ADA4932-1YCPZ-R7 datasheet(22/28 Pages) AD

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ADA4932-1/ADA4932-2

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input terminals due to negative voltage feedback and is in phase

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-x 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 59

indicates this open-circuit voltage.

1. The input impedance is calculated using the formula

665

)

499

(

499

)

(













−













−

Ω

2V p-p

RIN, se

665

Ω

ADA4932-x

RL VOUT, dm

+VS

–VS

499

Ω

499

Ω

499

Ω

499

Ω

VOCM

Figure 59. Calculating Single-Ended Input Impedance, RIN

2. 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-x

VOUT, dm

+VS

–VS

Ω

499

Ω

499

Ω

499

Ω

499

Ω

VOCM

2V p-p

RIN, se

Ω

53.6

Ω

Figure 60. Adding Termination Resistor, RT

3. Figure 60 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.

Ω

2V p-p

53.6

Ω

RTH

25.9

Ω

VTH

1.03V p-p

Figure 61. 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 62.

ADA4932-x

RL VOUT, dm

+VS

–VS

RTH

25.5

Ω

499

Ω

499

Ω

499

Ω

499

Ω

VOCM

VTH

1.03V p-p

RTS

25.5

Ω

Figure 62. Thevenin Equivalent and Matched Gain Resistors

Figure 62 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 termi-

nated 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 62.

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. This is discussed in Step 4.

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