Datasheet

Data Sheet AD8237
Rev. 0 | Page 23 of 28
Resistance at the reference pin does affect the gain of the AD8237;
however, if this resistance is constant, the gain setting resistors can be
adjusted to compensate. For example, the AD8237 can be driven
with a voltage divider, as shown in Figure 72.
AD8237
+IN
–IN
REF
FB
V
OUT
G = 1 +
R2 + R3
||R4
R1
R1
R2
R3
R4
V
S
10289-074
Figure 72. Using Voltage Divider to Set Reference Voltage
LAYOUT
Common-Mode Rejection Ratio over Frequency
Poor layout can cause some of the common-mode signal to be
converted to a differential signal before reaching the in-amp. This
conversion can occur when the path to the positive input pin has
a different frequency response than the path to the negative input
pin. For best CMRR vs. frequency performance, closely match the
impedance of each path. Place additional source resistance in the
input path (for example, for input protection) close to the in-amp
inputs to minimize interaction between the resistors and the parasitic
capacitance from the printed circuit board (PCB) traces.
Power Supplies
Use a stable dc voltage to power the instrumentation amplifier.
Noise on the supply pins can adversely affect performance. For
more information, see the PSRR performance curves in Figure 17
through Figure 20.
Place a 0.1 µF capacitor as close as possible to each supply pin.
As shown in Figure 73, a 10 µF tantalum capacitor can be used farther
away from the part. This capacitor, which is intended to be effective at
low frequencies, can usually be shared by other precision integrated
circuits. Keep the traces between these integrated circuits short to
minimize interaction of the trace parasitic inductance with the shared
capacitor. If a single supply is used, decoupling capacitors at −V
S
can be omitted.
R1 R2
AD8237
+V
S
+IN
–IN
0.1µF
10µF
0.1µF 10µF
–V
S
V
OUT
10289-075
REF
FB
Figure 73. Supply Decoupling, REF, and Output Referred to Local Ground
Reference
The output voltage of the AD8237 is developed with respect to
the potential on the reference terminal. Take care to tie REF to
the appropriate local ground.
INPUT BIAS CURRENT RETURN PATH
The input bias current of the AD8237 must have a return path to
ground. When the source, such as a thermocouple, cannot provide
a return current path, create one, as shown in Figure 74.