AD8420ARMZ-R7 datasheet(21/27 Pages) AD

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AD8420

Data Sheet

Rev. A | Page 20 of 26

GAIN ACCURACY

Unlike most instrumentation amplifiers, the relative match of

the two gain setting resistors determines the gain accuracy of

the AD8420 rather than a single resistor. For example, if two

resistors have exactly the same absolute error, there is no error

in gain. Conversely, two 1% resistors can cause approximately 2%

maximum gain error at high gains. Temperature coefficient

mismatch of the gain setting resistors increases the gain drift of

the instrumentation amplifier circuit, according to the gain

equation. Because these external resistors do not have to match

any on-chip resistors, resistors with good TC tracking can

achieve excellent gain drift. Even with standard thin film

resistors, the AD8420 can still achieve better gain drift than

most instrumentation amplifiers.

When the differential voltage at the inputs approaches the

differential input limit, the diodes start to conduct, limiting the

voltage seen by the inputs of amplifier gm1. This can look like

increased gain error at large differential inputs. Performance of the

AD8420 is specified for ±1 V differential from −40°C to +85°C.

However, at higher temperatures, the reduced forward voltage of

the diodes limits the differential input to a smaller voltage.

Figure 60 tracks 1% error across the operating temperature

range to show the effect of temperature on the input limit.

–40 –25 –10

110 125

TEMPERATURE (°C)

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

POSITIVE VOLTAGE

NEGATIVE VOLTAGE

VS = ±15V

Figure 60. Differential Input Limit vs. Temperature

INPUT VOLTAGE RANGE

The allowed input range of the AD8420 is much simpler than

traditional architectures. For the transfer function of the AD8420

to be valid, the input voltage should follow two rules:



Keep the differential input voltage within ±1 V.



Keep the voltage on the +IN, −IN, REF, and FB pins in the

specified input voltage range.

Because the output swing is completely independent of the

input common-mode voltage, there are no hexagonal figures

or complicated formulas to follow, and no limitation for the

output swing the amplifier has for input signals with changing

common mode.

INPUT PROTECTION

The current into the AD8420 inputs is limited internally. This

ensures that the diodes that limit the differential voltage seen by

the internal amplifier do not draw excessive current when they

turn on. The device can handle large differential input voltages,

regardless of the amount of gain applied, without damage. As a

result, the AD8420 inputs are protected from voltages beyond

the positive rail. If voltages beyond the negative rail are expected,

external protection must be used.

Keep all of the AD8420 terminals within the voltage range specified

in the Absolute Maximum Ratings section. All terminals of the

AD8420 are protected against ESD.

Input Voltages Beyond the Rails

For applications that require protection beyond the negative rail,

one option is to use an external resistor in series with each input

to limit current during overload conditions. In this case, size the

resistors to limit the current into the AD8420 to 6 mA.

RPROTECT ≥ (Negative Supply − VIN)/6 mA

Although the AD8420 inputs must still be kept within the −VS +

40 V limitation, the I × R drop across the protection resistor

increases the protection on the positive side to approximately

(40 V + Negative Supply) + 300 μA × RPROTECT

An alternate protection method is to 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

a minimum for normal operation, use low leakage diode clamps,

such as the BAV199. The AD8420 also combines well with TVS

diodes, such as the PTVSxS1UR.

SIMPLE METHOD

ALTERNATE METHOD

AD8420

RPROTECT

VIN+

–

VIN–

VIN+

VIN–

–

+VS

AD8420

–VS

+VS

–VS

+VS

–VS

+VS

–VS

–

Figure 61. Protection for Voltages Beyond the Rails

Large Differential Input Voltage

The AD8420 is able to handle large differential input voltage

without damage to the device. Refer to Figure 9, Figure 10,

Figure 11, and Figure 12 for overvoltage performance. The

AD8420 differential voltage is internally limited with diodes to

±1 V. If this limit is exceeded, the diodes start to conduct and

draw current, as shown in Figure 22. This current is limited

internally to a value that is safe for the AD8420, but if the input

current cannot be tolerated in the system, place resistors in

series with each input with the following value:

















MAX

DIFF

PROTECT

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