Datasheet

AD8237 Data Sheet
Rev. 0 | Page 22 of 28
INPUT PROTECTION
If no external protection is used, keep the inputs of the AD8237
within the voltages specified in the absolute maximum ratings. If the
application requires voltages beyond these ratings, input protection
resistors can be placed in series with the inputs of the AD8237 to
limit the current to 5 mA. For example, if +V
S
is 3 V and a 10 V
overload voltage can occur at the inputs, place a protection resistor of
at least (10 V − 3 V)/5 mA = 1.4 kΩ in series with the inputs.
AD8237
R
PROTECT
R
PROTECT
V
+IN
+
–
V
–IN
+
–
+V
S
–V
S
POSITIVE VOLTAGE PROTECTION:
R
PROTECT
>
V
IN
– +V
S
5mA
NEGATIVE VOLTAGE PROTECTION:
R
PROTECT
>
–V
S
– V
IN
5mA
10289-070
Figure 68. Protection Resistors for Large Input Voltages
FILTERING RADIO FREQUENCY INTERFERENCE
The AD8237 contains an on-chip RFI filter that is sufficient for
a majority of applications. For applications where additional radio
frequency immunity is needed, an external RFI filter can also be
applied as shown in Figure 69.
+IN
+V
S
–V
S
C
C
1nF
5%
C
D
10nF
C
C
1nF
5%
10µF
10µF
0.1µF
0.1µF
R
10kΩ
1%
R
10kΩ
1%
AD8237
–IN
10289-071
DIFFERENTIAL FILTER CUTOFF =
1
2 R (2C
D
+ C
C
)
COMMON-MODE FILTER CUTOFF =
1
2 R C
C
Figure 69. Adding Extra RFI Filtering
USING THE REFERENCE PIN
In general, instrumentation amplifier reference pins can be useful
for a few reasons. They provide a means of physically separating the
input and output grounds to reject ground bounce common to the
inputs. They can also be used to precisely level shift the output signal.
In the configuration shown in Figure 65 through Figure 67, the gain
of the reference pin to the output is unity, as is common in a typical
in-amp. Because the reference pin is functionally no different from
the positive input, it can be used with gain, as shown in Figure 70.
This configuration can be very useful in certain cases, such as dc
removal servo loops, which typically use an inverting integrator to
drive REF and compensate for a dc offset. This requires special
attention to the input range (especially at REF) and the output range.
All three input voltages are referred to the one ground shown, which
may need to be a low impedance midsupply.
AD8237
+IN
–IN
REF
FB
V
OUT
R2
R1
V
OUT
= (V
REF
+ V
+IN
– V
–IN
) (1 +
R2
R1
)
10289-072
Figure 70. Applying Gain to the Reference Voltage
Traditional instrumentation amplifier architectures require the
reference pin to be driven with a low impedance source. In these
traditional architectures, impedance at the reference pin degrades
both CMRR and gain accuracy. With the AD8237 architecture,
resistance at the reference pin has no effect on CMRR.
AD8237
+IN
–IN
REF
FB
V
OUT
G = 1 +
R2 + R
REF
R1
V
REF
R1 R2
R
REF
10289-073
Figure 71. Calculating Gain with Reference Resistance