
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 AV
DD
. The connections for the relevant
reference pins are shown in the typical connection diagrams. If
the internal reference is being used, the REF
IN
/REF
OUT
pin
should have a 100 nF capacitor connected to AGND very close
to the REF
IN
/REF
OUT
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 REF
IN
/REF
OUT
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
μ
V p-p.
AV
DD
DV
DD
C
REF1
C
REF2
REF
IN
/REF
OUT
ANALOG SUPPLY
+3V TO +5V
0.1
m
F
10
m
F
0.1
m
F
0.01
m
F
0.1
m
F
0.1
m
F
AD7853/AD7853L
Figure 18. Relevant Connections When Using Internal
Reference
The other option is that the REF
IN
/REF
OUT
pin be overdriven
by connecting it to an external reference. This is possible due to
the series resistance from the REF
IN
/REF
OUT
pin to the internal
reference. This external reference can have a range that includes
AV
DD
. When using AV
DD
as the reference source, the 100 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
level as the reference. The connections for this arrangement are
shown in Figure 19. When using AV
DD
it may be necessary to
add a resistor in series with the AV
DD
supply. This will have the
effect of filtering the noise associated with the AV
DD
supply.
AV
DD
DV
DD
C
REF1
C
REF2
REF
IN
/REF
OUT
ANALOG SUPPLY
+3V TO +5V
0.1
m
F
10
m
F
0.1
m
F
0.01
m
F
0.1
m
F
0.1
m
F
AD7853/AD7853L
Figure 19. Relevant Connections When Using AV
DD
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.
0
–120
–60
–100
20
–80
0
–20
–40
80
60
40
AV
DD
= DV
DD
= 3.3V
f
SAMPLE
= 200kHz
f
IN
= 10kHz
SNR = 72.04dB
THD = –88.43dB
FREQUENCY – kHz
Figure 20. FFT Plot
100
S
Figure 21 shows the SNR versus Frequency for different sup-
plies and different external references.
INPUT FREQUENCY – kHz
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 AV
DD
supply while the digital supply is left
unaltered. Both the 3.3 V and 5.0 V supply performances are
shown.
74
73
69
0
100
S
20
40
60
80
72
71
70
5.0V SUPPLIES, WITH 5V REFERENCE
5.0V SUPPLIES
5.0V SUPPLIES, L VERSION
3.3V SUPPLIES
AV
DD
= DV
DD
WITH 2.5V REFERENCE
UNLESS STATED OTHERWISE