Datasheet

AD7688
Rev. A | Page 12 of 28
SW+MSB
16,384C
IN+
LSB
COMP
CONTROL
LOGIC
SWITCHES CONTROL
BUSY
OUTPUT CODE
CNV
REF
GND
IN–
4C 2C C C32,768C
SW–MSB
16,384C
LSB
4C 2C C C32,768C
02973-023
Figure 24. ADC Simplified Schematic
CIRCUIT INFORMATION
The AD7688 is a fast, low power, single-supply, precise 16-bit
ADC using a successive approximation architecture.
The AD7688 is capable of converting 500,000 samples per
second (500 kSPS) and powers down between conversions.
When operating at 100 SPS, for example, it consumes 3.75 μW
typically, ideal for battery-powered applications.
The AD7688 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 AD7688 is specified from 4.5 V to 5.5 V and can be
interfaced to any of the 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 AD7685, AD7686, and
AD7687.
CONVERTER OPERATION
The AD7688 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 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.
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
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
/65536).
The control logic toggles these switches, starting with the MSB,
in order 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 AD7688 has an on-board conversion clock, the
serial clock, SCK, is not required for the conversion process.