AD7621AST Analog Devices Inc, AD7621AST Datasheet - Page 19

• Manufacturer Part Number: AD7621AST
• Description: IC ADC 16BIT 2MSPS DIFF 48LQFP
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of AD7621AST

• Number Of Bits: 16
• Sampling Rate (per Second): 2M
• Data Interface: Serial, Parallel
• Number Of Converters: 1
• Power Dissipation (max): 86mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 48-LQFP
• For Use With: EVAL-AD7621CBZ - BOARD EVALUATION FOR AD7621
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

External Reference (PDBUF = High, PRBUF = High)
To use an external reference directly on the REF pin, PDREF
and PDBUF should both be high.
For improved drift performance, an external reference, such as
the
using the external voltage reference are:
•
•
PDREF and PDBUF power down the internal reference and the
internal reference buffer, respectively.
Reference Decoupling
Whether using an internal or external reference, the AD7621
voltage reference input (REF) has a dynamic input impedance;
therefore, it should be driven by a low impedance source with
efficient decoupling between the REF and REFGND inputs.
This decoupling depends on the choice of the voltage reference,
but usually consists of a low ESR capacitor connected to REF
and REFGND with minimum parasitic inductance. A 10 μF
(X5R, 1206 size) ceramic chip capacitor (or 47 μF tantalum
capacitor) is appropriate when using either the internal
reference or one of these recommended reference voltages:
•
•
•
The placement of the reference decoupling is also important to
the performance of the AD7621. The decoupling capacitor
should be mounted on the same side as the ADC right at the
REF pin with a thick PCB trace. The REFGND should also
connect to the reference decoupling capacitor with the shortest
distance.
For applications that use multiple AD7621 devices, it is more
effective to use the internal reference buffer in order to buffer
the reference voltage.
The voltage reference temperature coefficient (TC) directly
impacts full scale; therefore, in applications where full-scale
accuracy matters, care must be taken with the TC. For instance,
a ±15 ppm/°C TC of the reference changes full-scale by ±1 LSB/°C.
AD780
SNR and dynamic range improvement (about 1.7 dB)
resulting from the use of a reference voltage very close to
the supply (2.5 V) instead of a typical 2.048 V reference
when the internal reference is used. This is calculated by
Power savings when the internal reference is powered
down (PBREF = PDBUF = high).
The low noise, low temperature drift
The low power
The low cost
SNR
or ADR431, can be used. The advantages of directly
=
20
AD1582
log
ADR291
⎛
⎜
⎝
. 2
. 2
048
50
⎞
⎟
⎠
ADR431
and
AD780
Rev. 0 | Page 19 of 32
Temperature Sensor
The TEMP pin measures the temperature of the AD7621. To
improve the calibration accuracy over the temperature range,
the output of the TEMP pin is applied to one of the inputs of the
analog switch (such as, ADG779), and the ADC itself is used to
measure its own temperature. This configuration is shown in
Figure 28.
POWER SUPPLY
The AD7621 uses three sets of power supply pins: an analog
2.5 V supply AVDD, a digital 2.5 V core supply DVDD, and a
digital input/output interface supply OVDD. The OVDD supply
allows direct interface with any logic working between 2.3 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 23.
Power Sequencing
The AD7621 is independent of power supply sequencing once
OVDD does not exceed DVDD by more than 0.3 V until
DVDD = 2.3 V during any time; for instance, at power-up or
power-down (see the Absolute Maximum Ratings section).
Additionally, it is very insensitive to power supply variations
over a wide frequency range as shown in Figure 29.
ANALOG INPUT
(UNIPOLAR)
75
70
65
60
55
50
45
1
ADG779
Figure 28. Use of the Temperature Sensor
Figure 29. PSRR vs. Frequency
10
EXT REF
AD8021
FREQUENCY (kHz)
C
100
C
INT REF
IN+
TEMP
1k
AD7621
TEMPERATURE
SENSOR
AD7621
10k