AD8420ARMZ AD [Analog Devices], AD8420ARMZ Datasheet - Page 9

• Manufacturer Part Number: AD8420ARMZ
• Description: Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

Preliminary Technical Data
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:
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.
1)
2)
Keep differential input voltage within ±1V.
Keep voltage on +IN, -IN, and REF pins in specified
input voltage range
Rev. PrD | Page 9 of 9
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.
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.
Figure 6. Using Resistor Divider to Set Reference Voltage
–IN
+IN
AD8420
G = 1+
Figure 5. Calculating Gain with Reference Resistance
R2+R3||R4
–IN
R1
+IN
AD8420
G = 1+
V
REF
FB
OUT
R2+R
R1
R2
R1
REF
V
REF
FB
V
OUT
S
R3
R4
R2
R1
R
V
REF
REF
filters noise
capacitor
Optional
from Vs
AD8420