EVAL-AD7986EBZ Analog Devices Inc, EVAL-AD7986EBZ Datasheet - Page 13

• Manufacturer Part Number: EVAL-AD7986EBZ
• Description: 18-Bit A/D Converter Eval. Board
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7986EBZ

• Silicon Manufacturer: Analog Devices
• Application Sub Type: ADC
• Kit Application Type: Data Converter
• Silicon Core Number: AD7986
• Kit Contents: Board
• Number Of Adc's: 1
• Number Of Bits: 18
• Sampling Rate (per Second): 2M
• Data Interface: Serial
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 29mW @ 2MSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7986
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7986 is a fast, low power, single-supply, precise, 18-bit
ADC using a successive approximation architecture. The AD7986
features different modes to optimize performance according to
the application. In turbo mode, the AD7986 is capable of convert-
ing 2,000,000 samples per second (2 MSPS).
The AD7986 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 AD7986 can be interfaced to any 1.8 V to 2.7 V digital logic
family. It is available in a 20-lead LFCSP (QFN) that allows space
savings and flexible configurations.
CONVERTER OPERATION
The AD7986 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 that 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 AGND 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
REFGND
REF
IN+
IN–
131,072C
131,072C
65,536C
65,536C
MSB
MSB
Figure 21. ADC Simplified Schematic
4C
4C
Rev. B | Page 13 of 28
2C
2C
C
C
two capacitor arrays are then disconnected from the analog
inputs and connected to the REFGND input. Therefore, the
differential voltage between Input IN+ and Input 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
REFGND and REF, the comparator input varies by binary-
weighted voltage steps (V
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 AD7986 has an on-board conversion clock, the
serial clock, SCK, is not required for the conversion process.
CONVERSION MODES OF OPERATION
The AD7986 features two conversion modes of operation: turbo
and normal. Turbo conversion mode (TURBO = high) allows
the fastest conversion rate of up to 2 MSPS, and does not power
down between conversions. The first conversion in turbo mode
should be ignored because it contains meaningless data. For
applications that require lower power and slightly slower
sampling rates, the normal mode (TURBO = low) allows a
maximum conversion rate of 1.5 MSPS, and powers down
between conversion. The first conversion in normal mode does
contain meaningful data.
C
C
LSB
LSB
SW+
SW–
COMP
SWITCHES CONTROL
REF
/2, V
CONTROL
LOGIC
CNV
REF
/4 … V
OUTPUT CODE
BUSY
REF
/262,144). The
AD7986