AD7853_15 datasheet(17/34 Pages) AD

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REV. B

–17–

AD7853/AD7853L

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 relevant

reference pins are shown in the typical connection diagrams. If

the internal reference is being used, the REFIN/REFOUT pin

should have a 100 nF capacitor connected to AGND very close

to the REFIN/REFOUT pin. These connections are shown in

Figure 18.

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

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.

AVDD

DVDD

CREF1

CREF2

REFIN/REFOUT

ANALOG SUPPLY

+3V TO +5V

0.1 F

10 F

0.1 F

0.01 F

0.1 F

AD7853/AD7853L

Figure 18. Relevant Connections When Using Internal

Reference

The other option is that the REFIN/REFOUT pin be overdriven

by connecting it to an external reference. This is possible due to

the series resistance from the REFIN/REFOUT pin to the internal

reference. This external reference can have a range that includes

AVDD. When using AVDD as the reference source, the 100 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

level as the reference. The connections for this arrangement are

shown in Figure 19. When using AVDD it may be necessary to

add a resistor in series with the AVDD supply. This will have the

effect of filtering the noise associated with the AVDD supply.

AVDD

DVDD

CREF1

CREF2

REFIN/REFOUT

ANALOG SUPPLY

+3V TO +5V

0.1 F

10 F

0.1 F

0.01 F

0.1 F

AD7853/AD7853L

Figure 19. Relevant Connections When Using AVDD as the

Reference

PERFORMANCE CURVES

Figure 20 shows a typical FFT plot for the AD7853 at 200 kHz

sample rate and 10 kHz input frequency.

–120

–60

–100

–80

–20

–40

AVDD = DVDD = 3.3V

fSAMPLE = 200kHz

fIN = 10kHz

SNR = 72.04dB

THD = –88.43dB

FREQUENCY – kHz

100

Figure 20. FFT Plot

Figure 21 shows the SNR versus Frequency for different sup-

plies and different external references.

INPUT FREQUENCY – kHz

100

5.0V SUPPLIES, WITH 5V REFERENCE

5.0V SUPPLIES

5.0V SUPPLIES, L VERSION

3.3V SUPPLIES

AVDD = DVDD WITH 2.5V REFERENCE

UNLESS STATED OTHERWISE

Figure 21. SNR vs. Frequency

Figure 22 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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