LTC1042 查看數據表（PDF） - Linear Technology

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LTC1042

Window Comparator

Linear Technology 

LTC1042

APPLICATIO S I FOR ATIO

To synchronize the sampling of the LTC1042 to an external

frequency source, the OSC pin can be driven by a CMOS

gate. A CMOS gate is necessary because the input trip

points of the oscillator are close to the supply rails and TTL

does not have enough output swing. Externally driven,

there will be a delay from the rising edge of the OSC input

and the start of the sampling cycle of approximately 5µs.

Input Impedance

The input impedance of the LTC1042 does not look like a

classic linear comparator; CMOS switches and a precision

capacitor array form the dual differential input structure.

Input impedance characteristics can be determined from

the equivalent circuit shown in Figure 2. The input

capacitance will charge with a time constant of RS • CIN. It

is critical, in determining errors caused by the input

charging current, that CIN be fully charged during the

“active” time.

For RS ≤ 10kΩ

For Rs less than or equal to 10kΩ, CIN fully charges and no

error is caused by the charging current.

For RS > 10kΩ

For source resistances greater than 10kΩ, CIN cannot fully

charge, causing voltage errors. To minimize these errors

an input bypass capacitor, CS should be used. Charge is

shared between CIN and CS causing a voltage error. The

magnitude of this error is ∆V = VIN x CIN/(CIN + CS). This

error can be made arbitrarily small by increasing CS.

The averaging effect of the bypass capacitor CS causes

another error term. Each time the input switches cycle

between the plus and minus inputs, CIN is charged and

discharged. The average input current due to this is

lAVG = VIN x CIN x fS, where fS is the sampling frequency.

Because the input current is directly proportional to the

differential input voltage, the LTC1042 can be said to have

an average input resistance of RIN = VIN/IAVG = 1/(fS x CIN).

Since two comparator inputs are connected in parallel, RIN

is one half this value (see typical curve of RIN vs Sampling

Frequency). This finite input resistance causes an error

due to voltage divided between RS and RIN.

The input error caused by both of these effects is

VERROR = VIN[2CIN/(2CIN + CS) + RS/(RS + RIN)].

EXAMPLE: Assume fS = 10Hz, RS = 1MΩ, CS = 1µF and

VIN = 1V. Then VERROR = 1V(66µV + 660µV) = 726µV. If the

sampling frequency is reduced to 1Hz, the voltage error

from input impedance effects is reduced to 136µV.

Input Voltage Range

The input switches of the LTC1042 are capable of

switching either to the V+ supply or ground. Consequently,

the input voltage range includes both supply rails. This is

a further benefit of the input sampling structure.

Error Specifications

The only measurable errors on the LTC1042 are the

deviations from “ideal” of the upper and lower window

limits [Figure 1(B)]. The critical parameters for a window

comparator are the width and center of the window. These

errors may be expressed in terms of VU and VL.

center error = [(VU + VL)/2] – CENTER

width error = (VU – VL) – 2 x (WIDTH/2)

The specified error limits (see Electrical Characteristics)

include error due to offset, power supply variation, gain,

time and temperature.

RS

S1

+

CIN

(~33pF)

VIN

CS

S2

–

V–

LTC1042 DIFFERENTIAL INPUT

LTC1042 • AI02

Figure 2. Equivalent Input Circuit

1042fa

5

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