5

LTC1040

1040fa

The LTC1040 uses sampled data techniques to achieve its

unique characteristics. Some of the experience acquired

using classic linear comparators does not apply to this

circuit, so a brief description of internal operation is

essential to proper application.

The most obvious difference between the LTC1040 and

other comparators is the dual differential input structure.

Functionally, when the sum of inputs is positive, the

comparator output is high and when the sum of the inputs

is negative, the output is low. This unique input structure

is achieved with CMOS switches and a precision capacitor

array. Because of the switching nature of the inputs, the

concept of input current and input impedance needs to be

examined.

The equivalent input circuit is shown in Figure 1. Here, the

not be needed, depending on the size of the source

APPLICATIO S I FOR ATIO

Figure 1. Equivalent Input Circuit

LTC1040 • AI01

S1

S2

≈ 33pF

LTC1040 DIFFERENTIAL INPUT

+

–

sampling interval (

exceeds 10k

Ω, a bypass capacitor is necessary to mini-

mize errors.

=

1

=

1

This represents an error and can be made arbitrarily small

finite input resistance of the LTC1040 must be considered.

Switches S1 and S2 alternately open and close, charging

flow into the positive input and out of the negative input.

The magnitude of this current is:

current is directly proportional to input voltage, the LTC1040

can be said to have an average input resistance of:

sampling frequency is reduced to 1Hz, the voltage error is

reduced to 66

µV.

(see typical curve of Input Resistance vs Sampling Fre-

causing error.

The input voltage error caused by these two effects is:

()

+

= 33

µV + 330µV = 363µV.

()

+