LMF60 datasheet(13/20 Pages) NSC

13 / 20 page

10 LMF60 Application Hints (Continued)

TLH9294 – 13

TLH9294 – 14

FIGURE 5 VOS Adjust Schemes

13 OFFSET ADJUST

The VOSADJ pin is used in adjusting the output offset level

of the filter section If this pin is not used it must be tied to

the analog ground (AGND) level either mid-supply for single

ended supply operation or ground for split supply operation

This pin sets the zero reference for the output of the filter

The implementation of this pin can be seen in

Figure 5 In

5(a) DC offset is adjusted using a potentiometer in 5(b) the

Op-Amp integrator circuit keeps the average DC output lev-

el at AGND The circuit in

5(b) is therefore appropriate only

for AC-coupled signals and signals biased at AGND

14 INPUT IMPEDANCE

The LMF60 lowpass filter input (FILTER IN pin) is not a high

impedance buffer input This input is a switched capacitor

resistor equivalent and its effective impedance is inversely

proportional to the clock frequency The equivalent circuit of

the input to the filter can be seen in

Figure 6 The input

capacitor charges to the input voltage (VIN) during one half

of the clock period during the second half the charge is

transferred to the feedback capacitor The total transfer of

charge in one clock cycle is therefore Q e CINVIN and

since current is defined as the flow of charge per unit time

the average input current becomes

IIN e QT

(where T equals one clock period) or

IIN e

CINVIN

CINVINfCLK

The equivalent input resistor (RIN) then can be defined as

RIN e VIN IIN e

CINfCLK

The input capacitor is 2 pF for the LMF60-50 and 1 pF for

the LMF60-100 so for the LMF60-100

RIN e

1 c 1012

fCLK

1 c 1012

fC c 100

1 c 1010

and

RIN e

5 c 1011

fCLK

5 c 1011

fC c 50

1 c 1010

for the LMF60-50 As shown in the above equations for a

given cutoff frequency (fC) the input impedance remains the

same for the LMF60-50 and the LMF60-100 The higher the

clock to cutoff frequency ratio the greater equivalent input

resistance for a given clock frequency As the cutoff fre-

quency increases the equivalent input impedance decreas-

es This input resistance will form a voltage divider with the

source impedance (RSOURCE) Since RIN is inversely pro-

portional to the cutoff frequency operation at higher cutoff

frequencies will be more likely to load the input signal which

would appear as an overall decrease in gain at the output of

the filter Since the filter’s ideal gain is unity its overall gain

is given by

AV e

RIN

RIN a RSOURCE

TLH9294 – 15

a) Equivalent Circuit for LMF60 Filter Input

TLH9294 – 16

b) Actual Circuit for LMF60 Filter Input

FIGURE 6 LMF60 Filter Input

http

www nationalcom

LMF60 Datasheet(PDF) 13 Page - National Semiconductor (TI)

LMF60 Datasheet(HTML) 13 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL