AD7685CRM Analog Devices Inc, AD7685CRM Datasheet - Page 13

• Manufacturer Part Number: AD7685CRM
• Description: IC ADC 16BIT PSEUDO-DIFF 10-MSOP
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of AD7685CRM

• Design Resources: Parametric Measurement Unit and Supporting Components for PAD Appls Using AD5522 and AD7685 (CN0104) Integrated Device Power Supply for PAD with Output Voltage Range 0 V to 25 V (CN0130)
• Number Of Bits: 16
• Sampling Rate (per Second): 250k
• Data Interface: DSP, MICROWIRE™, QSPI™, Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 15mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-TFSOP (0.118", 3.00mm Width)
• For Use With: EVAL-AD7685CBZ - BOARD EVAL FOR AD7685
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7685 is a fast, low power, single-supply, precise 16-bit
ADC using a successive approximation architecture.
The AD7685 is capable of converting 250,000 samples per
second (250 kSPS) and powers down between conversions.
When operating at 100 SPS, for example, it consumes typically
1.35 μW with a 2.5 V supply, ideal for battery-powered
applications.
The AD7685 provides the user with on-chip, track-and-hold
and does not exhibit any pipeline delay or latency, making it
ideal for multiple multiplexed channel applications.
The AD7685 is specified from 2.3 V to 5.5 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 combines
space savings and allows flexible configurations.
It is pin-for-pin-compatible with the AD7686, AD7687, and
AD7688.
GND
REF
IN+
IN–
32,768C
32,768C
16,384C
16,384C
MSB
MSB
Figure 25. ADC Simplified Schematic
4C
4C
Rev. B | Page 13 of 28
2C
2C
C
C
CONVERTER OPERATION
The AD7685 is a successive approximation ADC based on a
charge redistribution DAC. Figure 25 shows the simplified
schematic of the ADC. The capacitive DAC consists of two
identical arrays of 16 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.
Therefore, 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
inputs IN+ and IN− 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 powers down and
returns to the acquisition phase, and the control logic generates
the ADC output code and a BUSY signal indicator.
Because the AD7685 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
AD7685
/65536).