AD8422ACPZ-R7 datasheet(25/28 Pages) AD

25 / 28 page

Data Sheet

AD8422

THEORY OF OPERATION

analog.com

Rev. C | 25 of 28

Figure 62. Input Protection Options for Input Voltages Beyond Absolute

Maximum Ratings

At the expense of some noise performance, another solution is to

use series resistors. In the overvoltage case, current into the inputs

of the AD8422 is internally limited to a safe value for the amplifier.

Although the AD8422 inputs must still be kept within the Absolute

Maximum Ratings, the I × R drop across the protection resistor

increases the maximum voltage that the system can withstand to

the following values:

For positive input signals,

VMAX_NEW = (40 V + Negative Supply) + IIN × RPROTECT

(4)

For negative input signals,

VMIN_NEW = (Positive Supply – 40 V) − IOUT × RPROTECT

(5)

Overvoltage performance is shown in Figure 15, Figure 16, Figure

17, and Figure 18. With gains greater than 100 and supply voltages

less than ±2.5 V, overdrive voltages beyond the rails may cause the

output to invert as far as the REF pin voltage.

RF INTERFERENCE (RFI)

RF rectification is often a problem when amplifiers are used in

applications that have strong RF signals. The disturbance can

appear as a small dc offset voltage. High frequency signals can

be filtered with a low-pass RC network placed at the input of the

instrumentation amplifier, as shown in Figure 63.

Figure 63. RFI Suppression

The filter limits the input signal bandwidth, according to the follow-

ing relationship:

FilterFrequencyDIFF= 12πR(2CD+CC)

(6)

FilterFrequencyCM= 12πRCC

(7)

where CD ≥ 10 CC.

CD affects the difference signal, and CC affects the common-mode

signal. Choose values of R and CC that minimize RFI. A mismatch

between R × CC at the positive input and R × CC at the negative

input degrades the CMRR of the AD8422. By using a value of CD

that is one order of magnitude larger than CC, the effect of the

mismatch is reduced, and performance is improved.

Resistors add noise; therefore, the choice of the resistor and capac-

itor values depends on the desired tradeoff between noise, input

impedance at high frequencies, and RF immunity. The resistors

used for the RFI filter can be the same as those used for input

protection.

AD8422ACPZ-R7 Datasheet(PDF) 25 Page - Analog Devices

AD8422ACPZ-R7 Datasheet(HTML) 25 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL