參數(shù)資料
型號(hào)： EVAL-AD7854CB
廠商： Analog Devices, Inc.
英文描述： 3 V to 5 V Single Supply, 200 kSPS 12-Bit Sampling ADCs
中文描述： 3 V至5 V單電源，200 kSPS的12位采樣ADC
文件頁數(shù)： 16/28頁
文件大?。?/td> 264K
代理商： EVAL-AD7854CB
AD7854/AD7854L
–
16
–
REV. B
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 5V 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
AD7854/
AD7854L
ANALOG
SUPPLY
+3V TO +5V
0.1 F
0.1 F
10 F
0.1 F
0.01 F
0.1 F
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 k
from
the REF
IN
/REF
OUT
pin to the internal 2.5 V reference. Thus, a
low source impedance external reference is recommended.
AV
DD
DV
DD
C
REF1
C
REF2
REF
IN
/REF
OUT
AD7854/
AD7854L
ANALOG
SUPPLY
+3V TO +5V
0.1 F
0.1 F
10 F
0.1 F
0.01 F
0.01 F
Figure 17. Relevant Connections, AV
DD
as the Reference
AD7854/AD7854L PERFORMANCE CURVES
Figure 18 shows a typical FFT plot for the AD7854 at 200 kHz
sample rate and 10 kHz input frequency.
FREQUENCY
–
kHz
0
–
20
–
1200
100
20
S
–
40
60
–
40
–
60
–
80
80
–
100
AV
DD
= DV
DD
= 3.3V
F
SAMPLE
= 200kHz
F
SNR = 72.04dB
THD =
–
88.43dB
Figure 18. FFT Plot
Figure 19 shows the SNR versus frequency for different supplies
and different external references.
INPUT FREQUENCY
–
kHz
74
73
690
100
20
S
–
40
80
72
71
70
60
AV
DD
= DV
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 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
= 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.
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