AD7851AR datasheet(18/36 Pages) AD

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AD7851

–18–

REV. B

REFERENCE SECTION

For specified performance, it is recommended that when using

an external reference this reference should be between 4 V and

the analog supply AVDD. The connections for the relevant refer-

ence 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

capacitor 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.

REFIN/REFOUT

AD7851

ANALOG

SUPPLY

AVDD

DVDD

0.01 F

0.1 F

10 F

CREF1

CREF2

0.01 F

0.1 F

470nF

0.1 F

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.

REFIN/REFOUT

AD7851

ANALOG

SUPPLY

AVDD

DVDD

0.01 F

0.1 F

10 F

CREF1

CREF2

0.01 F

470nF

0.1 F

Figure 19. Relevant Connections When Using AVDD

as the Reference

AD7851 PERFORMANCE CURVES

Figure 20 shows a typical FFT plot for the AD7851 at 333 kHz

sample rate and 10 kHz input frequency.

FREQUENCY (kHz)

–20

–120

100

–40

–60

–80

–100

AVDD = DVDD = 5V

SAMPLE = 333kHz

IN = 10kHz

SNR = 79.5dB

THD = –95.2

Figure 20. FFT Plot

Figure 21 shows the SNR versus frequency for a 5 V supply and

a 4.096 external reference (5 V reference is typically 1 dB better

performance).

INPUT FREQUENCY (kHz)

166

120

140

100

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 defined

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 is the power at frequency f in ADC output, Pfs is the 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.

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