ADA4932-1YCPZ-R2 datasheet(22/27 Pages) AD

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Data Sheet

ADA4932-1/ADA4932-2

APPLICATIONS INFORMATION

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Rev. F | 22 of 27

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 ADA4932-1/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.

1. Calculate the input impedance by using the following formula:

RIN,se= RG

1− RF

2× RG+RF = 499

1− 499

2× 499+499

=665 Ω

Figure 58. 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.

Figure 59. Adding Termination Resistor, RT

3. 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.

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.

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).

4. The feedback resistor value is modified as a final gain adjust-

ment to obtain the desired output voltage.

To make the output voltage VOUT = 1 V p-p, calculate RF by

using the following formula:

RF = DesiredVOUT,dm RG+RTS

VTH

1Vp−p 524.5 Ω

1.03Vp−p

= 509Ω

The closest standard 1% value to 509 Ω is 511 Ω, which gives a

differential output voltage of 1.00 V p-p.

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