AD7476 データシートの表示（PDF） - Analog Devices

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AD7476

1 MSPS, 12-/10-/8-Bit ADCs in 6-Lead SOT-23

Analog Devices 

AD7476/AD7477/AD7478

Alternatively, because the supply current required by the

AD7476/AD7477/AD7478 is so low, a precision reference can be

used as the supply source to the AD7476/AD7477/AD7478. 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 6). 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 AD7476/AD7477/AD7478. If the low

dropout REF193 is used, the current it typically needs to supply

to the AD7476/AD7477/AD7478 is 1 mA. When the ADC is

converting at a rate of 1 MSPS, the REF193 will need to supply a

maximum of 1.6 mA to the AD7476/AD7477/AD7478. The load

regulation of the REF193 is typically 10 ppm/mA (REF193, VS =

5 V), which results in an error of 16 ppm (48 µV) for the 1.6 mA

drawn from it. This corresponds to a 0.065 LSB error for the

AD7476 with VDD = 3 V from the REF193, a 0.016 LSB error for

the AD7477, and a 0.004 LSB error for the AD7478. For applica-

tions 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.

the substrate. These diodes can conduct a maximum of 10 mA

without causing irreversible damage to the part. The capacitor

C1 in Figure 7 is typically about 4 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 typically has a capacitance of 30 pF. 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, 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.

VDD

VIN

C1

4pF

D1

C2

R1 30pF

D2

680nF

1mA

1␮F

TANT

3V

0.1␮F

REF193

10␮F

0V TO VDD

INPUT

VDD

VIN

GND

AD7476/

AD7477/

AD7478

SCLK

SDATA

CS

0.1␮F

5V

SUPPLY

␮C/␮P

SERIAL

INTERFACE

Figure 6. REF193 as Power Supply to AD7476/AD7477/

AD7478

Table I provides some typical performance data with various

references used as a VDD source with a low frequency analog

input. Under the same setup conditions, the references were

compared and the AD780 proved the optimum reference.

Reference Tied

to VDD

AD780 @ 3 V

REF193

AD780 @ 2.5 V

REF192

AD1582

Table I.

AD7476 SNR Performance

1 kHz Input (dB)

71.17

70.4

71.35

70.93

70.05

Analog Input

Figure 7 shows an equivalent circuit of the analog input structure

of the AD7476/AD7477/AD7478. 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. This will cause these diodes

to become forward-biased and start conducting current into

CONVERSION PHASE - SWITCH OPEN

TRACK PHASE - SWITCH CLOSED

Figure 7. Equivalent Analog Input Circuit

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

impedance should be limited to low values. The maximum source

impedance will depend on the amount of total harmonic distortion

(THD) that can be tolerated. The THD will increase as the source

impedance increases and performance will degrade. Figure 8

shows a graph of the total harmonic distortion versus source

impedance for different analog input frequencies when using

a supply voltage of 2.7 V and sampling at a rate of 605 kSPS.

Figures 9 and 10 each show a graph of the total harmonic

distortion versus analog input signal frequency for various supply

voltages while sampling at 993 kSPS with an SCLK frequency of

20 MHz and 605 kSPS with an SCLK frequency of 12 MHz,

respectively.

0

–10

VDD = 2.7V

fS = 605kSPS

–20

–30

–40

fIN = 200kHz

–50

fIN = 300kHz

–60

–70

–80

–90

–100

1

fIN = 100kHz

fIN = 10kHz

10

100

1k

10k

SOURCE IMPEDANCE – ⍀

Figure 8. THD vs. Source Impedance for Various Analog

Input Frequencies

REV. D

–11–

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