AD7660AST Analog Devices Inc, AD7660AST Datasheet - Page 13

• Manufacturer Part Number: AD7660AST
• Description: IC ADC 16BIT UNIPOLAR 48-LQFP
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of AD7660AST

• Rohs Status: RoHS non-compliant
• Number Of Bits: 16
• Sampling Rate (per Second): 100k
• Data Interface: Serial, Parallel
• Number Of Converters: 1
• Power Dissipation (max): 25mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 48-LQFP
• For Use With: EVAL-AD7660CBZ - BOARD EVALUATION FOR AD7660

The AD8519, OP162, or the OP184 meet these requirements
and are usually appropriate for almost all applications. As an
alternative, in very high speed and noise-sensitive applications,
the AD8021 with an external compensation capacitor of 10 pF
or the AD829 with an external compensation capacitor of 82 pF
can be used. This capacitor should have good linearity as an
NPO ceramic or mica type. Moreover, the use of a noninverting
+1 gain arrangement is recommended and helps to obtain the
best signal-to-noise ratio.
Voltage Reference Input
The AD7660 uses an external 2.5 V voltage reference.
The voltage reference input REF of the AD7660 has a dynamic
input impedance; it should therefore be driven by a low impedance
source with an efficient decoupling between REF and REFGND
inputs. This decoupling depends on the choice of the voltage
reference but usually consists of a 1 mF ceramic capacitor and a
low ESR tantalum capacitor connected to the REF and REFGND
inputs with minimum parasitic inductance. 47 mF is an appropriate
value for the tantalum capacitor when used with one of the
recommended reference voltages:
For applications using multiple AD7660s, it is more effective to
buffer the reference voltage with a low noise, very stable op amp
like the AD8031.
Care should also be taken with the reference temperature coeffi-
cient of the voltage reference that directly affects the full-scale
accuracy if this parameter matters. For instance, a ± 15 ppm/∞C
tempco of the reference changes the full scale by ± 1 LSB/∞C.
V
to AVDD – 1.85 V. The benefit here is the increased SNR
obtained as a result of this increase. Since the input range is
defined in terms of V
range to make it a ±3 V input range with an AVDD above 4.85 V.
The theoretical improvement as a result of this increase in
reference is 1.58 dB (20 log [3/2.5]). Due to the theoretical
quantization noise, however, the observed improvement is
approximately 1 dB. The AD780 can be selected with a 3 V
reference voltage.
Power Supply
The AD7660 uses three sets of power supply pins: an analog 5 V
supply AVDD, a digital 5 V core supply DVDD, and a digital
input/output interface supply OVDD. The OVDD supply allows
direct interface with any logic working between 2.7 V and 5.25 V.
To reduce the number of supplies needed, the digital core
(DVDD) can be supplied through a simple RC filter from the
analog supply, as shown in Figure 5. The AD7660 is independent
REV. D
REF
∑ The low noise, low temperature drift ADR421 and AD780
∑ The low power ADR291 voltage reference
∑ The low cost AD1582 voltage reference
voltage references
, as mentioned in the specification table, could be increased
REF
, this would essentially increase the
–13–
of power supply sequencing and thus free from supply voltage
induced latch-up. Additionally, it is very insensitive to power supply
variations over a wide frequency range, as shown in Figure 9.
POWER DISSIPATION VS. THROUGHPUT
The AD7660 automatically reduces its power consumption at
the end of each conversion phase. During the acquisition phase,
the operating currents are very low, which allows a significant
power saving when the conversion rate is reduced, as shown in
Figure 10. This feature makes the AD7660 ideal for very low
power battery applications. It should be noted that the digital
interface remains active even during the acquisition phase. To
reduce the operating digital supply currents even further, the
digital inputs need to be driven close to the power rails (i.e.,
DVDD and DGND for all inputs except EXT/INT, INVSYNC,
INVSCLK, RDC/SDIN, and OVDD or OGND for the last
four inputs.
100000
10000
–50
–55
–60
–65
–70
–75
–80
Figure 10. Power Dissipation vs. Sample Rate
1000
100
10
1k
1
10
Figure 9. PSRR vs. Frequency
100
INPUT FREQUENCY – Hz
10k
THROUGHPUT – SPS
1000
100k
10000
AD7660
1M
100000