LTC1040CSW datasheet(8/12 Pages) LINER

8 / 12 page

LTC1040

1040fa

Output Logic

In addition to the normal outputs (AOUT and BOUT), two

additional outputs, A + B and ON/0FF, are provided (see

Figure 8 and Table 1). All logic is powered from V+ and

ground, thus input and output logic levels are independent

of the V – supply. The LTC1040 is directly compatible with

CMOS logic and is TTL compatible for 4.75V

≤ V+ ≤ 5.25V.

No external pull-up resistors are required.

ΣA INPUTS

ΣB INPUTS

AOUT

BOUT

A + B

ON/OFF

+–

–+

––

*I = indeterminate. When both A and B outputs are low, the ON/OFF output

remains in the state it was in prior to entering AOUT = BOUT = L.

Using External Strobe

A positive pulse on the strobe input, with the 0SC input tied

to ground, will initiate a comparison cycle. The STROBE

input is edge-sensitive and pulse widths of 50ns will

typically trigger the device.

APPLICATIO S I FOR ATIO

Because of the sampling nature of the LTC1040, some

sensitivity exists between the offset voltage and the falling

edge of the input strobe. When the falling edge of the

strobe signal falls within the comparator’s active time

(80

µsafterrisingedge),offsetchangesofasmuchas2mV

can occur. To eliminate this problem, make sure the strobe

pulse width is greater than the response time, tD.

Using Internal Strobe

An internal oscillator allows the LTC1040 to strobe itself.

The frequency of oscillation, and hence sampling rate, is

set by an external RC network (see typical curve of

Sampling Rate vs REXT, CEXT).

For self-oscillation, the STROBE pin must be tied to

ground. The external RC network is connected as shown

in Figure 9.

To assure oscillation, REXT must be between 100k and

10M. There is no limit to the size of CEXT.

REXT is very important in determining the power

consumption. The average voltage at the oscillator pin is

approximately V+/2. The power consumed by REXT is then:

PREXT = (V +/2)2/REXT.

LTC1040 • AI08

BOUT

AOUT

A + B

ON/OFF

µs

COMPARATOR A

OUTPUT

COMPARATOR B

OUTPUT

STROBE

REXT

CEXT

LTC1040 • AI09

LTC1040

V+

Figure 9. External RC Connection

Figure 8. LTC1040 Logic Diagram

Table 1. Output Logic Truth Table

Example: REXT = 1M, V + 5V, PREXT = (2.5)2/106 =

6.25 • 10–6W.

This is about four times the power consumed by the

LTC1040 at V + = 5V and fS = 1 sample/second. Where

power is a premium REXT should be made as large as

possible. Note that the power consumed by REXT is not a

function of fS or CEXT.

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