ADC150 datasheet(4/9 Pages) ETC

4 / 9 page

THEORY OF OPERATION

The timing control circuitry governs the counters that

measure the integration time in both directions.

The ADC150's on-board microprocessor is used to

calculate the results of the integration equation and

perform

error

corrections.

Note

that

the

µP

automatically performs an auto zero function at start-

up, but it is recommended to achieve maximum

accuracy, that an auto zero be performed again after

the ADC150 is fully warmed up.

When the µP detects a convert signal, it lowers the

status lines to indicate that the ADC is involved in a

conversion.

When it detects a change in slope

direction, the µP will collect the counts for the

integration time. When sufficient counts have been

collected, the µP performs the calculations described

above.

When the calculations are complete, the µP places

the most significant byte in the output buffer and

raises the S

0 flag.

When another pulse is placed on

the convert line, the middle byte is placed on the

output, the S

0 flag is lowered and the S1 flag raised.

When the last pulse is placed in the convert line, the

least significant byte is placed in the output buffer and

both status flags are high indicating that the ADC150

is ready for another conversion.

Status line summary:

FIGURE 1. BLOCK DIAGRAM

Conversion in progress.

Conversion complete. MSB in output.

Middle byte in output register.

LSB in output. Ready for next conversion.

Tp=

In the ADC150 block diagram (see Figure 1), V

and V

low are the inputs. Both are buffered and fed

into a differential, voltage controlled, single output

current source.

This current is added to the

reference current at the input of the op amp

integrator. The output of the integrator is fed into

a Schmitt trigger, which in turn, is fed into the

ADC's

timing

control

circuitry.

When

the

integrator output actuates the Schmitt trigger, the

timing

circuit

changes

the

direction

the

reference current source and the integrator

begins integrating in the opposite direction. This

continues until the Schmitt trigger is actuated

again by the integrator and reverses the direction

of the reference current.

The equation for integration times are:

V X C

I ref + I inp

Tm=

V X C

-I ref + I inp

Resolving these equations produces:

I inp = I ref

Tp - Tm

Tp + Tm

Auto

Zero

Switch

Schmitt

Trigger

Bidirectional

Reference

Current Source

Current

Directional

Switch

Timing

Control

and

Counter

Microprocessor

Output

Buffer

Clock

Differential

Voltage Controlled

Current Source

low

+15V

-15V

Auto

Zero

Convert

Status

Lines

Output Enable

Data

Output

V = Voltage

C= Integration Capacitor Value

I ref = Reference Current

I inp = Input Current

Tp = Time Positive

Tm = Time Negative

ADC150DS REV. F MAR 00

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