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AD7982 Datasheet(PDF) 12 Page - Analog Devices

Part No. AD7982
Description  16-Bit, 100 kSPS PulSAR, Differential ADC in MSOP
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

AD7982 Datasheet(HTML) 12 Page - Analog Devices

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AD7684
Rev. A | Page 12 of 16
APPLICATION INFORMATION
SW+
MSB
16,384C
+IN
LSB
COMP
CONTROL
LOGIC
SWITCHES CONTROL
BUSY
OUTPUT CODE
CNV
REF
GND
–IN
4C
2C
C
C
32,768C
SW–
MSB
16,384C
LSB
4C
2C
C
C
32,768C
Figure 20. ADC Simplified Schematic
CIRCUIT INFORMATION
The AD7684 is a low power, single-supply, 16-bit ADC using a
successive approximation architecture. It is capable of converting
100,000 samples per second (100 kSPS) and powers down
between conversions. When operating at 10 kSPS, for example,
it consumes typically 150 μW with a 2.7 V supply, ideal for
battery-powered applications.
The AD7684 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 AD7684 is specified from 2.7 V to 5.5 V. It is housed in an
8-lead MSOP.
CONVERTER OPERATION
The AD7684 is a successive approximation ADC based on a
charge redistribution DAC. Figure 20 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
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 CS
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 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
(VREF/2, VREF/4...VREF/65,536). 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.
TRANSFER FUNCTIONS
The ideal transfer function for the AD7684 is shown in
Figure 21 and Table 8.
100...000
100...001
100...010
011...101
011...110
011...111
ANALOG INPUT
+FSR – 1.5 LSB
+FSR – 1 LSB
–FSR + 1 LSB
–FSR
–FSR + 0.5 LSB
Figure 21. ADC Ideal Transfer Function
Table 8. Output Codes and Ideal Input Voltages
Description
Analog Input
VREF = 5 V
Digital Output Code Hex
FSR − 1 LSB
+4.999847 V
7FFF1
Midscale + 1 LSB
+152.6 μV
0001
Midscale
0 V
0000
Midscale – 1 LSB
−152.6 μV
FFFF
−FSR + 1 LSB
−4.999847 V
8001
−FSR
−5 V
80002
1 This is also the code for an overranged analog input (V+IN − V−IN above
VREF − VGND).
2 This is also the code for an underranged analog input (V+IN − V−IN below
−VREF + VGND).


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