AD7475ARM-REEL7 查看數據表（PDF） - Analog Devices

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AD7475ARM-REEL7

1 MSPS, 12-Bit ADCs

Analog Devices 

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

impedance should be limited to low values. The maximum

source impedance depends on the amount of total harmonic

distortion (THD) that can be tolerated. The THD increases as

the source impedance increases and performance degrades.

Figure 16 shows a graph of the total harmonic distortion vs.

source impedance for various analog input frequencies.

–10

–20

–30

fIN = 10kHz

–40

fIN = 500kHz

–50

fIN = 200kHz

–60

–70

fIN = 100kHz

–80

–90

1

10

100

1000

SOURCE IMPEDANCE (Ω)

10000

Figure 16. THD vs. Source Impedance for Various Analog Input Frequencies

Figure 17 shows a graph of total harmonic distortion vs. analog

input frequency for various supply voltages while sampling at

1 MSPS with an SCLK of 20 MHz.

–75

–77

VDD = VDRIVE = 5.25V

–79

VDD = VDRIVE = 2.70V

–81

–83

–85

VDD = VDRIVE = 3.60V

–87

VDD = VDRIVE = 4.75V

–89

–91

–93

–95

10

100

INPUT FREQUENCY (kHz)

1000

Figure 17. THD vs. Analog Input Frequency for Various Supply Voltages

Digital Inputs

The digital inputs applied to the AD7475/AD7495 are not

limited by the maximum ratings, which limit the analog inputs.

Instead, the digital inputs applied can go to 7 V and are not

restricted by the VDD + 0.3 V limit as on the analog inputs.

Another advantage of SCLK and CS not being restricted by the

VDD + 0.3 V limit is that power supply sequencing issues are

avoided. If CS or SCLK are applied before VDD, there is no risk

of latch-up as there would be on the analog inputs if a signal

greater than 0.3 V were applied prior to VDD.

AD7475/AD7495

VDRIVE

The AD7475/AD7495 also has the VDRIVE feature. This feature

controls the voltage at which the serial interface operates. VDRIVE

allows the ADC to easily interface to both 3 V and 5 V

processors.

For example, if the AD7475/AD7495 were operated with a VDD

of 5 V, the VDRIVE pin could be powered from a 3 V supply. The

AD7475/AD7495 have better dynamic performance with a VDD

of 5 V, while still being able to interface to 3 V digital parts.

Care should be taken to ensure VDRIVE does not exceed VDD by

more than 0.3 V. (See the Absolute Maximum Ratings section.)

Reference Section

An external reference source should be used to supply the 2.5 V

reference to the AD7475. Errors in the reference source result

in gain errors in the AD7475 transfer function and add the

specified full-scale errors on the part. The AD7475 voltage

reference input, REF IN, has a dynamic input impedance. A

small dynamic current is required to charge the capacitors in

the capacitive DAC during the bit trials. This current is typically

50 μA for a 2.5 V reference. A capacitor of at least 0.1 μF should

be placed on the REF IN pin. Suitable reference sources for the

AD7475 are the AD780, AD680, AD1582, ADR391, ADR381,

ADR431, and ADR03.

The AD7495 contains an on-chip 2.5 V reference. As shown in

Figure 18, the voltage that appears at the REF OUT pin is

internally buffered before being applied to the ADC; the output

impedance of this buffer is typically 10 Ω. The reference is

capable of sourcing up to 2 mA. The REF OUT pin should be

decoupled to AGND using a 100 nF or greater capacitor.

If the 2.5 V internal reference is used to drive another device

that is capable of glitching the reference at critical times, then

the reference has to be buffered before driving the device. To

ensure optimum performance of the AD7495, it is recom-

mended that the internal reference not be over driven. If an

ADC with external reference capability is required, the AD7475

should be used.

V

160kΩ

40kΩ

REF OUT

25Ω

Figure 18. AD7495 Reference Circuit

Rev. B | Page 15 of 24

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