ADA4945-1ACPZ-R7 datasheet(43/57 Pages) AD

43 / 57 page

Data Sheet

ADA4945-1

analog.com

Rev. A

43 of 57

THEORY OF OPERATION

Figure 99. ADA4945-1 Architectural Block Diagram

The ADA4945-1 is a high speed, low power differential amplifier fabricated on Analog Devices advanced dielectrically

isolated SiGe bipolar process. The device provides two closely balanced differential outputs in response to either

differential or single-ended input signals. An external feedback network that is similar to a voltage feedback

operational amplifier sets the differential gain. The output common-mode voltage is independent of the input

common-mode voltage and is set by an external voltage at the VOCM terminal. The PNP input stage allows input

common-mode voltages between the negative supply and 1.3 V less than the positive supply. A rail-to-rail output

stage supplies a wide output voltage range. The DISABLE pin can reduce the supply current (Iq) of the amplifier to 50

µA.

Fully Differential and Common-Mode Signal Paths

Figure 99 shows a simplified diagram of the ADA4945-1 architecture. The differential feedback loop consists of the

differential transconductance (GDIFF) working through the GO output buffers and the RF/RG feedback networks. The

common-mode feedback loop is set up with a voltage divider across the two differential outputs to create an output

voltage midpoint (VOUT(CM)) and a common-mode transconductance (GCM).

The differential feedback loop forces the voltages at +IN and −IN to equal each other. This voltage equalization sets

the following relationships:

+DIN

= −

V−OUT

−DIN

= −

V+OUT

Subtracting the previous equations gives the relationship that shows RF and RG setting the differential gain.

(V+OUT − V−OUT) = (+DIN − (−DIN)) ×

GCM

GDIFF

–OUT

+OUT

+DIN

–DIN

+IN

–IN

VOCM

OUTPUT

CLAMP

+VCLAMP

–VCLAMP

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