ESD

IN

+

ESD

ESD

IN

-

ESD

V

+

V

-

V

-

V

+

+

-

+

-

+

-

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

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

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Product Folder Links: LMP7707 LMP7708 LMP7709