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
LMP7711
www.ti.com
SNOSAP4F –SEPTEMBER 2005–REVISED MAY 2013
Figure 55. 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 causes 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 56 is simply −R
2
/R
1
.
Figure 56. Compensating for Input Capacitance
For the time being, ignore C
F
. The AC gain of the circuit in Figure 56 can be calculated as follows:
(1)
This equation is rearranged to find the location of the two poles:
(2)
Copyright © 2005–2013, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LMP7711