ADA4841-X AD [Analog Devices], ADA4841-X Datasheet - Page 11

• Manufacturer Part Number: ADA4841-X
• Manufacturer: AD [Analog Devices]

Preliminary Technical Data
THEORY OF OPERATION
OVERVIEW
The AD7689 is an 8-channel, 16-bit, charge redistribution
successive approximation register (SAR), analog-to-digital
converter (ADC). The AD7689 is capable of converting 250,000
samples per second (250 kSPS) and powers down between
conversions. For example, when operating with an external
reference at 1 kSPS, it consumes TBD μW typically, ideal for
battery-powered applications.
The AD7689 contains all of the components for use in a multi-
channel, low power, data acquisition system including:
all of which are configured through a SPI compatible, 14-bit
register.
The AD7689 provides the user with an on-chip track-and-hold
and does not exhibit pipeline delay or latency.
The AD7689 uses a simple SPI interface for configuring and
receiving conversion results.
The AD7689 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 20-lead, 4mm x 4mm LFCSP that combines space savings and
allows flexible configurations. It is pin-for-pin compatible with
the 16-bit AD7682, AD7699 and 14-bit AD7949.
IN- or
GND
COM
• 16-bit SAR ADC with no missing codes
• 8-channel, low crosstalk multiplexer
• Internal low drift reference and buffer
• Temperature sensor
• Selectable 1-pole filter
• Channel sequencer
CAP
IN+
32,768C
32,768C
16,384C
16,384C
MSB
MSB
Figure 11. ADC Simplified Schematic
4C
4C
Rev. PrC | Page 11 of 20
2C
2C
C
C
CONVERTER OPERATION
The AD7689 is a successive approximation ADC based on a
charge redistribution DAC. Figure 11 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− (or COM) 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 IN+ and IN- (or COM) 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 GND and CAP, the
comparator input varies by binary-weighted voltage steps
(V
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 AD7689 has an on-board conversion clock, the
serial clock, SCK, is not required for the conversion process.
REF
/2, V
C
C
REF
LSB
LSB
/4 ... V
SW+
SW–
REF
COMP
/32,768). The control logic toggles these
SWITCHES CONTROL
CONTROL
LOGIC
CNV
BUSY
OUTPUT CODE
AD7689