AD7274 datasheet(17/28 Pages) AD

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AD7273/AD7274

Rev. 0 | Page 16 of 28

TYPICAL CONNECTION DIAGRAM

Figure 27 shows a typical connection diagram for the AD7273/

AD7274. An external reference must be applied to the ADC.

This reference can be in the range of 1.4 V to VDD. A precision

reference, such as the REF19x family or the ADR421, can be

used to supply the reference voltage to the AD7273/AD7274.

The conversion result is output in a 16-bit word with two leading

zeros followed by the 12-bit or 10-bit result. The 12-bit result from

the AD7274 is followed by two trailing zeros, and the 10-bit result

from the AD7273 is followed by four trailing zeros.

Table 7 provides some typical performance data with various

references under the same setup conditions for the AD7274.

Table 7. AD7274 Performance (Various Voltage Reference IC)

Voltage Reference

AD7274 SNR Performance

1 MHz Input

AD780 @ 2.5 V

71.3 dB

AD780 @ 3 V

70.1 dB

REF195

70.9 dB

AD7273/

AD7274

VDD

VIN

SERIAL

INTERFACE

0V TO VREF

INPUT

DSP/

μC/μP

VREF

AGND/DGND

SCLK

SDATA

0.1

μF

10pF

0.1

μF

2.5V

3.6V

SUPPLY

4.6 mA

REF195

Figure 27. AD7273/AD7274 Typical Connection Diagram

ANALOG INPUT

Figure 28 shows an equivalent circuit of the analog input

structure of the AD7273/AD7274. The two diodes, D1 and D2,

provide ESD protection for the analog inputs. Care must be

taken to ensure that the analog input signal never exceeds the

supply rails by more than 300 mV. Signals exceeding this value

cause these diodes to become forward biased and to start

conducting current into the substrate. These diodes can

conduct a maximum current of 10 mA without causing

irreversible damage to the part. Capacitor C1 in Figure 28 is

typically about 4 pF and can primarily be attributed to pin

capacitance. Resistor R1 is a lumped component made up of the

on resistance of a switch. This resistor is typically about 75 Ω.

Capacitor C2 is the ADC sampling capacitor and has a capacitance

of 32 pF typically. For ac applications, removing high frequency

components from the analog input signal is recommended by

using a band-pass filter on the relevant analog input pin. In

applications where harmonic distortion and signal-to-noise

ratio are critical, the analog input should be driven from a low

impedance source. Large source impedances significantly affect

the ac performance of the ADCs. This may necessitate the use

of an input buffer amplifier. The AD8021 op amp is compatible

with this device; however, the choice of the op amp is a function

of the particular application.

4pF

CONVERSION PHASE–SWITCH OPEN

TRACK PHASE–SWITCH CLOSED

VDD

VIN

Figure 28. Equivalent Analog Input Circuit

When no amplifier is used to drive the analog input, the source

impedance should be limited to a low value. The maximum source

impedance depends on the amount of THD that can be tolerated.

The THD increases as the source impedance increases and perfor-

mance degrades. Figure 14 shows a graph of the THD vs. the

analog input frequency for different source impedances when

using a supply voltage of 3 V and sampling at a rate of 3 MSPS.

DIGITAL INPUTS

The digital inputs applied to the AD7273/AD7274 are not

limited by the maximum ratings that limit the analog inputs.

Instead, the digital inputs can be applied at up to 6 V and are

not restricted by the VDD + 0.3 V limit of the analog inputs. For

example, if the AD7273/AD7274 were operated with a VDD of

3 V, then 5 V logic levels could be used on the digital inputs.

However, it is important to note that the data output on SDATA

still has 3 V logic levels when VDD = 3 V. Another advantage of

SCLK and CS not being restricted by the VDD + 0.3 V limit is

that power supply sequencing issues are avoided. For example,

unlike with the analog inputs, with the digital inputs, if CS or

SCLK are applied before VDD, there is no risk of latch-up.

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