AD7652 Analog Devices, AD7652 Datasheet - Page 18

• Manufacturer Part Number: AD7652
• Manufacturer: Analog Devices

Specifications of AD7652

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 500kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: Uni (Vref),Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

AD7652
Voltage Reference Input
The AD7652 allows the choice of either a very low temperature
drift internal voltage reference or an external 2.5 V reference.
Unlike many ADCs with internal references, the internal
reference of the AD7652 provides excellent performance and
can be used in almost all applications.
To use the internal reference along with the internal buffer,
PDREF and PDBUF should both be LOW. This will produce a
1.207 V voltage on REFBUFIN which, amplified by the buffer,
will result in a 2.5 V reference on the REF pin.
The output impedance of REFBUFIN is 11 k
the internal reference is enabled.
REFBUFIN with a 100 nF ceramic capacitor. Thus, the 100 nF
capacitor provides an RC filter for noise reduction.
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 the 2.5 V
reference to be applied to REFBUFIN.
To use an external reference directly on REF pin, PDREF and
PDBUF should both be HIGH.
PDREF and PDBUF respectively power down the internal
reference and the internal reference buffer. Note that the PDREF
and PDBUF input current should never exceed 20 mA. This
could eventually occur when input voltage is above AVDD (for
instance at power up). In this case, a 100 Ω series resistor is
recommended.
The internal reference is temperature compensated to 2.5 V ±
20 mV. The reference is trimmed to provide a typical drift of 7
ppm/°C
For improved drift performance, an external reference such as
the
The AD7652 voltage reference input REF has a dynamic input
impedance; it should therefore 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 tanta-
lum capacitor) is appropriate when using either the internal
reference or one of these recommended reference voltages:
•
•
•
AD780
The low noise, low temperature drift
The low power
The low cost
. This typical drift characteristic is shown in
can be used.
AD1582
ADR291
It is useful to decouple
ADR421
Ω (minimum) when
and
Figure 17
AD780
Rev. 0 | Page 18 of 28
.
For applications that use multiple AD7652s, it is more effective
to use the internal buffer to buffer the reference voltage.
Care should be taken with the voltage reference’s temperature
coefficient, which directly affects the full-scale accuracy if this
parameter matters. For instance, a ±15 ppm/°C temperature
coefficient 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 3 V with an AVDD above 4.85 V.
The
The TEMP pin, which measures the temperature of the AD7652,
can be used as shown in
applied to one of the inputs of the analog switch (e.g., ADG779),
and the ADC itself is used to measure its own temperature. This
configuration is very useful for improving the calibration
accuracy over the temperature range.
Power Supply
The AD7652 uses three power supply pins: an analog 5 V supply
AVDD, a digital 5 V core supply DVDD, and a digital
input/output interface supply OVDD. OVDD allows direct
interface with any logic between 2.7 V and DVDD + 0.3 V. To
reduce the supplies needed, the digital core (DVDD) can be
supplied through a simple RC filter from the analog supply, as
shown in
supply sequencing once OVDD does not exceed DVDD by
more than 0.3 V, and is thus free of supply voltage induced
latch-up.
ANALOG INPUT
(UNIPOLAR)
AD780
Figure 22
Figure 24. Temperature Sensor Connection Diagram
REF
can be selected with a 3 V reference voltage.
can be increased to AVDD – 1.85 V. Since the
ADG779
. The AD7652 is independent of power
AD8021
Figure 24
C
C
. The output of TEMP pin is
REF
IN
, this would essentially
TEMP
AD7652
TEMPERATURE
SENSOR
02965-0-024