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THS4012 EVM Inverting Operation
1-8
General
The gain of the EVM can easily be changed to support a particular application
by changing the ratio of resistors R6 and R4 (channel 1) and R14 and R12
(channel 2) as described in the following equation:
Inverting Gain
–R
F
R
G
–R
6
R
4
and
–R
14
R
12
In addition, some applications, such as those for video, may require the use
of 75-Ω cable and 75-Ω EVM input termination and output isolation resistors.
Because the noninverting terminals are at ground potential, the inverting
terminal becomes a
virtual ground
and is held to 0 V. This causes the input
impedance to ground at the input terminal to look like two resistors in parallel
(R1 and R4 for channel 1, and R9 and R12 for channel 2). As a result, if the
source termination is changed, R1 and R9 must be adjusted in accordance
with the following equations:
R
1
R
4
R
T
R
4
–R
T
(
Channel
1) and
R
9
R
12
R
T
R
4
–R
T
(
Channel
2)
where R
T
is the source impedance.
Any resistor on the EVM board can be replaced with a resistor of a different
value; however, care must be taken because the surface-mount solder pads
on the board are somewhat fragile and will not survive many
desoldering/soldering operations.
External factors can significantly affect the effective gain of the EVM. For
example, connecting test equipment with 50-Ω input impedance to the EVM
output will divide the output signal level by a factor of 2 (assuming the output
isolation resistor on the EVM board remains 50 Ω). Similar effects can occur
at the input, depending upon how the input signal sources are configured. The
gain equations given above assume no signal loss in either the input or the
output.
Frequency compensation capacitors C3 and C6 may need to be installed to
improve stability at lower gains. The appropriate value depends on the
particular application.
The EVM circuit board is an excellent example of proper board layout for
high-speed amplifier designs and can be used as a guide for user application
board layouts.