EVAL-AD7859CB View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD7859CB' PDF : 28 Pages View PDF
AD7859/AD7859L
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 PERFORMANCE CURVES
Figure 18 shows a typical FFT plot for the AD7859 at 200 kHz
sample rate and 10 kHz input frequency.
0
AVDD = DVDD = 3.3V
–20
FSAMPLE = 200kHz
FIN = 10kHz
SNR = 72.04dB
–40
THD = –88.43dB
–60
–80
ANALOG SUPPLY
+3V TO +5V
10µF
0.1µF
0.1µF
0.1µF
0.01µF
CREF1 AVDD
DVDD
AD7859/AD7859L
CREF2
–100
–120
0
20
40
60
80
100
FREQUENCY – kHz
Figure 18. FFT Plot
Figure 19 shows the SNR versus Frequency for different sup-
plies and different external references.
0.1µF
REFIN/REFOUT
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 kΩ from
the REFIN/REFOUT pin to the internal 2.5 V reference. Thus, a
low source impedance external reference is recommended.
ANALOG SUPPLY
+3V TO +5V
10µF
0.1µF
0.1µF
0.1µF
0.01µF
AVDD
CREF1
DVDD
AD7859/AD7859L
CREF2
74
AVDD = DVDD WITH 2.5V REFERENCE
UNLESS STATED OTHERWISE
73
5.0V SUPPLIES, WITH 5V REFERENCE
72
5.0V SUPPLIES
5.0V SUPPLIES,
71
L VERSION
3.3V SUPPLIES
70
69
0
20
40
60
80
100
INPUT FREQUENCY – kHz
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.
0.01µF
REFIN/REFOUT
Figure 17. Relevant Connections, AVDD as the Reference
REV. A
–17–
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