AD7477AARMZ datasheet(16/28 Pages) AD

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AD7476A/AD7477A/AD7478A

Rev. F | Page 16 of 28

TYPICAL CONNECTION DIAGRAM

Figure 18 shows a typical connection diagram for the AD7476A/

AD7477A/AD7478A. VREF is taken internally from VDD and, as

such, VDD should be well decoupled. This provides an analog

input range of 0 V to VDD. The conversion result is output in a

16-bit word with four leading zeros followed by the MSB of the

12-bit, 10-bit, or 8-bit result. The 10-bit result from the AD7477A

is followed by two trailing zeros, and the 8-bit result from the

AD7478A is followed by four trailing zeros. Alternatively, because

the supply current required by the AD7476A/AD7477A/AD7478A

is so low, a precision reference can be used as the supply source

to the AD7476A/AD7477A/AD7478A. A REF19x voltage refer-

ence (REF195 for 5 V or REF193 for 3 V) can be used to supply

the required voltage to the ADC (see Figure 18). This configuration

is especially useful if the power supply is quite noisy, or if the

system supply voltages are at some value other than 5 V or 3 V

(for example, 15 V).

The REF19x outputs a steady voltage to the AD7476A/

AD7477A/AD7478A. If the low dropout REF193 is used, the

current it needs to supply to the AD7476A/AD7477A/ AD7478A is

typically 1.2 mA. When the ADC is converting at a rate of 1

MSPS, the REF193 needs to supply a maximum of 1.7 mA to the

AD7476A/AD7477A/AD7478A. The load regulation of the

REF193 is typically 10 ppm/mA (VS = 5 V), resulting in an error

of 17 ppm (51 µV) for the 1.7 mA drawn from it. This corresponds

to a 0.069 LSB error for the AD7476A with VDD = 3 V from the

REF193, a 0.017 LSB error for the AD7477A, and a 0.0043 LSB

error for the AD7478A.

For applications where power consumption is a concern, use the

power-down mode of the ADC and the sleep mode of the

REF19x reference to improve power performance. See the

Modes of Operation section.

AD7476A/

AD7477A/

AD7478A

SCLK

SDATA

VIN

GND

0V TO VDD

INPUT

VDD

µC/µP

SERIAL

INTERFACE

0.1

µF

TANT

REF193

1.2mA

680nF

µF

0.1

µF

SUPPLY

Figure 18. REF193 as Power Supply to AD7476A/

AD7477A/AD7478A

Table 7 provides typical performance data with various

references used as a VDD source for a 100 kHz input tone at

room temperature under the same setup conditions.

Table 7. AD7476A Typical Performance for Various Voltage

References

Reference Tied to VDD

AD7476A SNR Performance (dB)

AD780 @ 3 V

72.65

REF193

72.35

AD780 @ 2.5 V

72.5

REF192

72.2

REF43

72.6

ANALOG INPUT

Figure 19 shows an equivalent circuit of the analog input

structure of the AD7476A/AD7477A/AD7478A. The two

diodes, D1 and D2, provide ESD protection for the analog

input. Care must be taken to ensure that the analog input signal

never exceeds the supply rails by more than 300 mV. This

causes the diodes to become forward-biased and start

conducting current into the substrate. The maximum current

these diodes can conduct without causing irreversible damage

to the part is 10 mA. The Capacitor C1 in Figure 19 is typically

about 6 pF and can primarily be attributed to pin capacitance.

The Resistor R1 is a lumped component made up of the on

resistance of a switch. This resistor is typically about 100 Ω. The

Capacitor C2 is the ADC sampling capacitor and has a

capacitance of 20 pF typically.

For ac applications, removing high frequency components from

the analog input signal is recommended by use of a band-pass

filter on the relevant analog input pin. In applications where

harmonic distortion and signal-to-noise ratio are critical, drive

the analog input from a low impedance source. Large source

impedances significantly affect the ac performance of the ADC,

necessitating the use of an input buffer amplifier. The choice of

the op amp is a function of the particular application.

20pF

VDD

VIN

6pF

CONVERSION PHASE – SWITCH OPEN

TRACK PHASE – SWITCH CLOSED

Figure 19. Equivalent Analog Input Circuit

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