ADA4932-2YCPZ-R2 datasheet(22/27 Pages) AD

22 / 27 page

ADA4932-1/ADA4932-2

Data Sheet

Rev. E | Page 22 of 27

with the input signal, thus reducing the effective voltage across

RG in the upper loop and partially bootstrapping RG.

Terminating a Single-Ended Input

This section describes how to properly terminate a single-ended

input to the ADA4932-1/ADA4932-2 with a gain of 1, RF = 499 Ω,

and RG = 499 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 four 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 58 indicates this open-circuit voltage.

Calculate the input impedance by using the following

formula:

665

)

499

(

499

)

(











































50Ω

2V p-p

RIN, se

665Ω

ADA4932-1/

ADA4932-2

RL VOUT, dm

+VS

–VS

499Ω

VOCM

Figure 58. Calculating Single-Ended Input Impedance, RIN

To match the 50 Ω source resistance, calculate the

termination resistor, RT, using RT||665 Ω = 50 Ω. The

closest standard 1% value for RT is 53.6 Ω.

ADA4932-1/

ADA4932-2

VOUT, dm

+VS

–VS

50Ω

499Ω

VOCM

2V p-p

RIN, se

50Ω

53.6Ω

Figure 59. Adding Termination Resistor, RT

Figure 59 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.

50Ω

2V p-p

53.6Ω

RTH

25.9Ω

VTH

1.03V p-p

Figure 60. Calculating the Thevenin Equivalent

RTS = RTH = RS||RT = 25.9 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 (closest 1% value used for

RTH) of the terminated source and RTS in the lower feedback

loop is shown in Figure 61.

ADA4932-1/

ADA4932-2

RL VOUT, dm

+VS

–VS

RTH

25.5Ω

499Ω

VOCM

VTH

1.03V p-p

RTS

25.5Ω

Figure 61. Thevenin Equivalent and Matched Gain Resistors

Figure 61 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 61.

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

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

closed-loop gain = 1. The actual differential output voltage,

however, is equal to (1.03 V p-p)(499/524.5) = 0.98 V p-p.

To obtain the desired output voltage of 1 V p-p, a final gain

adjustment can be made by increasing RF without modifying

any of the input circuitry (see Step 4).

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