AD1671
85
80
75
70
65
60
55
50
45
40
Figure 4 plots both S/(N+D) and Effective Number of Bits
(ENOB) for a 100 kHz input signal sampled from 666 kHz to
1.25 MHz.
11.75
11.50
11.25
11.00
72.5
72
71.5
71
70.5
70
–25
–50 –45 –40 –35 –30
–20 –15 –10 –5
0
69.5
69
ANALOG INPUT – dB
68.5
68
Figure 7. Spurious Free Dynamic Range vs. Input
Amplitude, fIN = 250 kHz
666
714
769
833
909
1000
1111
1250
SAMPLING FREQUENCY – kHz
APPLYING THE AD1671
GROUNDING AND DECOUPLING RULES
Figure 4. S/(N/D) vs. Sampling Frequency, fIN = 100 kHz
Proper grounding and decoupling should be a primary design
objective in any high speed, high resolution system. The
AD1671 separates analog and digital grounds to optimize the
management of analog and digital ground currents in a system.
The AD1671 is designed to minimize the current flowing from
REF COM (Pin 20) by directing the majority of the current
from VCC (+5 V–Pin 28) to VEE (–5 V–Pin 1). Minimizing ana-
log ground currents hence reduces the potential for large ground
voltage drops. This can be especially true in systems that do not
utilize ground planes or wide ground runs. REF COM is also
configured to be code independent, therefore reducing input de-
pendent analog ground voltage drops and errors. Code depen-
dent ground current is diverted to ACOM (Pin 27). Also critical
in any high speed digital design is the use of proper digital
grounding techniques to avoid potential CMOS “ground
bounce.” Figure 3 is provided to assist in the proper layout,
grounding and decoupling techniques.
Figure 5 is a THD plot for a full-scale 100 kHz input signal with
the sample frequency swept from 666 kHz to 1.25 MHz.
–68
–70
–72
–74
–76
–78
–80
–82
–84
–86
666
714
769
833
909
1000
1111
1250
SAMPLING FREQUENCY – kHz
Figure 5. THD vs. Sampling Rate, fIN = 100 kHz
+5V
–5V
+5V
The AD1671’s SFDR performance is ideal for use in communi-
cation systems such as high speed modems and digital radios.
The SFDR is better than 84 dB with sample rates up to 1.11 MHz
and increases as the input signal amplitude is attenuated by ap-
proximately 3 dB. Note also the SFDR is typically better than
80 dB with input signals attenuated by up to –7 dB.
0.1µF
0.1µF 10µF
0.1µF 10µF
10µF
28
1
18
V
V
V
LOGIC
CC
EE
23
22
20
27
BIT 1 13
AIN1
–68
–70
–72
AD1671
V
(±5V)
2
BIT 12
AIN2
IN
REF COM
–74
–76
–78
–80
–82
–84
–86
–88
–90
ACOM
AGP*
ENCODE 17
19
25
DCOM
DGP*
16
15
14
DAV
OTR
MSB
SHA OUT
26 BPO/UPO
24
21
REF IN
666
714
769
833
909
1000
1111
1250
SAMPLING FREQUENCY – kHz
REF OUT
1µF
Figure 6. Spurious Free Dynamic Range vs. Sampling
Rate, fIN = 100 kHz
*GROUND PLANE RECOMMENDED
Figure 8. AD1671 Grounding and Decoupling
–8–
REV. B
ADI [ ADI ]
