EL2140C 2141C
150 MHz Differential Twisted Pair Driver
Applications Information
EL2141C
EL2140C
R1 a R2 a R3
GAIN e
R2
2140-13
GAIN e 2
2140-14
VOUT
a
2
VOUTB
e
VREF
(common mode)
Choice of feedback resistor
There is little to be gained from choosing resistor
R2 values below 400X and in fact it would only
result in increased power dissipation and signal
distortion Above 400X the bandwidth response
will develop some peaking (for a gain of two) but
substantially higher resistor R2 values may be
used for higher voltage gains such as up to 2 kX
at a gain of eight before peaking will develop R1
and R3 are selected as needed to set the voltage
gain and while R1 e R3 is suggested the gain
equation above holds for any values (see distor-
tion for further suggestions)
Capacitance considerations
As with many high bandwidth amplifiers the
EL2140C 2141C prefer not to drive highly capac-
itive loads It is best if the capacitance on VOUT
and VOUTB is kept below 10 pF if the user does
not want gain peaking to develop
In addition on the EL2141C the two feedback
nodes FBP and FBN should be laid out so as to
minimize stray capacitance else an additional
pole will potentially develop in the response with
possible gain peaking
The amount of capacitance tolerated on any of
these nodes in an actual application will also be
dependent on the gain setting and the resistor
values in the feedback network
Distortion considerations
The harmonics that these amplifiers will poten-
tially produce are the 2nd 3rd 5th and 6th
Their amplitude is application dependent All
other harmonics should be negligible by compari-
son Each should be considered separately
H2 The second harmonic arises from the input
stage and the lower the applied differential sig-
nal amplitude the lower the magnitude of the
second harmonic For practical considerations of
required output signal and input noise levels the
user will end up choosing a circuit gain Refer-
ring to Figure 1 it is best if the voltage at the
negative feedback node tracks the VREF node
and the voltage at the positive feedback node
tracks the VIN node respectively This would the-
oretically require that R1 a R2 e R3 although
the lowest distortion is found at about R3 e R1
a (0 7 R2) With this arrangement the second
harmonic should be suppressed well below the
value of the third harmonic
6