Service manual

PT300, Service Manual

Rev B, October, 2000

Auto Zero

Integrate

Deintegrate

U17 Pin 1

Auto Zero

Figure 5 Conversion Cycle

Let’s examine the operation of the converter. Start with C7 at zero volts. The

+input voltage charges C7 for a fixed period called Time-integrate. Then the

integrator input is switched to the reference voltage and C7 is discharged for an

amount of time called Time-deintegrate; this time is related to the input voltage.

At sometime the voltage on C7 crosses zero, and the output of the comparator

changes state to reflect this. Another way of looking at the discharge cycle is

that the reference voltage does not change so the rate of discharge should be a

certain number of volts per second. The higher the input voltage, the higher the

voltage that will be driven onto the integrating capacitor. The higher the voltage

on the integrating capacitor, the longer the time of discharge. When the

integrator is not charging or discharging it is held in an "auto-zero" condition. At

this time any internal error voltages cancels inside U17. Note that the reference

voltage is simply the power supply divided by a resistor network. This is the

same supply used to power the load cells, so any change in the power supply

excitation to the load cells is canceled in the analog to digital converter. This

feature, adjusting the reference by the same amount as the excitation voltage, is

called ratiometric conversion.