Datasheet

AD623 Data Sheet
Rev. G | Page 28 of 32
INPUT DIFFERENTIAL AND COMMON-MODE
RANGE vs. SUPPLY AND GAIN
Figure 83 shows a simplified block diagram of the AD623. The
voltages at the outputs of Amplifier 1 (A1) and Amplifier 2 (A2)
are given by
V
A2
= V
CM
+ V
DIFF
/2 + 0.6 V + V
DIFF
× R
F
/R
G
= V
CM
+ 0.6 V + V
DIFF
× Gain/2
V
A1
= V
CM
− V
DIFF
/2 + 0.6 V + V
DIFF
× R
F
/R
G
= V
CM
+ 0.6 V − V
DIFF
× Gain/2
+V
S
7
4
–V
S
+V
S
7
4
–V
S
2
3
–IN
+IN
R
F
50kΩ
50kΩ 50kΩ
R
F
50kΩ
50kΩ 50kΩ
OUTPUT
REF
6
5
8
1
GAIN
R
G
A1
A2
A3
V
DIFF
2
–
+
V
DIFF
2
–
+
V
CM
00778-055
Figure 83. Simplified Block Diagram
The voltages on these internal nodes are critical in determining
whether the output voltage is clipped. The V
A1
and V
A2
voltages
can swing from approximately 10 mV above the negative supply
(−V
S
or ground) to within approximately 100 mV of the positive
rail before clipping occurs. Based on this, and from the previous
equations, the maximum and minimum input common-mode
voltages are given by the following equations:
V
CMMAX
= +V
S
− 0.7 V − V
DIFF
× Gain/2
V
CMMIN
= −V
S
− 0.590 V + V
DIFF
× Gain/2
These equations can be rearranged to give the maximum possible
differential voltage (positive or negative) for a particular common-
mode voltage, gain, and power supply. Because the signals on
A1 and A2 can clip on either rail, the maximum differential
voltage is the lesser of the two equations.
|V
DIFFMAX
| = 2 (+V
S
− 0.7 V − V
CM
)/Gain
|V
DIFFMAX
| = 2 (V
CM
− −V
S
+ 0.590 V)/Gain
However, the range on the differential input voltage range is
also constrained by the output swing. Therefore, the range of
V
DIFF
may need to be lower according to the following equation:
Input Range ≤ Available Output Swing/Gain
For a bipolar input voltage with a common-mode voltage that is
roughly half way between the rails, V
DIFFMAX
is half the value that
the previous equations yield because the REF pin is at midsupply.
Note that the available output swing is given for different supply
conditions in the Specifications section.
The equations can be rearranged to result in the maximum gain
for a fixed set of input conditions. The maximum gain is the
lesser of the two equations.
Gain
MAX
= 2 (+V
S
− 0.7 V − V
CM
)/V
DIFF
Gain
MAX
= 2 (V
CM
− −V
S
+ 0.590 V)/V
DIFF
Again, it is recommended that the resulting gain multiplied by
the input range is less than the available output swing. If this is
not the case, the maximum gain is given by
Gain
MAX
= Available Output Swing/Input Range
Also, for bipolar inputs (that is, input range = 2 V
DIFF
), the
maximum gain is half the value yielded by the previous equations
because the REF pin must be at midsupply.
The maximum gain and resulting output swing for different input
conditions is shown in Table 10. Output voltages are referenced
to the voltage on the REF pin.
For the purposes of computation, it is necessary to break down the
input voltage into its differential and common-mode components.
Therefore, when one of the inputs is grounded or at a fixed
voltage, the common-mode voltage changes as the differential
voltage changes. An example of this is the thermocouple
amplifier in Figure 82. The inverting input on the AD623 is
grounded. Therefore, when the input voltage is −10 mV, the
voltage on the noninverting input is −10 mV. For the purpose of
the signal swing calculations, this input voltage must be
composed of a common-mode voltage of −5 mV (that is, (+IN
+ −IN)/2) and a differential input voltage of −10 mV (that is,
+IN − −IN).