Datasheet
1
OUTER SHIELD/GROUND
SIGNAL CONDUCTOR
GUARD
LMP7721
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SNOSAW6D –JANUARY 2008–REVISED MARCH 2013
Figure 46. The Structure of a Triax
The signal conductor and the guard of the triax should be kept at the same potential; therefore, the leakage
current between them is practically zero. Since triax has an extra layer of insulation and a second conducting
sheath, it offers greater rejection of interference than coaxial cable/connector.
COMPENSATING INPUT CAPACITANCE
The high input resistance of the LMP7721 allows the use of large feedback and source resistor values without
losing gain accuracy due to loading. However, the circuit will be especially sensitive to its layout when these
large-value resistors are used.
Every amplifier has some capacitance between each input and AC ground, and also some differential
capacitance between the inputs. When the feedback network around an amplifier is resistive, this input
capacitance (along with any additional capacitance due to circuit board traces, the socket, etc.) and the feedback
resistors create a pole in the feedback path. In the General Operational Amplifier circuit, Figure 47 the frequency
of this pole is
(1)
where C
S
is the total capacitance at the inverting input, including amplifier input capacitance and any stray
capacitance from the IC socket (if one is used), circuit board traces, etc., and R
P
is the parallel combination of R
F
and R
IN
. The typical input capacitance of the LMP7721 is about 10pF. This formula, as well as all formulas
derived below, apply to inverting and non-inverting op amp configurations.
When the feedback resistors are smaller than a few kΩ, the frequency of the feedback pole will be quite high,
since C
S
is generally less than 15 pF. If the frequency of the feedback pole is much higher than the “ideal”
closed-loop bandwidth (the nominal closed-loop bandwidth in the absence of C
S
), the pole will have a negligible
effect on stability, as it will add only a small amount of phase shift.
However, if the feedback pole is less than approximately 6 to 10 times the “ideal” −3 dB frequency, a feedback
capacitor, C
F
, should be connected between the output and the inverting input of the op amp. This condition can
also be stated in terms of the amplifier’s low-frequency noise gain: To maintain stability a feedback capacitor will
probably be needed if
(2)
where
(3)
is the amplifier’s low-frequency noise gain and GBW is the amplifier’s gain bandwidth product. An amplifier’s low
frequency noise gain is represented by the formula
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