Datasheet

AD7822/AD7825/AD7829
Rev. C | Page 12 of 28
5V
4V
3V
2V
1V
V
DD
= 5V
INPUT SIGNAL RANGE
FOR VARIOUS V
MID
V
MID
= N/C (1.25V)
V
MID
= 2.5V
V
MID
= 3.75V
3V
2V
1V
V
DD
= 3V
INPUT SIGNAL RANGE
FOR VARIOUS V
MID
V
MID
= N/C (1V)
V
MID
= 1.5V
V
MID
= 2V
0
1321-011
Figure 11. Analog Input Span Variation with V
MID
V
MID
can be used to remove offsets in a system by applying the
offset to the V
MID
pin as shown in Figure 12, or it can be used to
accommodate bipolar signals by applying V
MID
to a level-shifting
circuit before V
IN
, as shown in Figure 13. When V
MID
is being
driven by an external source, the source can be directly tied to
the level-shifting circuitry (see
Figure 13). However, if the
internal V
MID
, that is, the default value, is being used as an
output, it must be buffered before applying it to the level-
shifting circuitry because the V
MID
pin has an impedance of
approximately 6 kΩ (see
Figure 14).
V
IN
V
MID
AD7822/
AD7825/
AD7829
V
MID
V
IN
V
MID
01321-012
Figure 12. Removing Offsets Using V
MID
V
REF
V
MID
V
IN
R3
R4
R2
R1
V
V
0V
V
IN
0V
2.5V
AD7822/
AD7825/
AD7829
2.5V
01321-013
Figure 13. Accommodating Bipolar Signals Using External V
MID
R2
V
REF
V
MID
V
IN
EXTERNAL
2.5V
R3
R4
R1
V
V
0
V
V
IN
0V
V
MID
AD7822/
AD7825/
AD7829
01321-014
Figure 14. Accommodating Bipolar Signals Using Internal V
MID
NOTE: Although there is a V
REF
pin from which a voltage
reference of 2.5 V can be sourced, or to which an external
reference can 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/AD7829, the input
voltage range at this pin is 2.5 V ± 2%.
Analog Input Structure
Figure 15 shows an equivalent circuit of the analog input
structure of the AD7822/AD7825/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. Doing so causes
these diodes to become forward biased and start conducting
current into the substrate. A maximum current of 20 mA can be
conducted by these diodes without causing irreversible damage
to the part. However, it is worth noting that a small amount of
current (1 mA) being conducted into the substrate, due to an
overvoltage on an unselected channel, can cause inaccurate
conversions on a selected channel.