ADA4945-1ACPZ-R7 datasheet(50/57 Pages) AD

50 / 57 page

Data Sheet

ADA4945-1

analog.com

Rev. A

50 of 57

Figure 108. Thevenin Equivalent with Matched Gain Resistors

Figure 108 shows an easily manageable circuit with matched feedback loops that can be easily evaluated.

5. The modified gain resistor, RG1, changes the input impedance. Repeat Step 1 through Step 4 several times

using the modified value of RG1 from the previous iteration until the value of RT does not change from the

previous iteration. After three additional iterations, the change in RG1 is less than 0.1%. The final circuit is

shown in Figure 109 with the closest 1% resistor values.

Figure 109.Terminated Single-Ended-to-Differential System with G =1

Input Common-Mode Voltage Range

The input common-mode range at the summing nodes of the ADA4945-1 is specified as −VS to +VS − 1.3 V. By extending

the input common-mode range down to −VS, the ADA4945-1 is especially well suited to dc-coupled, single-ended-to-

differential, and single-supply applications, such as ADC driving.

Input and Output Capacitive AC Coupling

Although the ADA4945-1 is best suited to dc-coupled applications, it is possible to use the device in ac-coupled

circuits. Input ac coupling capacitors can be inserted between the source and RG. This ac coupling blocks the flow of

the dc common-mode feedback current and causes the ADA4945-1 dc input common-mode voltage to equal the dc

output common-mode voltage. These ac coupling capacitors must be placed in both loops to keep the feedback

factors matched. Output ac coupling capacitors can be placed in series between each output and the respective load

of each output.

Setting the Output Common-Mode Voltage

The VOCM pin of the ADA4945-1 is internally biased at a voltage approximately equal to the midway between the

output voltage clamps, ((+VCLAMP) + (−VCLAMP))/2. Relying on this internal bias results in an output common-mode

voltage that is within approximately 100 mV of the expected value.

ADA4945-1

RL VOUT, dm

+VS

–VS

RTH

25.976Ω

RG1

233.265Ω

RG2

259.241Ω

RF2

499Ω

RF1

499Ω

VOCM

VTH

1.039V p-p

ADA4945-1

VOUT, dm

1.990V p-p

+VS

–VS

50Ω

243Ω

RG2

267Ω

RF1

499Ω

RF2

499Ω

VOCM

2V p-p

0.998V p-p

57.6Ω

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