AD8203
APPLICATIONS
+V
+V
S
The AD8203 difference amplifier is intended for applications
that require extracting a small differential signal in the presence
of large common-mode voltages. The input resistance is nominally
320 kΩ, and the device can tolerate common-mode voltages
higher than the supply voltage and lower than ground.
OUT
+IN
NC
OUT
S
V
V
DIFF
2
10kΩ
10kΩ
14R
EXT
GAIN =
R
+ 100kΩ
EXT
AD8203
GAIN
14 – GAIN
DIFF
2
R
= 100kΩ
V
The open collector output stage sources current to within
20 mV of ground and to within 200 mV of VS.
100kΩ
EXT
CM
–IN GND
A1
A2
CURRENT SENSING
R
EXT
High Line, High Current Sensing
Basic automotive applications making use of the large common-
mode range are shown in Figure 2 and Figure 3. The capability
of the device to operate as an amplifier in primary battery sup-
ply circuits is shown in Figure 2. Figure 3 illustrates the ability
of the device to withstand voltages below system ground.
NC = NO CONNECT
Figure 43. Adjusting for Gains < 14
The overall bandwidth is unaffected by changes in gain by using
this method, although there may be a small offset voltage due to
the imbalance in source resistances at the input to the buffer.
This can often be ignored, but if desired, it can be nulled by
inserting a resistor equal to 100 kΩ minus the parallel sum of
EXT and 100 kΩ, in series with Pin 4. For example, with
EXT = 100 kΩ (yielding a composite gain of ×7), the optional
offset nulling resistor is 50 kΩ.
Low Current Sensing
The AD8203 is also used in low current sensing applications,
such as the 4 to 20 mA current loop shown in Figure 42. In such
applications, the relatively large shunt resistor can degrade the
common-mode rejection. Adding a resistor of equal value on the
low impedance side of the input corrects this error.
R
R
Gains Greater Than 14
10Ω
1%
5V
Connecting a resistor from the output of the buffer amplifier to
its noninverting input, as shown in Figure 44, increases the
OUTPUT
+IN
+VS
NC OUT
gain. The gain is now multiplied by the factor REXT/(REXT
−
100 kΩ); for example, the gain is doubled for REXT = 200 kΩ.
Overall gains as high as 50 are achievable this way. Note that the
accuracy of the gain becomes critically dependent on the
resistor value at high gains. Also, the effective input offset
voltage at Pin 1 and Pin 8 (about six times the actual offset of
A1) limits the part’s use in high gain, dc-coupled applications.
+
10Ω
1%
AD8203
–IN GND A1
A2
+V
S
OUT
NC = NO CONNECT
+IN
+VS
NC
OUT
Figure 42. 4 to 20 mA Current Loop Receiver
V
V
DIFF
2
10kΩ
10kΩ
14R
EXT
GAIN =
R
– 100kΩ
EXT
GAIN ADJUSTMENT
AD8203
R
EXT
GAIN
GAIN – 14
DIFF
2
R
= 100kΩ
V
100kΩ
EXT
CM
The default gain of the preamplifier and buffer are ×7 and ×2,
respectively, resulting in a composite gain of ×14. With the
addition of external resistor(s) or trimmer(s), the gain can be
lowered, raised, or finely calibrated.
–IN GND
A1
A2
NC = NO CONNECT
Gains Less Than 14
Figure 44. Adjusting for Gains > 14
Since the preamplifier has an output resistance of 100 kΩ, an
external resistor connected from Pin 3 and Pin 4 to GND
decreases the gain by a factor REXT/(100 kΩ + REXT), as shown
in Figure 43.
Rev. B | Page 14 of 20
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