AD7851 Analog Devices, AD7851 Datasheet - Page 18

• Manufacturer Part Number: AD7851
• Description: 14-Bit, 333 kSPS, Serial Sampling A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7851

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 333kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: DIP,SOIC,SOP

AD7851
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 AV
ence pins are shown in the typical connection diagrams. If the
internal reference is being used, the REF
have a 100 nF capacitor connected to AGND very close to the
REF
If the internal reference is required for use external to the ADC,
it should be buffered at the REF
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.
The other option is that the REF
by connecting it to an external reference. This is possible due to
the series resistance from the REF
reference. This external reference can have a range that includes
AV
capacitor from the REF
close as possible to the REF
pin should be connected to AV
level as the reference. The connections for this arrangement are
shown in Figure 19. When using AV
add a resistor in series with the AV
effect of filtering the noise associated with the AV
ANALOG
SUPPLY
Figure 19. Relevant Connections When Using AV
as the Reference
DD
IN
. When using AV
Figure 18. Relevant Connections When Using
Internal Reference
/REF
5V
ANALOG
SUPPLY
10 F
OUT
0.01 F
0.1 F
5V
10
pin. These connections are shown in Figure 18.
DD
470nF
0.1 F
10 F
DD
IN
. The connections for the relevant refer-
0.01 F
0.01 F
/REF
0.01 F
as the reference source, the 100 nF
IN
C
C
REF
REF1
REF2
/REF
0.01 F
OUT
DD
IN
IN
/REF
IN
470nF
0.1 F
AV
/REF
IN
to keep this pin at the same
/REF
DD
OUT
pin to AGND should be as
/REF
DD
OUT
DD
supply. This will have the
10
OUT
pin, and also the C
it may be necessary to
AD7851
OUT
C
C
REF
IN
OUT
REF1
REF2
AV
/REF
IN
pin and a 100 nF
pin be overdriven
DD
/REF
0.1 F
pin to the internal
10
OUT
OUT
DV
AD7851
DD
DD
pin should
supply.
0.1 F
DV
DD
REF1
DD
–18–
AD7851 PERFORMANCE CURVES
Figure 20 shows a typical FFT plot for the AD7851 at 333 kHz
sample rate and 10 kHz input frequency.
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).
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.
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 AV
unaltered.
–100
–120
–20
–40
–60
–80
79
78
77
76
75
0
0
0
10
Figure 21. SNR vs. Frequency
PSRR (dB) = 10 log (Pf/Pfs)
20
20
Figure 20. FFT Plot
DD
INPUT FREQUENCY (kHz)
supply while the digital supply is left
FREQUENCY (kHz)
50
40
80
60
100
AV
f
f
SNR = 79.5dB
THD = –95.2
SAMPLE
IN
DD
= 10kHz
120
= DV
= 333kHz
80
DD
140
= 5V
166
100
REV. B