ADA4945-1ACPZ-R2 datasheet(35/44 Pages) AD

35 / 44 page

Data Sheet

ADA4945-1

Rev. 0 | Page 35 of 44

THEORY OF OPERATION

GCM

GDIFF

–OUT

+OUT

+DIN

–DIN

+IN

–IN

VOCM

OUTPUT

CLAMP

+VCLAMP

–VCLAMP

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 (IS) 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:

OUT

−

OUT

−

Subtracting the previous equations gives the relationship that

shows RF and RG setting the differential gain.

(V+OUT − V−OUT) = (+DIN – (−DIN)) ×

The common-mode feedback loop drives the output common-

mode voltage that is sampled at the midpoint of the output

voltage divider to equal the voltage at VOCM. This voltage

equalization results in the following relationships:

V+OUT = VOCM +

OUT,

V−OUT = VOCM −

OUT,

Note that the summing junction input voltages of the

differential amplifier (+IN and −IN in Figure 99) are set by both

the output voltages and the input voltages.

























−

OUT

























−

OUT

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