AD7634 Analog Devices, AD7634 Datasheet - Page 22

• Manufacturer Part Number: AD7634
• Manufacturer: Analog Devices

Specifications of AD7634

• Resolution (bits): 18bit
• # Chan: 1
• Sample Rate: 670kSPS
• Interface: Byte,Par,Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: 10V p-p,20 V p-p,40 V p-p
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

AD7634
This circuit can also be made discretely, and thus more flexible,
using any of the recommended low noise amplifiers in Table 9.
Again, to preserve the SNR of the converter, the resistors, R
and R
VOLTAGE REFERENCE INPUT/OUTPUT
The AD7634 allows the choice of either a very low temperature
drift internal voltage reference, an external reference, or an
external buffered reference.
The internal reference of the AD7634 provides excellent perform-
ance and can be used in almost all applications. However, the
linearity performance is guaranteed only with an external reference.
Internal Reference (REF = 5 V) (PDREF = Low,
PDBUF = Low)
To use the internal reference, the PDREF and PDBUF inputs
must be low. This enables the on-chip, band gap reference, buffer,
and TEMP sensor, resulting in a 5.00 V reference on the REF pin.
The internal reference is temperature-compensated to 5.000 V
± 35 mV. The reference is trimmed to provide a typical drift of
3 ppm/°C. This typical drift characteristic is shown in Figure 19.
External 2.5 V Reference and Internal Buffer (REF = 5 V)
(PDREF = High, PDBUF = Low)
To use an external reference 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 producing 5 V on the REF pin. The internal ref-
erence buffer is useful in multiconverter applications because
a buffer is typically required in these applications to avoid
reference coupling amongst the different converters.
External 5 V Reference (PDREF = High, PDBUF = High)
To use an external reference directly on the REF pin, PDREF
and PDBUF should both be high. PDREF and PDBUF power
down the internal reference and the internal reference buffer,
respectively. For improved drift performance, an external refer-
ence, such as the
Reference Decoupling
Whether using an internal or external reference, the AD7634
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 22 μF (X5R,
1206 size) ceramic chip capacitor (or 47 μF low ESR tantalum
capacitor) is appropriate when using either the internal refer-
ence or the ADR445/ADR435 external reference.
The placement of the reference decoupling is also important to
the performance of the AD7634. 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
G
, should be kept low.
ADR445
or
ADR435,
is recommended.
F
Rev. A | Page 22 of 32
the reference decoupling capacitor with the shortest distance
and to the analog ground plane with several vias.
For applications that use multiple AD7634 or other PulSAR
devices, it is more effective to use the internal reference buffer
to buffer the external 2.5 V 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 ±4 ppm/°C TC of the reference changes full scale by ±1 LSB/°C.
Temperature Sensor
The TEMP pin measures the temperature of the AD7634. 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 32.
POWER SUPPLIES
The AD7634 uses five sets of power supply pins:
•
•
•
•
•
Core Supplies
The AVDD and DVDD supply the AD7634 analog and digital
cores, respectively. Sufficient decoupling of these supplies is
required consisting of at least a 10 μF capacitor and a 100 nF
capacitor on each supply. The 100 nF capacitors should be
placed as close as possible to the AD7634. To reduce the number
of supplies needed, the DVDD can be supplied through a simple
RC filter from the analog supply, as shown in Figure 27.
High Voltage Supplies
The high voltage bipolar supplies, VCC and VEE, are required
and must be at least 2 V larger than the maximum input voltage.
For example, if using the ±10 V range, the supplies should be
±12 V minimum. This allows for 40 V p-p fully differential
input (±10 V on each input IN+ and IN−). Sufficient decoup-
ling of these supplies is also required consisting of at least a
10 μF capacitor and a 100 nF capacitor on each supply. For
unipolar operation, the VEE supply can be grounded with
some slight THD performance degradation.
ANALOG INPUT
AVDD: analog 5 V core supply
VCC: analog high voltage positive supply
VEE: high voltage negative supply
DVDD: digital 5 V core supply
OVDD: digital input/output interface supply
ADG779
Figure 32. Use of the Temperature Sensor
C
C
IN+
TEMP
AD7634
TEMPERATURE
SENSOR