ADA4940-1ACPZ-R7 datasheet(25/30 Pages) AD

25 / 30 page

Data Sheet

ADA4940-1/ADA4940-2

Rev. D | Page 25 of 30

Terminating a Single-Ended Input

This section describes how to properly terminate a single-ended

input to the ADA4940-1/ADA4940-2 with a gain of 1, RF = 1 kΩ

and RG = 1 kΩ. An example using an input source with a terminated

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

the three steps that must be followed. 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 66

indicates this open-circuit voltage.

50Ω

2V p-p

RIN, se

1.33kΩ

ADA4940-1

ADA4940-2

RL VOUT, dm

+VS

–VS

1kΩ

VOCM

Figure 66. Calculating Single-Ended Input Impedance, RIN

The input impedance is calculated by

.33

)

1000

(

1000

)

(











































To match the 50 Ω source resistance, calculate the

termination resistor, RT, using RT||1.33 kΩ = 50 Ω.

The closest standard 1% value for RT is 52.3 Ω.

ADA4940-1

ADA4940-2

VOUT, dm

+VS

–VS

50Ω

1kΩ

VOCM

2V p-p

RIN, se

50Ω

52.3Ω

Figure 67. Adding Termination Resistor RT

Figure 67 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

52.3Ω

RTH

25.5Ω

VTH

1.02V p-p

Figure 68. Calculating the Thevenin Equivalent

RTS = RTH = RS||RT = 25.5 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 69.

ADA4940-1

ADA4940-2

RL VOUT, dm

+VS

–VS

RTH

25.5Ω

1kΩ

VOCM

VTH

1.02V p-p

RTS

25.5Ω

Figure 69. Thevenin Equivalent and Matched Gain Resistors

Figure 69 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

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

evaluating Figure 69.

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 (1.02 V p-p)(1000/1025.5) = 0.996 V p-p.

This is within the tolerance of the resistors, so no change to

the feedback resistor, RF, is required.

INPUT COMMON-MODE VOLTAGE RANGE

The ADA4940-1/ADA4940-2 input common-mode range is

shifted down by approximately 1 VBE, in contrast to other ADC

drivers with centered input ranges, such as the ADA4939-1/

ADA4939-2. The downward-shifted input common-mode range

is especially suited to dc-coupled, single-ended-to-differential,

and single-supply applications.

For ±2.5 V or +5 V supply operation, the input common-mode

range at the summing nodes of the amplifier is specified as −2.7 V

to +1.3 V or −0.2 V to 3.8 V, and is specified as −0.2 V to +1.8 V

with a +3 V supply.

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