AD8237ARMZ-R7 datasheet(23/29 Pages) AD

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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 +VS 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

RPROTECT

V+IN

–

V–IN

–

+VS

–VS

POSITIVE VOLTAGE PROTECTION:

RPROTECT >

VIN – +VS

5mA

NEGATIVE VOLTAGE PROTECTION:

RPROTECT >

–VS – VIN

5mA

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

+VS

–VS

1nF

10nF

1nF

10µF

0.1µF

10kΩ

AD8237

–IN

DIFFERENTIAL FILTER CUTOFF =

2 R (2CD + CC)

COMMON-MODE FILTER CUTOFF =

2 R CC

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

VOUT

VOUT = (VREF + V+IN – V–IN) (1 +

)

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

VOUT

G = 1 +

R2 + RREF

VREF

RREF

Figure 71. Calculating Gain with Reference Resistance

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