ADA4097-1BUJZ-R5 datasheet(20/31 Pages) AD

20 / 31 page

Data Sheet

ADA4097-1/ADA4097-2

THEORY OF OPERATION

analog.com

Rev. A | 20 of 31

The ADA4097-1/ADA4097-2 are robust, voltage feedback amplifi-

ers that combine unity-gain stability with low offset, low offset drift,

and 53 nV/√Hz of input voltage noise. Figure 55 shows a simplified

schematic of the devices. The ADA4097-1/ADA4097-2 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 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 V. These input stages

result in two distinct operating regions, as shown in Figure 53.

Figure 53. Input Bias Current vs. Input Common-Mode Voltage over

Temperature, VSY = 5 V

For common-mode input voltages that are approximately 1 V below

the +VS supply, where Q1 and Q2 are active (see Figure 53),

the common emitter PNP input stage is active and the input bias

current is typically <0.3 nA. When the common-mode input is above

+VS − 1 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 ~0.8 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 54.

Figure 54. Offset Voltage vs. Input Common-Mode Voltage over Temperature,

VSY = 5 V

Figure 55. Simplified ADA4097-1/ADA4097-2 Schematic

ADA4097-1BUJZ-R5 датащи(PDF) 20 Page - Analog Devices