ADA4945-1ACPZ-R7 datasheet(49/57 Pages) AD

49 / 57 page

Data Sheet

ADA4945-1

analog.com

Rev. A

49 of 57

Figure 105. Single-Ended Input Impedance RIN

2. Add a termination resistor (RT) to match the 50 Ω source resistance. Because RT||665.33 Ω = 50 Ω,

RT = 54.06 Ω.

Figure 106. Adding Termination Resistor, RT

3. Replace the source termination resistor combination with the Thevenin equivalent. The Thevenin equivalent

of the source resistance, RS, and the termination resistance, RT, is RTH = RS||RT = 25.976 Ω. The Thevenin

equivalent of the source voltage is

�� = ��

��

�� + ��

= 1.039 ��−��

Figure 107. Thevenin Equivalent Circuit

4. Set RF1 = RF2 = RF to maintain a balanced system. Compensate the imbalance caused by RTH. There are two

methods available to compensate, as follows:



Add RTH to RG2 to maintain balanced gain resistances and increase RF1 and RF2 to RF =

VTH

Gain(RG + RTH) to

maintain the system gain.



Decrease RG2 to RG2 =

RF×VTH

VS×Gain

to maintain system gain and decrease RG1 to (RG2 − RTH) to maintain balanced

gain resistances.

The first compensation method is used in the Analog Devices DiffAmpCalc™ tool. Using the second compensation

method, RG2 =259.241 Ω and RG1 = 259.241 – 25.976 = 233.265 Ω. The modified circuit is shown in Figure 108

50Ω

2V p-p

RIN

665.333Ω

ADA4945-1

RL VOUT, dm

+VS

–VS

RG1

499Ω

RG2

499Ω

RF2

499Ω

RF1

499Ω

VOCM

ADA4945-1

RL VOUT, dm

+VS

–VS

50Ω

RG1

499Ω

RG2

499Ω

RF2

499Ω

RF1

499Ω

VOCM

2V p-p

RIN

50Ω

54.06Ω

50Ω

2V p-p

54.06Ω

RTH

25.976Ω

VTH

1.039V p-p

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