Datasheet

Data Sheet AD8208
Rev. C | Page 13 of 20
4 mA to 20 mA Current Loop Receiver
The AD8208 can also be used in low current-sensing applica-
tions, such as the 4 mA to 20 mA current loop receiver shown
in Figure 29. In such applications, the relatively large shunt
resistor may degrade the common-mode rejection. Adding a
resistor of equal value on the low impedance side of the input
corrects this error.
GND
NC
–IN
+IN
A1
V
S
A2
OUT
AD8208
BATTERY
10Ω
1%
10Ω
1%
NC = NO CONNECT
08714-028
C
F
OUTPUT
5V
+
–
Figure 29. 4 mA to 20 mA Current Loop Receiver
GAIN ADJUSTMENT
The default gain of the preamplifier and buffer are 10 V/V and
2 V/V, respectively, resulting in a composite gain of 20 V/V. With
the addition of external resistor(s) or trimmer(s), the gain can
be lowered, raised, or finely calibrated.
Gains Less than 20
Because 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 the following factor (see Figure 30):
R
EXT
/(100 kΩ + R
EXT
)
GND
NC
–IN
+IN
A1
V
S
A2
OUT
AD8208
V
DIFF
V
CM
NC = NO CONNECT
08714-029
R
EXT
OUTPUT
GAIN =
20R
EXT
R
EXT
+ 100kΩ
5V
R
EXT
= 100kΩ
GAIN
20 – GAIN
+
–
+
–
Figure 30. Adjusting for Gains Less than 20
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. In
many cases, this can be ignored, but if desired, the offset voltage can
be nulled by inserting a resistor in series with Pin 4. The resistor
used should be equal to 100 kΩ minus the parallel sum of R
EXT
and 100 kΩ. For example, with R
EXT
= 100 kΩ (yielding a composite
gain of 10 V/V), the optional offset nulling resistor is 50 kΩ.
Gains Greater than 20
Connecting a resistor from the output of the buffer amplifier to
its noninverting input, as shown in Figure 31, increases the gain.
The gain is now multiplied by the factor
R
EXT
/(R
EXT
− 100 kΩ)
For example, it is doubled for R
EXT
= 200 kΩ. Overall gains as
high as 50 are achievable in this way. Note that the accuracy of
the gain becomes critically dependent on the resistor value at
high gains. In addition, the effective input offset voltage at Pin 1
and Pin 8 (which is about six times the actual offset of A1) limits
the use of the part in high gain, dc-coupled applications.
GND
NC
–IN
+IN
A1
V
S
A2
OUT
AD8208
V
DIFF
V
CM
NC = NO CONNECT
08714-030
R
EXT
OUTPUT
GAIN =
20R
EXT
R
EXT
– 100kΩ
5V
R
EXT
= 100kΩ
GAIN
GAIN – 20
+
–
+
–
Figure 31. Adjusting for Gains Greater than 20
GAIN TRIM
Figure 32 shows a method for incremental gain trimming by
using a trim potentiometer and an external resistor, R
EXT
.
The following approximation is useful for small gain ranges:
ΔG ≈ (10 MΩ ÷ R
EXT
)%
For example, using this equation, the adjustment range is ±2%
for R
EXT
= 5 MΩ and ±10% for R
EXT
= 1 MΩ.
GND
NC
–IN
+IN
A1
V
S
A2
R
EXT
OUT
AD8208
5V
V
DIFF
V
CM
NC = NO CONNECT
08714-031
OUTPUT
GAIN TRIM
20kΩ MIN
+
–
+
–
Figure 32. Incremental Gain Trimming