ADA4098-1BUJZ-R5 datasheet(20/33 Pages) AD

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Data Sheet

ADA4098-1/ADA4098-2

THEORY OF OPERATION

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Rev. B | 20 of 33

The ADA4098-1 and ADA4098-2 are robust, voltage feedback

amplifiers that combine unity-gain stability with low offset, low offset

drift, and 17 nV/√Hz of input voltage noise. Figure 54 shows a sim-

plified schematic of the devices. The ADA4098-1 and ADA4098-2

have two input stages: a common emitter differential input stage

consisting of the Q1 and Q2 PNP transistors that operate with the

inputs biased between −VS and 1.25 V below +VS, and a common

base input stage that consists of the Q3 to Q6 PNP transistors

that operate when the common-mode input is biased >+VS − 1.25

V. These input stages result in two distinct operating regions, as

shown in Figure 52.

Figure 52. Input Bias Current vs. Input Common-Mode Voltage over

Temperature, VSY = 5 V

For common-mode input voltages that are approximately 1.25 V

below the +VS supply, where Q1 and Q2 are active (see Figure

52), the common emitter PNP input stage is active and the input

bias current is typically <700 pA. When the common-mode input is

above +VS − 1.25 V, the Q9 transistor turns on, which diverts bias

current away from the common emitter differential input pair to the

mirror that consists of M3 and M4. The current from M4 biases the

common base differential input pair (Q3 to Q6). The Over-The-Top

input pair operates in a common base configuration and the input

bias current increases to ~8 µA. The offset voltages of both input

stages are tightly trimmed and are specified in Table 1 and Table 2.

As the input common-mode transitions to the Over-The-Top region,

the input CMRR degrades slightly when compared to the rest of the

input common-mode range, as shown in Figure 53.

Figure 53. Offset Voltage vs. Input Common-Mode Voltage over Temperature,

VSY = 5 V

Figure 54. Simplified ADA4098-1 and ADA4098-2 Schematic

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