AD7829 AD [Analog Devices], AD7829 Datasheet - Page 8

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

AD7822/AD7825/AD7829
ADC TRANSFER FUNCTION
The output coding of the AD7822, AD7825, and AD7829 is
straight binary. The designed code transitions occur at succes-
sive integer LSB values (i.e., 1 LSB, 2 LSBs, etc.). The LSB size
is = V
(V
AD7825, and AD7829 is shown in Figure 6, below.
ANALOG INPUT
The AD7822 has a single input channel and the AD7825 and
AD7829 have four and eight input channels respectively. Each
input channel has an input span of 2.5 V or 2.0 V, depending on
the supply voltage (V
up by an on-chip “V
ADCs is detected when V
detected when V
a degree of glitch rejection; for example, a glitch from 5.5 V to
2.7 V up to 60 ns wide will not trip the V
The V
range AGND to V
default input range is AGND to 2.0 V (V
centered about 1.0 V, or AGND to 2.5 V (V
i.e., centered about 1.25 V. When using the default input range,
the V
decoupled to AGND with a 0.1 F capacitor.
If, however, an external V
will be from V
or from V
The range of values of V
value of V
can be applied to V
to V
evant ranges of V
V
various values of V
V
5.5
5.0
4.5
3.3
3.0
2.7
DD
DD
DD
DD
. Figure 7 illustrates the input signal range available with
MID
= 3 V). The ideal transfer characteristic for the AD7822,
REF
MID
V
Internal Max
1.25
1.25
1.25
1.00
1.00
1.00
– 1.25 V when V
MID
/256 (V
pin can be left unconnected or, in some cases, it can be
MID
pin is used to center this input span anywhere in the
DD
. For V
– 1.25 V to V
MID
11111111
00000000
(V
(V
Figure 6. Transfer Characteristic
10000000
111...110
111...000
000...010
000...001
000...111
DD
DD
DD
DD
MID
= 5V) V
= 3V) V
– 1.0 V to V
V
4.25
3.75
3.25
2.3
2.0
1.7
DD
MID
MID
falls below 3.8 V. This circuit also possesses
MID
DD
= 5 V) or the LSB size = (0.8 V
. If no input voltage is applied to V
DD
DD
and the input span for various values of
.
is from 1.0 V to V
MID
MID
). This input span is automatically set
= 3 V ± 10%, the range of values that
Ext
DD
Detector” circuit. 5 V operation of the
MID
1LSB
1LSB = V
DD
MID
– 1.25V
– 1V
ANALOG INPUT VOLTAGE
MID
= 5 V ± 10%. Table I shows the rel-
(V
Table I.
that can be applied depends on the
exceeds 4.1 V and 3 V operation is
DD
is applied, the analog input range
V
3.0 to 5.5
2.5 to 5.0
2.0 to 4.5
1.3 to 3.3
1.0 to 3.0
0.7 to 2.7
MID
+ 1.25 V (V
= 5V)
REF
IN
V
MID
+ 1.0 V (V
Span
/256
1LSB = 0.8V
V
V
MID
MID
(V
+ 1.25V – 1LSB
+ 1V – 1LSB
DD
DD
DD
DD
= 3V)
– 1.0 V and is 1.25 V
V
Min
1.25
1.25
1.25
1.00
1.00
1.00
DD
REF
= 3 V ± 10%) i.e.,
DD
detector.
MID
DD
/256
= 5 V ± 10%).
= 5 V ± 10%)
= 3 V ± 10%),
Ext
REF
V
0 to 2.5
0 to 2.5
0 to 2.5
0 to 2.0
0 to 2.0
0 to 2.0
MID
IN
)/256
Span
, the
–8–
V
offset to the V
accommodate bipolar signals by applying V
circuit before V
driven by an external source, the source may be directly tied to
the level-shifting circuitry (see Figure 9); however, if the internal
V
be buffered before applying it to the level-shifting circuitry, as
the V
Figure 10).
MID
MID
, i.e., the default value, is being used as an output, it must
5V
4V
3V
2V
1V
3V
2V
1V
Figure 7. Analog Input Span Variation with V
may be used to remove offsets in a system by applying the
MID
V
MID
V
IN
pin has an impedance of approximately 6 kΩ (see
V
Figure 8. Removing Offsets Using V
V
V
MID
V
MID
DD
DD
MID
= N/C (1.25V)
= 3V
= 5V
= N/C (1V)
IN
, as shown in Figure 9. When V
pin as shown in Figure 8, or it may be used to
V
MID
V
MID
= 1.5V
V
MID
= 2.5V
V
V
MID
IN
V
MID
= 2V
V
MID
INPUT SIGNAL RANGE
INPUT SIGNAL RANGE
MID
FOR VARIOUS V
FOR VARIOUS V
= 3.75V
AD7822/
AD7825/
AD7829
to a level-shifting
MID
MID
MID
MID
is being
MID
REV. B