AD8142ACPZ-RL(Rev0) View Datasheet(PDF) - Analog Devices
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Description
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AD8142ACPZ-RL Datasheet PDF : 24 Pages
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AD8141/AD8142
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
VACM
=
VOCM
+ 2VICM
3
where VICM is the common-mode voltage of the input signal,
that is,
VICM
=
VIP
+ VIN
2
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.
1. The single-ended input impedance is calculated as
RIN = 1.5 kΩ.
VIDEO SOURCE
RIN
RS 1.5kΩ
RG
75Ω
1kΩ
VS
1.4V p-p
RG
1kΩ
RF
2kΩ
+5V
+
AD8141/
AD8142
–
RL VOUT, dm
VS
1.4V p-p
RIN
RS 75Ω
RG
75Ω
1kΩ
RT
80.6Ω
RG
1kΩ
RF
2kΩ
+VS
+
AD8141/
AD8142
–
RL VOUT, dm
–VS
RF
2kΩ
Figure 38. Adding Termination Resistor RT
3. 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.
VS
1.4V p-p
RS
75Ω
RT
80.6Ω
VTH
0.725V p-p
RTH
38.8Ω
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.
RF
2kΩ
RTH
RG
38.8Ω 1kΩ
VTH
0.725V p-p
RG
1kΩ
RTS
38.3Ω
+VS
+
AD8141/
AD8142
–
–VS
RF
2kΩ
RL VOUT, dm
Figure 40. Thevenin Equivalent and Matched Gain Resistors
RF
Figure 40 presents a tractable circuit with matched feedback
2kΩ
loops that can be easily evaluated.
Figure 37. Calculating Single-Ended Input Impedance, RIN
2. 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.
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.
Rev. 0 | Page 16 of 24
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