LMV797MMSLASHNOPB datasheet(14/30 Pages) TI

14 / 30 page

-1

2CIN

P1,2 =

4 A0CIN

-R2/R1

1 +

CIN R2

VOUT

VIN

(s) =

A0 R1

R1 + R2

CIN

VOUT

VIN

VOUT

VIN

AV = -

= -

LMV796, LMV797

SNOSAU9D – MARCH 2006 – REVISED MARCH 2013

www.ti.com

INPUT CAPACITANCE AND FEEDBACK CIRCUIT ELEMENTS

The LMV796 family has a very low input bias current (100 fA) and a low 1/f noise corner frequency (400 Hz),

which makes it ideal for sensor applications. However, to obtain this performance a large CMOS input stage is

used, which adds to the input capacitance of the op amp, CIN. Though this does not affect the DC and low

frequency performance, at higher frequencies the input capacitance interacts with the input and the feedback

impedances to create a pole, which results in lower phase margin and gain peaking. This can be controlled by

being selective in the use of feedback resistors, as well as, by using a feedback capacitance, CF. For example, in

the inverting amplifier shown in Figure 46, if CIN and CF are ignored and the open loop gain of the op amp is

considered infinite then the gain of the circuit is

−R2/R1. An op amp, however, usually has a dominant pole, which

causes its gain to drop with frequency. Hence, this gain is only valid for DC and low frequency. To understand

the effect of the input capacitance coupled with the non-ideal gain of the op amp, the circuit needs to be

analyzed in the frequency domain using a Laplace transform.

Figure 46. Inverting Amplifier

For simplicity, the op amp is modeled as an ideal integrator with a unity gain frequency of A0 . Hence, its transfer

function (or gain) in the frequency domain is A0/s. Solving the circuit equations in the frequency domain, ignoring

CF for the moment, results in an expression for the gain shown in Equation 1.

(1)

It can be inferred from the denominator of the transfer function that it has two poles, whose expressions can be

obtained by solving for the roots of the denominator and are shown in Equation 2.

(2)

Equation 2 shows that as the values of R1 and R2 are increased, the magnitude of the poles, and hence the

bandwidth of the amplifier, is reduced. This theory is verified by using different values of R1 and R2 in the circuit

shown in Figure 45 and by comparing their frequency responses. In Figure 47 the frequency responses for three

different values of R1 and R2 are shown. When both R1 and R2 are 1 kΩ, the response is flattest and widest;

whereas, it narrows and peaks significantly when both their values are changed to 10 k

Ω or 30 kΩ. So it is

advisable to use lower values of R1 and R2 to obtain a wider and flatter response. Lower resistances also help in

high sensitivity circuits since they add less noise.

Submit Documentation Feedback

Product Folder Links: LMV796 LMV797

LMV797MM/NOPB Datasheet(PDF) 14 Page - Texas Instruments

LMV797MM/NOPB Datasheet(HTML) 14 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Datasheet Download

Link URL