Datasheet

AD7678
Rev. A | Page 15 of 28
CIRCUIT INFORMATION
IN+
REF
REFGND
IN–
MSB
4C 2C C C
LSB
SW+
SWITCHES
CONTROL
262,144C 131,072C
MSB
4C 2C C C
LSB
SW–
BUSY
OUTPUT
CODE
CNVST
CONTROL
LOGIC
COMP
262,144C 131,072C
03084–0–025
Figure 19. ADC Simplified Schematic
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.
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
REF
/2, V
REF
/4...V
REF
/262144). The control logic toggles these
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.