EVAL-AD7621CB View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD7621CB' PDF : 32 Pages View PDF
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 AD780 or ADR431, can be used. The advantages of directly
using the external voltage reference are:
ï‚· 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
SNR

20
log

2.048
2.50


ï‚· Power savings when the internal reference is powered
down (PBREF = PDBUF = high).
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:
ï‚· The low noise, low temperature drift ADR431 and AD780
ï‚· The low power ADR291
ï‚· The low cost AD1582
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.
AD7621
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 29.
ADG779
TEMP
ANALOG INPUT
(UNIPOLAR)
IN+
AD8021 CC
TEMPERATURE
SENSOR
AD7621
Figure 29. Use of the Temperature Sensor
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 24.
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 30.
75
70
65
EXT REF
60
55
INT REF
50
45
1
10
100
1k
10k
FREQUENCY (kHz)
Figure 30. PSRR vs. Frequency
Rev. A | Page 19 of 32
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