AD7822 查看數據表（PDF） - Analog Devices
零件编号
产品描述 (功能)
生产厂家
AD7822 Datasheet PDF : 20 Pages
| |||
2.5V
R4
R3
VREF
VMID AD7822/
AD7825/
AD7829
V
R2
V
VIN
R1
0V
VIN
2.5V
0V
Figure 9. Accommodating Bipolar Signals Using
External VMID
AD7822/AD7825/AD7829
being conducted into the substrate due to an over voltage on an
unselected channel, can cause inaccurate conversions on a
selected channel. The capacitor C2 in Figure 11 is typically
about 4 pF and can be primarily attributed to pin capacitance.
The resistor, R1, is a lumped component made up of the on
resistance of several components, including that of the multi-
plexer and the track and hold. This resistor is typically about
310 Ω. The capacitor C1 is the track-and-hold capacitor and
has a capacitance of 0.5 pF. Switch 1 is the track-and-hold switch,
while Switch 2 is that of the sampling capacitor as shown in
Figures 2 and 3.
VDD
VIN
C2
4pF
D1
R1
310⍀
D2
SW1
C1
0.5pF A SW2
B
EXTERNAL
2.5V
R4
R3
V
R2
V
VREF
VMID
VIN
AD7822/
AD7825/
AD7829
R1
0V
VIN
VMID
0V
Figure 10. Accommodating Bipolar Signals Using
Internal VMID
NOTE: Although there is a VREF pin from which a voltage refer-
ence of 2.5 V may be sourced, or to which an external reference
may be applied, this does not provide an option of varying
the value of the voltage reference. As stated in the specifications
for the AD7822, AD7825, and AD7829, the input voltage range
at this pin is 2.5 V ± 2%.
Analog Input Structure
Figure 11 shows an equivalent circuit of the analog input structure
of the AD7822, AD7825, and the AD7829. The two diodes, D1
and D2, provide ESD protection for the analog inputs. Care
must be taken to ensure that the analog input signal never exceeds
the supply rails by more than 200 mV. This will cause these
diodes to become forward biased and start conducting current into
the substrate. 20 mA is the maximum current these diodes can
conduct without causing irreversible damage to the part. How-
ever, it is worth noting that a small amount of current (1 mA)
Figure 11. Equivalent Analog Input Circuit
When in track phase, Switch 1 is closed and Switch 2 is in
Position A; when in hold mode, Switch 1 opens while Switch
2 remains in Position A. The track-and-hold remains in hold
mode for 120 ns—see Circuit Description—after which it returns
to track mode and the ADC enters its conversion phase. At this
point, Switch 1 opens and Switch 2 moves to Position B. At the
end of the conversion, Switch 2 moves back to Position A.
Analog Input Selection
On power-up, the default VIN selection is VIN1. When returning
to normal operation from power-down, the VIN selected will be
the same one that was selected prior to power-down being initiated.
Table II below shows the multiplexer address corresponding to
each analog input from VIN1 to VIN4(8) for the AD7825 or AD7829.
Table II.
A2
A1
0
0
0
0
0
1
0
1
1
0
1
0
1
1
1
1
A0
Analog Input Selected
0
VIN1
1
VIN2
0
VIN3
1
VIN4
0
VIN5
1
VIN6
0
VIN7
1
VIN8
Channel selection on the AD7825 and AD7829 is made without
the necessity of a write operation. The address of the next channel
to be converted is latched at the start of the current read operation,
i.e., on the falling edge of RD while CS is low, as shown in Figure
12. This allows for improved throughput rates in “channel
hopping” applications.
REV. B
–9–
Share Link: 