AD7690 Analog Devices, AD7690 Datasheet - Page 12
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AD7690
Manufacturer Part Number
AD7690
Description
Manufacturer
Analog Devices
Datasheet
1.AD7690.pdf
(24 pages)
Specifications of AD7690
Resolution (bits)
18bit
# Chan
1
Sample Rate
400kSPS
Interface
Ser,SPI
Analog Input Type
Diff-Uni
Ain Range
(2Vref) p-p
Adc Architecture
SAR
Pkg Type
CSP,SOP
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AD7690
THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7690 is a fast, low power, single-supply, precise, 18-bit
ADC using a successive approximation architecture.
The AD7690 is capable of converting 400,000 samples per
second (400 kSPS) and powers down between conversions.
When operating at 1 kSPS, for example, it consumes 50 μW
typically, ideal for battery-powered applications.
The AD7690 provides the user with an on-chip track-and-hold
and does not exhibit pipeline delay or latency, making it ideal
for multiple multiplexed channel applications.
The AD7690 is specified from 4.75 V to 5.25 V and can be
interfaced to any 1.8 V to 5 V digital logic family. It is housed in
a 10-lead MSOP or a tiny 10-lead QFN (LFCSP) that allows
space savings and flexible configurations.
It is pin-for-pin compatible with the 18-bit
and the 16-bit AD7687, AD7688, and AD7693.
GND
REF
IN+
IN–
131,072C
131,072C
AD7691
65,536C
65,536C
MSB
MSB
and
Figure 24. ADC Simplified Schematic
AD7982
4C
4C
Rev. B | Page 12 of 24
2C
2C
C
C
CONVERTER OPERATION
The AD7690 is a successive approximation ADC based on a
charge redistribution DAC. Figure 24 shows the simplified
schematic of the ADC. The capacitive DAC consists of two
identical arrays of 18 binary-weighted capacitors, which are
connected to the two comparator inputs.
During the acquisition phase, terminals of the array tied to the
comparator’s input are connected to GND via SW+ and SW−.
All independent switches are connected to the analog inputs.
Thus, the capacitor arrays are used as sampling capacitors and
acquire the analog signal on the IN+ and IN− inputs. When the
acquisition phase is complete and the CNV input goes high, a
conversion phase is initiated. When the conversion phase
begins, SW+ and SW− are opened first. The two capacitor
arrays are then disconnected from the inputs and connected to
the GND input. Therefore, the differential voltage between the
IN+ and IN− inputs captured at the end of the acquisition phase
is applied to the comparator inputs, causing the comparator to
become unbalanced. By switching each element of the capacitor
array between GND and REF, the comparator input varies by
binary-weighted voltage steps (V
The control logic toggles these switches, starting with the MSB,
to bring the comparator back into a balanced condition. After
the completion of this process, the part returns to the
acquisition phase, and the control logic generates the ADC
output code and a busy signal indicator.
Because the AD7690 has an on-board conversion clock, the
serial clock, SCK, is not required for the conversion process.
C
C
LSB
LSB
SW+
SW–
COMP
SWITCHES CONTROL
CONTROL
LOGIC
CNV
REF
/2, V
BUSY
OUTPUT CODE
REF
/4 ... V
REF
/262,144).