Datasheet

AD7822/AD7825/AD7829
Rev. C | Page 13 of 28
Capacitor C2 in Figure 15 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 multiplexer and the track-
and-hold. This resistor is typically about 310 Ω. Capacitor C1
is the track-and-hold capacitor and has a capacitance of 0.5 pF.
Switch 1 is the track-and-hold switch, and Switch 2 is that of the
sampling capacitor, as shown in
Figure 6 and Figure 7.
V
IN
C2
4pF
D1
D2
R1
310
SW1
C1
0.5pF
A
B
SW2
V
DD
01321-015
Figure 15. 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 and Switch 2
remains in Position A. The track-and-hold remains in hold
mode for 120 ns (see the
Circuit Description section), 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
IN
selection is V
IN1
. When returning
to normal operation from power-down, the V
IN
selected is the
same one that was selected prior to initiation of power-down.
Table 6 shows the multiplexer address corresponding to each
analog input from V
IN1
to V
IN4(8)
for the AD7825 or AD7829.
Table 6.
A2 A1 A0 Analog Input Selected
0 0 0 V
IN1
0 0 1 V
IN2
0 1 0 V
IN3
0 1 1 V
IN4
1 0 0 V
IN5
1 0 1 V
IN6
1 1 0 V
IN7
1 1 1 V
IN8
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,
that is, on the falling edge of
RD
while
CS
is low, as shown in
Figure 16. This allows for improved throughput rates in “channel
hopping” applications.
CONVST
DB0 TO DB7
A0 TO A2
EOC
CS
RD
t
2
t
1
t
3
t
13
VALID
DATA
ADDRESS CHANNEL y
TRACK CHx
TRACK CHx
HOLD CHx
TRACK CHy
HOLD CHy
120ns
01321-016
Figure 16. Channel Hopping Timing
There is a minimum time delay between the falling edge of
RD
and the next falling edge of the
CONVST
signal, t
13
. This is the
minimum acquisition time required of the track-and-hold to
maintain 8-bit performance.
Figure 17 shows the typical perform-
ance of the AD7825 when channel hopping for various acquisition
times. These results are obtained using an external reference
and internal V
MID
while channel hopping between V
IN1
and V
IN4
with 0 V on Channel 4 and 0.5 V on Channel 1.
ACQUISITION TIME (ns)
8.0
5.0
10500 200
ENOB
100 50 40 30 20 15
7.5
7.0
6.5
6.0
5.5
8.5
01321-017
Figure 17. Effective Number of Bits vs. Acquisition Time for the AD7825
The on-chip track-and-hold can accommodate input frequencies
to 10 MHz, making the AD7822/AD7825/AD7829 ideal for
subsampling applications. When the AD7825 is converting a
10 MHz input signal at a sampling rate of 2 MSPS, the effective
number of bits typically remains above seven, corresponding to
a signal-to-noise ratio of 42 dBs, as shown in
Figure 18.