Datasheet

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 AV
DD
. The connections for the reference
pins are shown below. If the internal reference is being used,
the REF
IN
/REF
OUT
pin should be decoupled with a 100 nF
capacitor to AGND very close to the REF
IN
/REF
OUT
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 REF
IN
/REF
OUT
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
C
REF1
C
REF2
REF
IN
/REF
OUT
0.1µF
0.01µF
0.1µF
AV
DD
DV
DD
0.1µF 0.1µF10µF
ANALOG SUPPLY
+3V TO +5V
Figure 16. Relevant Connections Using Internal Reference
The REF
IN
/REF
OUT
pin may be overdriven by connecting it to
an external reference. This is possible due to the series resis-
tance from the REF
IN
/REF
OUT
pin to the internal reference.
This external reference can be in the range 2.3 V to AV
DD
.
When using AV
DD
as the reference source, the 10 nF capacitor
from the REF
IN
/REF
OUT
pin to AGND should be as close as
possible to the REF
IN
/REF
OUT
pin, and also the C
REF1
pin
should be connected to AV
DD
to keep this pin at the same volt-
age as the reference. The connections for this arrangement are
shown in Figure 17. When using AV
DD
it may be necessary to
add a resistor in series with the AV
DD
supply. This has the effect
of filtering the noise associated with the AV
DD
supply.
Note that when using an external reference, the voltage present
at the REF
IN
/REF
OUT
pin is determined by the external refer-
ence source resistance and the series resistance of 150 kΩ from
the REF
IN
/REF
OUT
pin to the internal 2.5 V reference. Thus, a
low source impedance external reference is recommended.
AD7859/AD7859L
C
REF1
C
REF2
REF
IN
/REF
OUT
0.1µF
0.01µF
0.01µF
AV
DD
DV
DD
0.1µF
0.1µF
10µF
ANALOG SUPPLY
+3V TO +5V
Figure 17. Relevant Connections, AV
DD
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
0
–20
–100
0
10020 40 60 80
–40
–60
–80
AV
DD
= DV
DD
= 3.3V
F
SAMPLE
= 200kHz
F
IN
= 10kHz
SNR = 72.04dB
THD = –88.43dB
–120
SNR – dB
Figure 18. FFT Plot
Figure 19 shows the SNR versus Frequency for different sup-
plies and different external references.
INPUT FREQUENCY – kHz
S(N+D) RATIO – dB
74
73
69
0
10020 40 60 80
72
71
70
AV
DD
= DV
DD
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 AV
DD
supply while the digital supply is left
unaltered. Both the 3.3 V and 5.0 V supply performances are
shown.