AD7859AS datasheet(18/29 Pages) AD

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AD7859/AD7859L

REV. A

–17–

REFERENCE SECTION

For specified performance, it is recommended that when using

an external reference, this reference should be between 2.3 V

and the analog supply AVDD. The connections for the reference

pins are shown below. If the internal reference is being used,

the REFIN/REFOUT pin should be decoupled with a 100 nF

capacitor to AGND very close to the REFIN/REFOUT pin. These

connections are shown in Figure 16.

If the internal reference is required for use external to the ADC,

it should be buffered at the REFIN/REFOUT pin and a 100 nF

capacitor should be connected from this pin to AGND. The typical

noise performance for the internal reference, with 5 V supplies is

150 nV/

√Hz @ 1 kHz and dc noise is 100 µV p-p.

AD7859/AD7859L

CREF1

CREF2

REFIN/REFOUT

0.1µF

0.01µF

0.1µF

AVDD

DVDD

0.1µF

10µF

ANALOG SUPPLY

+3V TO +5V

Figure 16. Relevant Connections Using Internal Reference

The REFIN/REFOUT pin may be overdriven by connecting it to

an external reference. This is possible due to the series resis-

tance from the REFIN/REFOUT pin to the internal reference.

This external reference can be in the range 2.3 V to AVDD.

When using AVDD as the reference source, the 10 nF capacitor

from the REFIN/REFOUT pin to AGND should be as close as

possible to the REFIN/REFOUT pin, and also the CREF1 pin

should be connected to AVDD to keep this pin at the same volt-

age as the reference. The connections for this arrangement are

shown in Figure 17. When using AVDD it may be necessary to

add a resistor in series with the AVDD supply. This has the effect

of filtering the noise associated with the AVDD supply.

Note that when using an external reference, the voltage present

at the REFIN/REFOUT pin is determined by the external refer-

ence source resistance and the series resistance of 150 k

Ω from

the REFIN/REFOUT pin to the internal 2.5 V reference. Thus, a

low source impedance external reference is recommended.

AD7859/AD7859L

CREF1

CREF2

REFIN/REFOUT

0.1µF

0.01µF

AVDD

DVDD

0.1µF

10µF

ANALOG SUPPLY

+3V TO +5V

Figure 17. Relevant Connections, AVDD as the Reference

AD7859/AD7859L PERFORMANCE CURVES

Figure 18 shows a typical FFT plot for the AD7859 at 200 kHz

sample rate and 10 kHz input frequency.

FREQUENCY – kHz

–20

–100

100

–40

–60

–80

AVDD = DVDD = 3.3V

FSAMPLE = 200kHz

FIN = 10kHz

SNR = 72.04dB

THD = –88.43dB

–120

Figure 18. FFT Plot

Figure 19 shows the SNR versus Frequency for different sup-

plies and different external references.

INPUT FREQUENCY – kHz

100

AVDD = DVDD WITH 2.5V REFERENCE

UNLESS STATED OTHERWISE

5.0V SUPPLIES, WITH 5V REFERENCE

5.0V SUPPLIES

5.0V SUPPLIES,

L VERSION

3.3V SUPPLIES

Figure 19. SNR vs. Frequency

Figure 20 shows the Power Supply Rejection Ratio versus Fre-

quency for the part. The Power Supply Rejection Ratio is de-

fined as the ratio of the power in ADC output at frequency f to

the power of a full-scale sine wave.

PSRR (dB) = 10 log (Pf/Pfs)

Pf = Power at frequency f in ADC output, Pfs = power of a full-

scale sine wave. Here a 100 mV peak-to-peak sine wave is

coupled onto the AVDD supply while the digital supply is left

unaltered. Both the 3.3 V and 5.0 V supply performances are

shown.

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