Datasheet

ESD
R
1
IN
+
ESD
D
1
D
2
R
2
ESD
IN
-
ESD
V
+
V
-
V
-
V
+
R
1
= R
2
= 130Ö
C
IN
R
1
R
2
V
OUT
+
-
+
-
V
IN
+
-
C
F
LMP7707, LMP7708, LMP7709
www.ti.com SNOSAW5B –JUNE 2007–REVISED MARCH 2013
INPUT CAPACITANCE
CMOS input stages inherently have low input bias current and higher input referred voltage noise. The
LMP7707/LMP7708/LMP7709 enhances this performance by having the low input bias current of only ±200 fA,
as well as a very low input referred voltage noise of 9 nV/√Hz. In order to achieve this a large input stage has
been used. This large input stage increases the input capacitance of the LMP7707/LMP7708/LMP7709. The
typical value of this input capacitance, C
IN
, for the LMP7707/LMP7708/LMP7709 is 25 pF. The 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
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 will also cause gain peaking. In order to compensate for the input
capacitance, care must be taken in choosing the 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.
Figure 46. Compensating for Input Capacitance
Using this compensation method will have an impact on the high frequency gain of the op amp, due to the
frequency dependent feedback of this amplifier. Low gain settings can, again, introduce instability issues.
DIODES BETWEEN THE INPUTS
The LMP7707/LMP7708/LMP7709 have a set of anti-parallel diodes between the input pins, as shown in
Figure 47. These diodes are present to protect the input stage of the amplifier. At the same time, they limit the
amount of differential input voltage that is allowed on the input pins. A differential signal larger than one diode
voltage drop might damage the diodes. The differential signal between the inputs needs to be limited to ±300 mV
or the input current needs to be limited to ±10 mA. Exceeding these limits will damage the part.
Figure 47. Input of the LMP7707
TOTAL NOISE CONTRIBUTION
The LMP7707/LMP7708/LMP7709 have very low input bias current, very low input current noise and very low
input voltage noise. As a result, these amplifiers are ideal choices for circuits with high impedance sensor
applications.
Figure 48 shows the typical input noise of the LMP7707/LMP7708/LMP7709 as a function of source resistance.
The total noise at the input can be calculated using Equation 1.
Copyright © 2007–2013, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LMP7707 LMP7708 LMP7709