ADA4622-1ARJZ-R2 datasheet(26/36 Pages) AD

26 / 36 page

Data Sheet

ADA4622-1/ADA4622-2/ADA4622-4

THEORY OF OPERATION

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Rev. F | 26 of 36

Figure 80. Simplified Circuit Diagram

INPUT CHARACTERISTICS

The ADA4622-1/ADA4622-2/ADA4622-4 input stage consists of

N-channel JFETs that provide low offset, low noise, and high impe-

dance. The minimum input common-mode voltage extends from

−0.2 mV below V− to 1 V less than V+. Driving the input closer to

the positive rail causes loss of amplifier bandwidth and increased

common-mode voltage error. Figure 81 shows the rounding of the

output due to the loss of bandwidth. The input and output are

superimposed.

Figure 81. Bandwidth Limiting due to Headroom Requirements

The ADA4622-1/ADA4622-2/ADA4622-4 do not exhibit phase re-

versal for input voltages up to V+. For input voltages greater than

V+, a 10 kΩ resistor in series with the noninverting input prevents

phase reversal at the expense of higher noise (see Figure 82).

Figure 82. No Phase Reversal

Because the input stage uses N-channel JFETs, the input current

during normal operation is negative. However, the input bias current

changes direction as the input voltage approaches V+ due to

internal junctions becoming forward-biased (see Figure 83).

Figure 83. Input Bias Current vs. Common-Mode Voltage with ±5 V Supply

ADA4622-1ARJZ-R2 Datasheet(PDF) 26 Page - Analog Devices