AD7475ARMZ datasheet(15/24 Pages) AD

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Data Sheet

AD7475/AD7495

Rev. D | Page 15 of 24

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.

SOURCE IMPEDANCE (

Ω)

–90

100

–80

–70

–60

–50

10000

–40

fIN = 500kHz

fIN = 10kHz

fIN = 100kHz

fIN = 200kHz

1000

–30

–20

–10

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.

INPUT FREQUENCY (kHz)

100

–95

–93

–91

–87

1000

–85

VDD = VDRIVE = 3.60V

VDD = VDRIVE = 2.70V

VDD = VDRIVE = 5.25V

VDD = VDRIVE = 4.75V

–83

–81

–79

–77

–75

–89

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.

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 devices.

Ensure VDRIVE does not exceed VDD by more than 0.3 V. (See the

Absolute Maximum Ratings section.)

Reference Section

Use an external reference source 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 device. 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.

Place a capacitor of at least 0.1 µF 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 internally

buffers before applied to the ADC; the output impedance of this

buffer is typically 10 Ω. The reference is capable of sourcing up to

2 mA. Decouple the REF OUT pin 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 recommended

that the internal reference not be over driven. If an ADC with

external reference capability is required, use the AD7475.

REF OUT

Ω

40k

Ω

160k

Ω

Figure 18. AD7495 Reference Circuit

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