EVAL-AD7678CB Analog Devices Inc, EVAL-AD7678CB Datasheet - Page 15

• Manufacturer Part Number: EVAL-AD7678CB
• Description: BOARD EVALUATION FOR AD7678
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7678CB

• Number Of Adc's: 1
• Number Of Bits: 18
• Sampling Rate (per Second): 100k
• Data Interface: Serial, Parallel
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 31mW @ 100kSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7678
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

CIRCUIT INFORMATION
The AD7678 is a very fast, low power, single-supply, precise
18-bit analog-to-digital converter (ADC) using successive
approximation architecture.
The AD7678’s linearity and dynamic range are similar or better
than many - ADCs. With the advantages of its successive
architecture, which ease multiplexing and reduce power with
throughput, it can be advantageous in applications that
normally use - ADCs.
The AD7678 provides the user with an on-chip track/hold,
successive approximation ADC that does not exhibit any
pipeline or latency, making it ideal for multiple multiplexed
channel applications.
The AD7678 can be operated from a single 5 V supply and can
be interfaced to either 5 V or 3 V digital logic. It is housed in a
48-lead LQFP, or a tiny 48-lead LFCSP package that offers space
savings and allows for flexible configurations as either a serial
or parallel interface. The AD7678 is pin-to-pin compatible with
the AD7674, AD7676, and AD7679.
REFGND
REF
IN–
IN+
262,144C 131,072C
262,144C 131,072C
MSB
MSB
Figure 19. ADC Simplified Schematic
Rev. A | Page 15 of 28
4C
4C
2C
2C
CONVERTER OPERATION
The AD7678 is a successive approximation ADC based on a
charge redistribution DAC. Figure 19 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 AGND 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 IN+ and IN– inputs. When the
acquisition phase is complete and the CNVST input goes low, 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 REFGND 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 REFGND and REF, the
comparator input varies by binary weighted voltage steps
(V
switches, starting with the MSB first, to bring the comparator
back into a balanced condition. After completing this process,
the control logic generates the ADC output code and brings the
BUSY output low.
REF
C
C
/2, V
REF
C
C
/4...V
LSB
LSB
REF
SW+
SW–
COMP
/262144). The control logic toggles these
SWITCHES
CONTROL
CONTROL
LOGIC
CNVST
OUTPUT
03084–0–025
BUSY
CODE
AD7678