Preliminary Technical Data

AD8426

Rev. PrD | Page 13 of 20

HIDDEN

PADDLE

BOTTOM VIEW

EXPOSED LEAD

FRAME TABS

NOTES

1. EXPOSED LEAD FRAME TABS AT THE FOUR CORNERS

OF THE PACKAGE ARE INTERNALLY CONNECTED TO

FURTHER INFORMATION ON PACKAGE AVAILABILITY.

Figure 10. Hidden Paddle Package, Bottom View

Common-Mode Rejection Ratio over Frequency

Poor layout can cause some of the common-mode signals to be

converted to differential signals before reaching the in-amp. Such

conversions occur when one input path has a frequency response

that is different from the other. To keep CMRR across frequency

high, the input source impedance and capacitance of each path

should be closely matched. Additional source resistance in the

input path (for example, for input protection) should be placed

close to the in-amp inputs, which minimizes their interaction

with parasitic capacitance from the PCB traces.

Parasitic capacitance at the gain setting pins can also affect CMRR

over frequency. If the board design has a component at the gain

setting pins (for example, a switch or jumper), the component

should be chosen so that the parasitic capacitance is as small as

possible.

Power Supplies

A stable dc voltage should be used to power the instrumenta-

tion amplifier. Noise on the supply pins can adversely affect

performance.

A 0.1 μF capacitor should be placed as close as possible to each

supply pin. As shown in Figure 11, a 10 μF capacitor can be used

farther away from the part. In most cases, it can be shared by

other precision integrated circuits.

AD8426

+IN

–IN

LOAD

REF

0.1µF

10µF

0.1µF

10µF

Figure 11. Supply Decoupling, REF, and Output Referred to Local Ground

References

the potential on the reference terminal. Care should be taken to

tie REF to the appropriate local ground. This should also help

minimize crosstalk between the two channels.

INPUT BIAS CURRENT RETURN PATH

to ground. When the source, such as a thermocouple, cannot

provide a return current path, one should be created, as shown

in Figure 12.

THERMOCOUPLE

REF

AD8426

CAPACITIVELY COUPLED

REF

C

C

AD8426

TRANSFORMER

REF

AD8426

INCORRECT

CAPACITIVELY COUPLED

REF

C

R

R

C

AD8426

1

THERMOCOUPLE

REF

10MΩ

AD8426

TRANSFORMER

REF

AD8426

CORRECT

Figure 12. Creating an Input Bias Current Return Path

INPUT PROTECTION

need additional input protection. Input voltages can be up to

40 V from the opposite supply rail. For example, with a +5 V

positive supply and a −8 V negative supply, the part can safely

withstand voltages from −35 V to +32 V. Unlike some other

instrumentation amplifiers, the part can handle large differen-

tial input voltages even when the part is in high gain.