ADA4620-2ARZ-R7 datasheet(33/49 Pages) AD

33 / 49 page

Data Sheet

ADA4620-1, ADA4620-2

analog.com

Rev. 0

33 of 49

limitation of current drive to the output transistors, along with appropriate user precautions, to maintain a safe

operating area (SOA).

Compensation

The wide gain bandwidth product of 16.5 MHz is achieved through internal compensation ensuring unity-gain

stable operation even for capacitive loads larger than 100 pF. Larger capacitive loads can be driven with the

assistance of an isolation resistor in series with the load.

An additional aspect of the compensation scheme is that capacitors are coupled to the positive and negative

supply rails to improve high-frequency power supply rejection ratio (PSRR).

No Phase Reversal

The ADA4620 does not suffer from output voltage phase reversal when driven beyond the specified input common-

mode range. In JFET amplifiers, phase reversal can happen when the input differential transistors go into their

triode region of operation. The input signal then couples directly into the transistors’ drains without undergoing

the normal inverting gain of the transistors. This is the usual source for phase reversal – a lack of the normal

inverting gain at the input. On the ADA4620, the high common-mode output stage is designed in such a way that

the amplification does not rely on an inverting gain of the input transistors. This avoids direct coupling into the

drain nodes when in the triode region. In this way, the ADA4620 circumvents the phase reversal phenomenon.

Additional clamping also ensures that phase reversal does not occur.

Electrical Overstress Protection

Figure 114. Electrical Overstress Protection Circuitry

The ADA4620 is provided with electrical overstress protection, as shown in Figure 114. The diode stack between V+

and V– provides the primary protection against overvoltage stress. The stack of diodes undergoes controlled

avalanche breakdown around 47.5 V, well above the absolute maximum rating of 40 V. If the supply voltage exceeds

that threshold, significant current begins to flow between the supplies. The series resistance of the diode stack is

approximately 11 Ω, providing some limitation to the current flow.

The signal pins (+IN, −IN, and OUT) of the amplifier are provided with a diode each to V+ and V–. In the case of a pin

overvoltage condition (greater than the V+ supply voltage), a diode becomes forward-biased. If the V+ supply is low

impedance, current flows from the signal pin to V+. The current is only limited by external resistance. If the V+

supply is high impedance, the pin voltage drags the V+ supply up. This continues until the diode stack breaks down.

Similarly, when there is a pin undervoltage condition (less than the V– supply voltage), a diode becomes forward-

biased. If the V– supply is low impedance, current flows from V– to the signal pin, again only limited by external

resistance. If the V– supply is high impedance, the pin voltage drags down the V– supply until the diode stack

ADA4620-1/

ADA4620-2

–

OUT

+IN

–IN

V–

V+

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