Instruction Manual

245364-V May 2002

Model 755A

5-8 DIGITAL OUTPUT CIRCUIT

Refer to Figure 5-10 on page 5-14. With front panel TEST Switch in position 1, the output signal from buffer amplifier U10 is routed through an attenuator and filter network to an integrating analog-to-digital converter. It converts the signal into an equivalent digital value in the range of 0.00% to 99.99%. Any value above 99.99% will be preceded by an over-range bit, for example, 1.1123.

The output of the ADC consists of binary-coded decimal characters that are input to the liquid crystal controller and display chip characters sequentially in time. The BCD characters are converted into seven-line codes to drive the bar segments of the liquid crystal display.

A separate regulator circuit, which operates from the +15VDC supply, provides a regulated 5VDC for the digital functions associated with the display.

5-9 ANALOG OUTPUT CIRCUITS FOR

RECORDER AND ALARMS

Refer to Figure 5-10 on page 5-14. The analog output circuits utilize two amplifiers:

First Stage Amplifier

This amplifier permits selecting the desired fullscale oxygen range for the recorder by an appropriate combination of scale expansion and zero suppression.

Scale expansion is accomplished by selecting the appropriate feedback resistor in a switch- selectable network, thus establishing one of six values for amplifier gain:

DESIRED OXYGEN

REQUIRED

SPAN FOR

AMPLIFIER GAIN

RECORDER

 

100%

1X

20%

5X

10%

10X

5%

20X

2%

50X

1%

100X

The desired zero suppression is obtained as the sum of (a) a jumper-selectable fixed value of 0%, 20%, 40%, or 80% oxygen and (b) a continuously adjustable value of 0% to 25% oxygen. Thus the total zero suppression may be set for any desired valued from 0% to 105% oxygen.

In order to establish the precise zero suppression required for the recorder output, the actual applied zero suppression may be read on the digital display by placing the front panel TEST Switch in position 4 and the recorder scale-expansion jumper in the 100% (i.e., 1X gain) position.

Note that, in this mode the signal input is disconnected, allowing only the input offset current for the zero-suppression function to flow into the amplifier summing node.

Example 1, Selection of a Zero Based Recorder Oxygen Range:

Desired oxygen range for recorder output: 0% to 100%

Required span is 100% oxygen; thus a gain of 1 is selected with the jumper.

Required zero suppression is 0%; thus, the Zero Suppression Selection Jumper is removed, and the Zero Suppression Potentiometer is set for 0%.

Example 2, Selection of a Zero Suppression Recorder Oxygen Range:

Desired oxygen range for recorder output: 90% to 100%.

Required span is 10% oxygen; thus, a gain of 10 is selected with the jumper.

Required zero suppression is 90% oxygen; thus the Zero Suppression Selection Jumper is placed in the 80% position, and the Zero Suppression Potentiometer is set for 10% oxygen.

5-12

Circuit Analysis

Rosemount Analytical Inc. A Division of Emerson Process Management