AD7859 Analog Devices, AD7859 Datasheet - Page 17

• Manufacturer Part Number: AD7859
• Description: 3 V to 5 V Single Supply, 200 kSPS 8-Channel, 12-Bit, Parallel Sampling ADCs
• Manufacturer: Analog Devices

Specifications of AD7859

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 200kSPS
• Interface: Byte,Par
• Analog Input Type: SE-Bip,SE-Uni
• Ain Range: (Vref) p-p,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: LCC,QFP

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