AD8141ACPZ-R7 datasheet(16/24 Pages) AD

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AD8141/AD8142

Rev. 0 | Page 16 of 24

INPUT COMMON-MODE VOLTAGE RANGE IN

SINGLE-SUPPLY APPLICATIONS

The driver inputs are designed to facilitate level-shifting of ground

referenced input signals on a single power supply. For a single-

ended input, this implies, for example, that the voltage at VIN in

Figure 36 is 0 V when the amplifier’s negative power supply

voltage is also set to 0 V.

It is important to ensure that the common-mode voltage at the

amplifier inputs, VAP and VAN, stays within its specified range.

Because the VAP and VAN voltages are driven to be essentially

equal by negative feedback, the amplifier’s input common-mode

voltage can be expressed as a single term, VACM. VACM can be

calculated as

2 ICM

OCM

ACM

where VICM is the common-mode voltage of the input signal,

that is,

ICM

TERMINATING A SINGLE-ENDED INPUT

Each driver has a nominal fixed gain of 2, with RF = 2.0 kΩ and

RG = 1.0 kΩ. A typical single-ended video signal source applied to

the AD8141/AD8142 input has a maximum terminated output

voltage of 0.7 V p-p and source resistance of 75 Ω. Because the

terminated output voltage of the source is 0.7 V p-p, the open-

circuit output voltage of the source is 1.4 V p-p. The source shown

in Figure 37 indicates this open-circuit voltage. The following

three steps illustrate how to terminate a signal from a typical

single-ended 75 Ω video source.

The single-ended input impedance is calculated as

RIN = 1.5 kΩ.

AD8141/

AD8142

+5V

VOUT, dm

–

VIDEO SOURCE

RIN

1.5kΩ

1kΩ

2kΩ

1kΩ

75Ω

1.4V p-p

Figure 37. Calculating Single-Ended Input Impedance, RIN

To match the 75 Ω source resistance, the termination resistor,

RT, is calculated using RT||1.125 kΩ = 75 Ω. The closest

standard 1% value for RT is 80.6 Ω.

AD8141/

AD8142

+VS

–VS

VOUT, dm

–

RIN

75Ω

80.6Ω

1kΩ

75Ω

2kΩ

1.4V p-p

Figure 38. Adding Termination Resistor RT

It can be seen from Figure 38 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, a

correction resistor (RTS) is added in series with RG in the

lower loop. RTS is the closest 1% resistor to the Thevenin

equivalent of the source resistance RS and the termination

resistance RT, equal to RS||RT.

80.6Ω

75Ω

RTH

38.8Ω

VTH

0.725V p-p

1.4V p-p

Figure 39. Calculating the Thevenin Equivalent

RTH = RS||RT = 38.8 Ω, and RTS = 38.3 Ω. Note that VTH is

greater than 0.7 V p-p, which was obtained with RT = 75 Ω

alone. The modified circuit with the Thevenin equivalent of

the terminated source and RTS in the lower feedback loop is

shown in Figure 40.

AD8141/

AD8142

+VS

–VS

VOUT, dm

–

RTS

38.3Ω

2kΩ

1kΩ

RTH

38.8Ω

2kΩ

VTH

0.725V p-p

Figure 40. Thevenin Equivalent and Matched Gain Resistors

Figure 40 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 0.7 V p-p, as it is if RT = 75 Ω alone. These

two effects have opposite impacts on the output voltage, and for

large resistor values in the feedback loops, the effects essentially

cancel each other out. For smaller RF and RG, however, the

diminished closed-loop gain is not canceled completely by the

increased VTH.

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