ADA4620-2ARZ-R7 datasheet(32/49 Pages) AD

32 / 49 page

Data Sheet

ADA4620-1, ADA4620-2

analog.com

Rev. 0

32 of 49

THEORY OF OPERATION

Figure 113. Simplified Schematic

The ADA4620 is a wide-input-range, low-power, low-distortion, rail-to-rail output, precision JFET input amplifier

that operates over a wide supply voltage range up to a maximum of 36 V. The amplifier employs a two-temperature

offset trim to achieve a low offset over a wide temperature range. The amplifier features a secondary input stage to

handle input common-mode operation near the positive supply rail. For enhanced slewing, the ADA4620 provides

an additional slew-boosting JFET input stage, which operates with large differential input voltages.

Input and Gain Stages

Figure 113 shows the simplified circuit diagram for the ADA4620. The low-noise architecture provides wide

common-mode input range optimized for low noise, low bias current, low offset voltage, and low distortion. The

use of a low-noise, offset-trimmed, bootstrapped N-channel JFET-based (nJFET) input stage enables a precision

common-mode range starting below the negative supply rail and extending to about 4.4 V below the positive

supply rail. The bootstrapping ensures low variation in leakage current versus input common-mode, extremely

high common-mode rejection ratio, and reduced harmonic distortion. When the input common-mode voltage rises

above about 4.4 V from the positive supply rail, the main input stage shuts off and an alternative nJFET-buffered

NPN bipolar differential pair, not shown in the figure, takes control. This alternative input stage maintains low

input leakage current and linear operation to within approximately 1 V of the positive rail. The offset voltage of this

alternative input stage is not trimmed. Internal clamps prevent phase inversion for signals ranging up to and

slightly exceeding the positive supply rail.

Novel slew boosting circuitry provides a high slew rate for fast settling and low distortion without compromising

stability. An additional nJFET differential input stage has a bias dependent on the applied differential voltage.

During high slew events, the large input differential voltage results in an increase of bias current, which assists the

amplifier in tracking rapidly moving signals. The result is a slew rate in excess of 30 V/µs, which improves settling

time and reduces distortion.

Output Stage

The rail-to-rail output stage receives a differential signal from the input stage. This input stage signal is applied

across a buffered H-bridge class A/B stage. The buffering ensures the input stage is not significantly loaded and

maintains a high gain. The H-bridge is advantageous for providing large slewing currents to the output transistors

and output stage compensation. The outputs of this stage are current mirrored to the common-emitter output

transistors and compensation. The output stage has no thermal shutdown and the device relies on an inherent

OUTPUT BIAS

CONTROL

SLEW BOOST AND

INPUT SELECT

OFFSET

AND

DRIFT TRIM

VOUT

COMMON-MODE

CONTROL

V+

V–

+IN

–IN

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