EVAL-AD7643CBZ Analog Devices Inc, EVAL-AD7643CBZ Datasheet - Page 19

• Manufacturer Part Number: EVAL-AD7643CBZ
• Description: BOARD EVALUATION FOR AD7643
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7643CBZ

• Number Of Adc's: 1
• Number Of Bits: 18
• Sampling Rate (per Second): 1.25M
• Data Interface: Serial, Parallel
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 65mW @ 1.25MSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7643
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Internal Reference (PDBUF = Low, PDREF = Low)
To use the internal reference, the PDREF and PDBUF inputs
must both be low. This produces a 1.2 V band gap output on
REFBUFIN, which is amplified by the internal buffer and
results in a 2.048 V reference on the REF pin.
The internal reference is temperature compensated to 2.048 V ±
10 mV. The reference is trimmed to provide a typical drift of
8 ppm/°C. This typical drift characteristic is shown in Figure 7.
The output resistance of REFBUFIN is 6.33 kΩ (minimum)
when the internal reference is enabled. It is necessary to
decouple this with a ceramic capacitor greater than 100 nF.
Therefore, the capacitor provides an RC filter for noise reduction.
Because the output impedance of REFBUFIN is typically
6.33 kΩ, relative humidity (among other industrial contaminates)
can directly affect the drift characteristics of the reference.
Typically, a guard ring is used to reduce the effects of drift
under such circumstances. However, because the AD7643 has a
fine lead pitch, guarding this node is not practical. Therefore, in
these industrial and other types of applications, it is recommended
to use a conformal coating, such as Dow Corning® 1-2577 or
HumiSeal® 1B73.
External 1.2 V Reference and Internal Buffer (PDBUF =
Low, PDREF = High)
To use an external reference along with the internal buffer,
PDREF should be high and PDBUF should be low. This powers
down the internal reference and allows an external 1.2 V
reference to be applied to REFBUFIN, producing 2.048 V
(typically) on the REF pin.
External 2.5 V Reference (PDBUF = High, PDREF = High)
To use an external 2.5 V 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. The input current
of PDREF and PDBUF should never exceed 20 mA. This can
AD780
The 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
The power savings when the internal reference is powered
down (PDREF high).
SNR
=
or ADR431, can be used. The advantages of directly
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048
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occur when the driving voltage is above AVDD (for instance, at
power-up). In this case, a 125 Ω series resistor is recommended.
Reference Decoupling
Whether using an internal or external reference, the AD7643
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 the recommended reference voltages.
The placement of the reference decoupling is also important to
the performance of the AD7643. 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 AD7643 devices, it is more
effective to use an external reference with the internal reference
buffer to buffer the reference voltage. However, because the
reference buffers are not unity gain, ratiometric, simultaneously
sampled designs should use an external reference and external
buffer, such as the AD8031/AD8032; therefore, preserving the
same reference level for all converters.
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 ±4 ppm/°C TC of the reference changes full scale by ±1 LSB/°C.
Note that V
input range is defined in terms of V
increase the range to 0 V to 2.8 V with an AVDD = 2.7 V.
Temperature Sensor
The TEMP pin measures the temperature of the AD7643. 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.
ANALOG INPUT
(UNIPOLAR)
REF
can be increased to AVDD + 0.1 V. Because the
Figure 28. Use of the Temperature Sensor
ADG779
AD8021
C
C
REF
IN+
, this would essentially
TEMP
AD7643
TEMPERATURE
SENSOR
AD7643