LTC1067-50CS datasheet(13/20 Pages) AD

13 / 20 page

LTC1067/LTC1067-50

Aliasing

Aliasing is an inherent phenomenon of switched-capacitor

filters and occurs when the frequency of the input signals

that produce the strongest aliased components have a

frequency, fIN, such as (fSAMPLING – fIN) that falls into the

filter’s passband. For both the LTC1067 and the

LTC1067-50, the sampling frequency is twice fCLK. If the

input signal spectrum is not band limited, aliasing may

occur.

Output Loading

The op amps on the LTC1067/LTC1067-50 have a rail-to-

rail output stage. The output loading issues can be divided

into resistive loading effects and capacitive loading ef-

fects.

Resistive loading effects the maximum output signal swing.

This effect is shown in the typical performance curves.

Note that the load on the output must include both the

feedback resistor and any external load resistor. For

example, consider the following situation: the part is

running on split power supplies, the section is configured

in Mode 3, the R4 resistor is 20k and an external 20k load

is connected from the LP node to ground. The load on the

LP output is 20k in parallel with 20k, or 10k. All testing on

the LTC1067/LTC1067-50 is done with a 10k load. For the

best results, the load resistance on all output pins should

be at least 10k.

Capacitive loading reduces the stability of the op amps.

The signal at the output of a switched-capacitor filter is

composed of a series of very small steps. The op amp

must respond to a step and fully settle before the next step.

As the stability of the op amp is decreased, the output step

response has increased ringing and a much longer settling

time. This longer settling time drastically lowers the maxi-

mum usable clock speed and introduces errors. If the

capacitive loading is sufficiently high, the stability will be

decreased to the point of oscillation at the output.

The LTC1067/LTC1067-50 are sensitive to capacitive load-

ing. Capacitive loading should be kept below 20pF. Good,

tight layout techniques should be maintained at all times.

These parts should not drive long traces and never drive

a long coaxial cable.

When probing the LTC1067 or

APPLICATIONS INFORMATION

LTC1067-50, always use a 10

× probe. Never use a 1×

probe. A standard 10

× probe has a capacitance of 10pF to

15pF while a 1

× probe’s capacitance can be as high as

150pF. The 1

× probe will probably cause oscillation.

What to Do with an Unused Section

If the LTC1067 or LTC1067-50 is used as a single 2nd

order filter, the other 2nd order section is not used. Do not

leave this section unconnected. If the section is uncon-

nected, inputs and outputs are left to float to undetermined

levels and oscillation may occur. The unused section

should be connected as shown in Figure 10.

Output Voltage Swing on a Single Supply Voltage

The typical performance curves show the output voltage

swing limitations. The curves show the output signal

swing, in volts peak-to-peak, versus the output load resis-

tance. The peak-to-peak swing is limited by the following

three considerations: the op amp’s output swings closer

to the negative supply than the positive supply, the AGND

pin is biased at the midpoint of the supplies and all

operating modes are inverting.

The op amps in the LTC1067/LTC1067-50 swing closer to

the negative supply rail than the positive supply rail. The

positive output voltage swing for single supply operation

is shown in Figures 11 and 12. The negative output voltage

swing is about 15mV for the LTC1067 and 10mV for the

LTC1067-50. The negative output voltage swing is nearly

independent of load resistance since the load in this case

is connected to the V – supply rail.

For single supply applications, the on-chip resistor divider

sets the voltage at the AGND pin to the midpoint of the V +

and V – potentials. The AGND voltage is the reference for

all internal op amps. If the input to the filter is at the V – rail,

–

V+

∑

1067 F10

INV

Figure 10. Connections for an Unused Section

LTC1067-50CS Datasheet(PDF) 13 Page - Analog Devices

LTC1067-50CS Datasheet(HTML) 13 Page - Analog Devices

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