7.3.4 Input Capacitance

CMOS input stages inherently have low input bias current and higher input-referred voltage noise. The

LMP7704-SP 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. To achieve these specifications, a larger input stage has been

used. This larger input stage increases the input capacitance of the LMP7704-SP. The typical value of this input

gain and feedback resistors, which are seen on the inputs of the amplifier, to form a pole. This pole has little or

no effect on the output of the amplifier at low frequencies and dc conditions, but plays a bigger role as the

frequency increases. At higher frequencies, the presence of this pole decreases phase margin and also causes

gain peaking. 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 7-2. Compensating for Input Capacitance

1 +

s

¨

©

§

+

s

2

¨

©

§

(s) =

(1)

This equation is rearranged to find the location of the two poles:

+

¨

©

§

-1

1

1

r

1

1

+

2

-

(2)

LMP7704-SP

SNOSDB6A – DECEMBER 2020 – REVISED JANUARY 2021

www.ti.com

18

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