Datasheet

+
¨
©
§
¨
©
§
-1
2C
IN
P
1,2
=
1
R
1
1
R
2
r
1
R
1
1
R
2
+
2
-
4 A
0
C
IN
R
2
-R
2
/R
1
1 +
s
¨
©
§
¨
©
§
+
s
2
A
0
C
IN
R
2
¨
©
§
¨
©
§
V
OUT
V
IN
(s) =
A
0
R
1
R
1
+ R
2
C
IN
R
1
R
2
V
OUT
+
-
+
-
V
IN
+
-
V
OUT
V
IN
R
2
R
1
A
V
=
-
=
-
C
F
0 1 2 3 4
0
5
10
15
20
25
C
CM
(pF)
V
CM
(V)
V
S
= 5V
LMP7715, LMP7716, LMP7716Q
www.ti.com SNOSAV0E –MARCH 2006–REVISED MARCH 2013
Figure 50. Input Common Mode Capacitance
This input capacitance will interact with other impedances, such as gain and feedback resistors which are seen
on the inputs of the amplifier, to form a pole. This pole will have little or no effect on the output of the amplifier at
low frequencies and under DC conditions, but will play a bigger role as the frequency increases. At higher
frequencies, the presence of this pole will decrease phase margin and also cause gain peaking. In order to
compensate for the input capacitance, care must be taken in choosing feedback resistors. In addition to being
selective in picking values for the feedback resistor, a capacitor can be added to the feedback path to increase
stability.
The DC gain of the circuit shown in Figure 51 is simply −R
2
/R
1
.
Figure 51. Compensating for Input Capacitance
For the time being, ignore C
F
. The AC gain of the circuit in Figure 51 can be calculated as follows:
(1)
This equation is rearranged to find the location of the two poles:
(2)
As shown in Equation 2, as the values of R
1
and R
2
are increased, the magnitude of the poles are reduced,
which in turn decreases the bandwidth of the amplifier. Figure 52 shows the frequency response with different
value resistors for R
1
and R
2
. Whenever possible, it is best to chose smaller feedback resistors.
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