Datasheet

AD7984
Rev. A | Page 12 of 24
THEORY OF OPERATION
COMP
CONTROL
LOGIC
SWITCHES CONTROL
BUSY
OUTPUT CODE
CNV
CC2C65,536C 4C131,072C
LSB
SW+
MSB
LSB
SW–
MSB
CC2C65,536C 4C131,072C
IN+
REF
G
ND
IN–
0
6973-011
Figure 21. ADC Simplified Schematic
CIRCUIT INFORMATION
The AD7984 is a fast, low power, single-supply, precise, 18-bit
ADC using a successive approximation architecture and is
capable of converting 1,330,000 samples per second (1.33 MSPS).
The AD7984 provides the user with an on-chip track-and-hold
and does not exhibit any pipeline delay or latency, making it
ideal for multiple multiplexed channel applications.
The AD7984 can be interfaced to any 1.8 V to 5 V digital logic
family. It is available 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 AD7982.
CONVERTER OPERATION
The AD7984 is a successive approximation ADC based on a
charge redistribution DAC. Figure 21 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
input of the comparator 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
REF
/2, V
REF
/4 ... V
REF
/262,144). 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 AD7984 has an on-board conversion clock, the
serial clock, SCK, is not required for the conversion process.