Preliminary Technical Data

AD8420

Rev. PrD | Page 9 of 9

INPUT VOLTAGE RANGE

Unlike traditional instrumentation amplifier architectures, the

allowed input range of the AD8420 is simplicity itself. For the

AD8420’s transfer function to be valid, the input voltage should

follow two rules:

1) Keep differential input voltage within ±1V.

2) Keep voltage on +IN, -IN, and REF pins in specified

input voltage range

No hexagonal figures. No complicated formulas.

INPUT PROTECTION

The AD8420 has very robust inputs and typically does not

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. The part can handle

large differential input voltages, even when the part is in high

gain, without damage.

The rest of the AD8420 terminals should be kept within the

supplies. All terminals of the AD8426 are protected against ESD.

For applications that require protection beyond the AD8420’s

limits, place diodes at the AD8420 inputs to limit voltage and

resistors in series with the inputs to limit the current into these

diodes. To keep input bias current at minimum, low leakage

diode clamps such as the BAV199 should be used. The AD8420

also combines well with TVS diodes such as the PTVSxS1UR.

DRIVING THE REFERENCE PIN

Traditional instrumentation amplifier architectures require the

reference pin to be driven with a low impedance source. In

traditional architectures, impedance at the reference pin

degrades both CMRR and gain accuracy. With the AD8420

architecture, resistance at the reference pin has no effect on

CMRR.

–IN

+IN

R2

R1

G = 1+

R1

AD8420

REF

FB

Figure 5. Calculating Gain with Reference Resistance

Resistance at the reference pin does affect the AD8420’s gain,

but if this resistance is constant, the gain setting resistors can be

adjusted to compensate. For example, the AD8420 can be

driven with a voltage divider as shown in Figure 6.

–IN

+IN

R2

R1

G = 1+

R2+R3||R4

R1

AD8420

REF

FB

R4

R3

Optional

capacitor

filters noise

from Vs

Figure 6. Using Resistor Divider to Set Reference Voltage