AD7829 AD [Analog Devices], AD7829 Datasheet - Page 9

• Manufacturer Part Number: AD7829
• Description: 3 V/5 V, 2 MSPS, 8-Bit, 1-, 4-, 8-Channel Sampling ADCs
• Manufacturer: AD [Analog Devices]

NOTE: Although there is a V
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)
REV. B
0V
0V
Figure 9. Accommodating Bipolar Signals Using
External V
Figure 10. Accommodating Bipolar Signals Using
Internal V
V
V
V
V
2.5V
MID
MID
R3
R2
R3
R2
R4
R1
R4
R1
REF
EXTERNAL
2.5V
V
V
V
MID
0V
0V
IN
IN
2.5V
pin from which a voltage refer-
V
V
V
V
V
V
IN
IN
REF
MID
REF
MID
AD7822/
AD7825/
AD7829
AD7822/
AD7825/
AD7829
–9–
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.
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 V
to normal operation from power-down, the V
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 V
A2
0
0
0
0
1
1
1
1
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.
V
IN
4pF
Figure 11. Equivalent Analog Input Circuit
C2
A1
0
0
1
1
0
0
1
1
V
DD
AD7822/AD7825/AD7829
D1
D2
IN1
310
A0
0
1
0
1
0
1
0
1
R1
IN
to V
Table II.
selection is V
IN4(8)
SW1
for the AD7825 or AD7829.
Analog Input Selected
V
V
V
V
V
V
V
V
IN1
IN2
IN3
IN4
IN5
IN6
IN7
IN8
0.5pF A
C1
IN1
IN
. When returning
SW2
selected will be
B