequency of 1.5 MHz, ignor-

ing all other noise sources, the result is an allowable jitter on the

CONVST falling edge of 1.06 ps. For a 14-bit converter (ideal

SNR = 86.04 dB), the allowable jitter is greater than 1.06 ps, but

due consider-ation must be given to the design of the CONVST

circuitry to achieve 14-bit performance with large analog input

frequencies.

Typical Connection

Figure 23 shows a typical connection diagram for the AD7484

operating in Parallel Mode 1. Conversion is initiated by a falling

edge on CONVST. When CONVST goes low, the BUSY signal

goes low, and at the end of conversion, the rising edge of BUSY

is used to activate an interrupt service routine. The CS and RD

lines are then activated to read the 14 data bits (15 bits if using

the overrange feature).

In Figure 23, the VDRIVE pin is tied to DVDD, which results in

logic output levels being either 0 V or DVDD. The voltage applied to

VDRIVE controls the voltage value of the output logic signals. For

example, if DVDD is supplied by a 5 V supply and VDRIVE is supplied

by a 3 V supply, the logic output levels are either 0 V or 3 V. This

feature allows the AD7484 to interface to 3 V devices while still

enabling the ADC to process signals at a 5 V supply.

DIGITAL

SUPPLY

4.75V TO 5.25V

ANALOG

SUPPLY

4.75V TO 5.25V

+

10µF

1nF

0.1µF

+

0.1µF

47µF

0.1µF

ADM809

VDRIVE DVDD AVDD

RESET

MODE1

CBIAS

MODE2

REFSEL

WRITE

CLIP

NAP

REFIN

STBY

AD7484

PARALLEL

INTERFACE

D0 TO D13

REFOUT

CS

CONVST

RD

VIN

BUSY

1nF

AD780 2.5V

REFERENCE

0.47µF

0.47µF

0V TO 2.5V

Figure 23. Typical Connection Diagram

Rev. C | Page 16 of 20

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