AD7608 Analog Devices, AD7608 Datasheet - Page 21

• Manufacturer Part Number: AD7608
• Description: 8-Channel DAS with 18-Bit, Bipolar, Simultaneous Sampling ADC
• Manufacturer: Analog Devices

Specifications of AD7608

• Resolution (bits): 18bit
• # Chan: 8
• Sample Rate: 200kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: Diff-Bip
• Ain Range: Bip 10V,Bip 5.0V
• Adc Architecture: SAR
• Pkg Type: QFP

Data Sheet
INTERNAL/EXTERNAL REFERENCE
The AD7608 contains an on-chip 2.5 V band gap reference. The
REFIN/REFOUT pin allows access to the 2.5 V reference that
generates the on-chip 4.5 V reference internally, or it allows an
external reference of 2.5 V to be applied to the AD7608. An
externally applied reference of 2.5 V is also gained up to 4.5 V,
using the internal buffer. This 4.5 V buffered reference is the
reference used by the SAR ADC.
The REF SELECT pin is a logic input pin that allows the user to
select between the internal reference or an external reference.
If this pin is set to logic high, the internal reference is selected
and enabled. If this pin is set to logic low, the internal reference
is disabled and an external reference voltage must be applied
to the REFIN/REFOUT pin. The internal reference buffer is
always enabled. After a reset, the AD7608 operates in the reference
mode selected by the REF SELECT pin. Decoupling is required
on the REFIN/REFOUT pin for both the internal and external
reference options. A 10 µF ceramic capacitor is required on the
REFIN/REFOUT pin.
The AD7608 contains a reference buffer configured to gain the
REF voltage up to ~4.5 V, as shown in Figure 37. The REFCAPA
and REFCAPB pins must be shorted together externally, and a
ceramic capacitor of 10 μF applied to REFGND, to ensure that
the reference buffer is in closed-loop operation. The reference
voltage available at the REFIN/REFOUT pin is 2.5 V.
When the AD7608 is configured in external reference mode,
the REFIN/REFOUT pin is a high input impedance pin. For
applications using multiple AD7608 devices, the following
configurations are recommended, depending on the application
requirements.
External Reference Mode
One
REFIN/REFOUT pins of all AD7608 devices (see Figure 38).
In this configuration, each REFIN/REFOUT pin of the AD7608
should be decoupled with at least a 100 nF decoupling capacitor.
ADR421
external reference can be used to drive the
Rev. A | Page 21 of 32
Internal Reference Mode
One AD7608 device, configured to operate in the internal refer-
ence mode, can be used to drive the remaining AD7608 devices,
which are configured to operate in external reference mode (see
Figure 39). The REFIN/REFOUT pin of the AD7608, configured
in internal reference mode, should be decoupled using a 10 µF
ceramic decoupling capacitor. The other AD7608 devices,
configured in external reference mode, should use at least a
100 nF decoupling capacitor on their REFIN/REFOUT pins.
ADR421
Figure 38. Single External Reference Driving Multiple AD7608 REFIN Pins
REFIN/REFOUT
Figure 39. Internal Reference Driving Multiple AD7608 REFIN Pins
AD7608
REF SELECT
REFIN/REFOUT
AD7608
REF SELECT
REFIN/REFOUT
0.1µF
+
10µF
2.5V
REF
100nF
V
DRIVE
Figure 37. Reference Circuitry
BUF
REFIN/REFOUT
REFIN/REFOUT
AD7608
AD7608
REF SELECT
REF SELECT
SAR
100nF
100nF
REFCAPB
REFCAPA
REFIN/REFOUT
REFIN/REFOUT
AD7608
AD7608
10µF
REF SELECT
REF SELECT
AD7608
100nF
100nF