ADA4950-2YCPZ-R2 datasheet(21/26 Pages) AD

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Data Sheet

ADA4950-1/ADA4950-2

Rev. B | Page 21 of 26

Terminating a Single-Ended Input

This section describes how to properly terminate a single-ended

input to the ADA4950-x with a gain of 1, RF = 500 Ω, and RG =

500 Ω. An example using an input source with a terminated output

voltage of 1 V p-p and source resistance of 50 Ω illustrates the

steps that must be followed. Note that because the terminated

output voltage of the source is 1 V p-p, the open-circuit output

voltage of the source is 2 V p-p. The source shown in Figure 56

indicates this open-circuit voltage.

1. The input impedance is calculated using the following

formula:

667

)

500

(

500

)

(













−













−

50Ω

2V p-p

RIN, se

667Ω

ADA4950-x

RL VOUT, dm

+VS

–VS

500Ω

VOCM

Figure 56. Calculating Single-Ended Input Impedance, RIN

2. To match the 50 Ω source resistance, calculate the

termination resistor, RT, using RT||667 Ω = 50 Ω. The

closest standard 1% value for RT is 53.6 Ω.

ADA4950-x

VOUT, dm

+VS

–VS

50Ω

500Ω

VOCM

2V p-p

RIN, se

50Ω

53.6

Ω

Figure 57. Adding Termination Resistor, RT

3. Figure 57 shows 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, add a correction resistor (RTS)

in series with RG in the lower loop. RTS is the Thevenin

equivalent of the source resistance, RS, and the termination

resistance, RT, and is equal to RS||RT.

RTS = RTH = RS||RT = 25.9 Ω

50Ω

2V p-p

53.6

Ω

RTH

25.9Ω

VTH

1.03V p-p

Figure 58. Calculating the Thevenin Equivalent

Note that VTH is greater than 1 V p-p, which was obtained

with RT = 50 Ω. The modified circuit with the Thevenin

equivalent (closest 1% value used for RTH) of the terminated

source and RTS in the lower feedback loop is shown in

Figure 59.

ADA4950-x

RL VOUT, dm

+VS

–VS

RTH

25.5Ω

500Ω

VOCM

VTH

1.03V p-p

RTS

25.5

Ω

Figure 59. Thevenin Equivalent and Matched Gain Resistors

Figure 59 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, or high

gains, however, the diminished closed-loop gain is not canceled

completely by the increased VTH. This can be seen by evaluating

Figure 59.

The desired differential output in this example is 1 V p-p

because the terminated input signal is 1 V p-p and the closed-

loop gain = 1. The actual differential output voltage, however, is

equal to (1.03 V p-p)(500/525.5) = 0.98 V p-p.

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