AN1812 datasheet(10/34 Pages) STMICROELECTRONICS

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6 Characteristics of different slope converters

AN1812

10/34

6.2

Dual-slope converter

Figure 7.

Dual-slope converter circuit diagram

6.2.1

Timing diagram

As shown in Figure 8 a dual-slope ADC has a charging phase followed by a fixed rate

discharging phase.

Figure 8.

Dual-slope converter timing diagram

The advantage of a dual-slope ADC is that it is not dependent on the correction factor for the

R*C product. However, the input offset voltage problem still persists here and this ADC also

requires a dual supply op-amp to be able to measure the positive and negative voltages.

6.3

Solution presented in this application note

In this converter, the input voltage is proportional to the inverse of the time measured. We can

see in Figure 9, as the input voltage becomes closer to V1, the conversion time also increases.

For an input of V1, the conversion time will be infinite (1/ Tm = 0 in Figure 9). So the input

voltage range depends on the value of V1 and the maximum delay that the application can

tolerate.

out

gnd

clk

cmp

Control

Logic

enbl

clk

Counter

ctr

clk

-Vin

Vref

Vin1

Vin2

-Vref

Charging

Phase

Fixed-Rate

Discharge

Vin

Vref

Tcharge

Tdischarge

Time

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