AD7477AARM datasheet(15/24 Pages) AD

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REV. C

AD7476A/AD7477A/AD7478A

–15–

TYPICAL CONNECTION DIAGRAM

Figure 7 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

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

the AD7478A will be 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 reference (REF195 for 5 V or REF193 for 3 V)

can be used to supply the required voltage to the ADC (see Figure 7).

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 (e.g., 15 V). The REF19x will output 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 will

need to supply a maximum of 1.7 mA to the AD7476A/

AD7477A/AD7478A. The load regulation of the REF193 is typi-

cally 10 ppm/mA (VS = 5 V), which results 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 of concern, the power-down mode of the ADC and the sleep

mode of the REF19x reference should be used to improve

power performance. See the Modes of Operation section.

AD7476A/

AD7477A/

AD7478A

SCLK

SDATA

VIN

GND

0V TO VDD

INPUT

VDD

C/ P

SERIAL

INTERFACE

0.1 F

1 F

TANT

REF193

1.2mA

680nF

10 F

0.1 F

SUPPLY

Figure 7. REF193 as Power Supply to AD7476A/

AD7477A/AD7478A

Table I provides some 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 I. AD7476A Typical Performance for Various

Voltage References IC

Reference Tied

AD7476A SNR Performance

to VDD

(dB)

AD780 @ 3 V

72.65

REF193

72.35

AD780 @ 2.5 V

72.5

REF192

72.2

REF43

72.6

Analog Input

Figure 8 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 will cause these 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 8 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 applica-

tions, 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, the analog input should be driven

from a low impedance source. Large source impedances will

significantly affect the ac performance of the ADC. This may

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

the op amp will be a function of the particular application.

20pF

VDD

VIN

6pF

CONVERSION PHASE – SWITCH OPEN

TRACK PHASE – SWITCH CLOSED

Figure 8. Equivalent Analog Input Circuit

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