ADA4939-2YCPZ-R2 datasheet(20/24 Pages) AD

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ADA4939-1/ADA4939-2

Rev. 0 | Page 20 of 24

In order to match the 50 Ω source resistance, the termi-

nation resistor, RT, is calculated using RT||300 Ω = 50 Ω.

The closest standard 1% value for RT is 60.4 Ω.

ADA4939

VOUT, dm

+VS

–VS

50Ω

200Ω

400Ω

VOCM

2V p-p

RIN

50Ω

60.4Ω

Figure 47. Adding Termination Resistor RT

It can be seen from Figure 47 that the effective RG in the

upper feedback loop is now greater than the RG in the

lower loop due to the addition of the termination resistors.

To compensate for the imbalance of the gain resistors,

a correction resistor (RTS) is added in series with RG in the

lower loop. RTS is equal to the Thevenin equivalent of the

source resistance RS and the termination resistance RT and

is equal to RS||RT.

50Ω

2V p-p

60.4Ω

RTH

27.4Ω

VTH

1.09V p-p

Figure 48. Calculating the Thevenin Equivalent

RTS = RTH = RS||RT = 27.4 Ω. Note that VTH is greater than

1 V p-p, which was obtained with RT = 50 Ω. The modified

circuit with the Thevenin equivalent of the terminated source

and RTS in the lower feedback loop is shown in Figure 49.

ADA4939

RL VOUT, dm

+VS

–VS

RTH

27.4Ω

200Ω

400Ω

VOCM

VTH

1.09V p-p

RTS

27.4Ω

Figure 49. Thevenin Equivalent and Matched Gain Resistors

Figure 49 presents a tractable circuit with matched

feedback loops that can be easily evaluated.

It is useful to point out two effects that occur with a

terminated input. The first is that the value of RG is increased

in both loops, lowering the overall closed-loop gain. The

second is that VTH is a little larger than 1 V p-p, as it would

be if RT = 50 Ω. These two effects have opposite impacts on

the output voltage, and for large resistor values in the feedback

loops (~1 kΩ), the effects essentially cancel each other out.

For small RF and RG, however, the diminished closed-loop

gain is not canceled completely by the increased VTH. This

can be seen by evaluating Figure 49.

The desired differential output in this example is 2 V p-p

because the terminated input signal was 1 V p-p and the

closed-loop gain = 2. The actual differential output voltage,

however, is equal to (1.09 V p-p)(400/227.4) = 1.92 V p-p.

To obtain the desired output voltage of 2 V p-p, a final gain

adjustment can be made by increasing RF without modifying

any of the input circuitry. This is discussed in Step 4.

The feedback resistor value is modified as a final gain

adjustment to obtain the desired output voltage.

To make the output voltage VOUT = 2 V p-p, RF must be

calculated using the following formula:

()() ()(

)

−

417

227

OUT

Desired

The closest standard 1 % values to 417 Ω are 412 Ω and

422 Ω. Choosing 422 Ω gives a differential output voltage

of 2.02 V p-p.

The final circuit is shown in Figure 50.

ADA4939

VOUT, dm

2.02V p-p

+VS

–VS

50Ω

200Ω

422Ω

VOCM

2V p-p

1V p-p

60.4Ω

RTS

27.4Ω

Figure 50. Terminated Single-Ended-to-Differential System with G = 2

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