ADA4099-2HCPZ datasheet(21/34 Pages) AD

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Data Sheet

ADA4099-1/ADA4099-2

THEORY OF OPERATION

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Rev. A | 21 of 34

INPUT PROTECTION

The inputs are protected against temporary voltage excursions to

10 V below –VS (see Figure 56) by an internal 250 Ω resistor (see

Figure 55). This resistor limits the current in the series D1 diode

and D2 diode that are tied to the bases of the Q1 and Q2 tran-

sistors, respectively. Adding additional external series resistance

extends the protection to >10 V below −VS, at the cost of stability

and added thermal noise. The input stage of the ADA4099-1 and

ADA4099-2 incorporates phase reversal protection to prevent the

output from phase reversing for inputs below −VS. The ADA4099-1

and ADA4099-2 op amps do not have clamping diodes between

the inputs and can be differentially overdriven up to 80 V without

damage, inducing parametric shifts, or drawing appreciable input

current. Figure 57 summarizes the input fault types that can be

applied to the ADA4099-1 and ADA4099-2 without compromising

input integrity.

Figure 56. ADA4099-1 and ADA4099-2 as Unity-Gain Buffers with

Noninverting Inputs Driven Beyond the Supply (VSY = 5 V)

Figure 57. ADA4099-1 and ADA4099-2 Fault Tolerant Conditions

OVER-THE-TOP OPERATION

CONSIDERATIONS

When the ADA4099-1 and ADA4099-2 input common-modes are

biased near or >+VS supply, the amplifiers operate in the Over-The-

Top configuration. The differential input pair that controls amplifier

operation is the common base pair, Q3 to Q6 (see Figure 55).

Input bias currents change from <±4 nA in normal operation to

approximately 82.5 μA in Over-The-Top operation when the input

stage transitions from common emitter to common base. The Over-

The-Top input bias currents are well matched, and the associated

offset is typically <250 nA. Ensure that the impedance connected to

the inverting and noninverting inputs is well matched to avoid any

input bias current induced voltage offsets.

Differential input impedance, RIN (see Figure 58), decreases from

>100 kΩ in normal operation to ~600 Ω in Over-The-Top operation

(see Table 1 and Table 2).

Figure 58. Difference Amplifier Configured for Normal Operation and Over-

The-Top Operation

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