11

It may also be possible to directly program the host’s serial

hardware for operation at nonstandard baud rates, allowing

HI-7159A operation at any arbitrary frequency. For example:

50Hz AC rejection requires a 2MHz clock. At this frequency

the “9600” baud rate becomes 7812.5 baud. The host’s

UART must be programmed with the proper divider to

operate at this baud rate. The data clock (see Figure 2) is

defined as 16 times the baud rate, so the data clock of this

configuration would be 125kHz. The data clock can also be

determined by dividing the oscillator (clock) frequency by the

correct divider from Table 12.

The following equation determines the divider needed to

operate the HI-7159A at any given crystal frequency:

Once determined, the new divider must be written directly to

the Host’s UART. Most PC compatibles use an 8250 UART with

a 1.8432MHz crystal, so the proper divider for the 2MHz

example given above would be 15. Again, these considerations

apply only to Serial Modes 1 and 2. Parallel and Serial Mode 0

communication rates are independent of crystal frequency.

Conversion Time

The conversion time of the HI-7159A is a function of the

crystal frequency and the type of conversion being made.

Table 13. At other clock frequencies the times may be

calculated from the following formula:

where the constant C is determined from Table 13.

Component Selection

Three external passive components must be chosen for the

chosen based on the crystal frequency, the reference voltage

the analog components necessary for the HI-7159A to function.

The reference capacitor and integrating components can

either be selected from Table 14, or calculated from the

following equations.

conversion. Its value is determined by two considerations: it

must be small enough to be fully charged from its

discharged state at power-on; yet it also must be large

enough to supply current to the circuit during conversion

without significantly drooping from its initial value. For

2.4MHz operation, a 1

µF capacitor is recommended. The

equation for other frequencies is:

maximum integration current and the maximum integrator

output voltage swing. The maximum integration current and

recommended integration current for the HI-7159A is

since:

TABLE 12. CRYSTAL DIVIDER RATIOS

BAUD RATE SELECTED

CRYSTAL DIVIDER

“300”

512

“1200”

128

“9600”

16

“19200”

8

TABLE 13. CONVERSION TIMES

CONVERSION TYPE

COMP

UNCOMP

COMP

UNCOMP

f = 2.4MHz

133ms

66.7ms

33.3ms

16.7ms

C

320,000

160,000

80,000

40,000

f

()

Divider 7159A

()

--------------------------------------------

f

()

Divider Host UART

()

----------------------------------------------------------------

Data Clock

==

t

CONV

C

f

CLOCK

---------------------

=

TABLE 14. RECOMMENDED COMPONENT VALUES vs

CLOCK FREQUENCY

2.4MHz

400k

Ω

0.01

µF

1.0

µF

1.2MHz

360k

Ω

0.022

µF

2.2

µF

600kHz

330k

Ω

0.047

µF

4.7

µF

performance may be degraded.

+5V

27

1

14

INT OUT

INT IN

BUF OUT

2

3

4

5

6

7

8

9

12

13

11

10

26

REFERENCE

CAPACITOR

GUARD

RINGS

REF LO

-5V

HI-7159A

FIGURE 8. ANALOG COMPONENTS AND INPUTS

C

REF

2.5

f

CLOCK

---------------------

=

I

INT

V

IN

R

INT

-------------

so

R

INT

V

IN

I

INT

-----------

,

==

HI-7159A