AN2615 datasheet(13/37 Pages) STMICROELECTRONICS

13 / 37 page

AN2615

Characteristics of different slope converters

13/37

6.3

Solution presented in this application note

In this application note, a single supply ADC for positive and negative input voltages is

described. It's input voltage is proportional to the inverse of the time measured. We can see

in Figure 9 below that as the input voltage becomes closer to V1, the conversion time also

increases. For an input of V1, the conversion time is 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.

Figure 9.

VIN versus time in AN2615 solution

Vin = V1 - (R * C) * (V3 -V2) /Tm

The significant advantage of this ADC is its ability to measure positive and negative input

voltages operating from single supply, while other solutions require a dual supply. Also this

converter does not require any negative voltage reference. Again, as in the single slope

converter, the major sources of error are the correction factor for R*C product and the input

offset voltage.

As shown Figure 9, the ADC is capable of measuring the input voltage ranging +Vref to -Vref,

where the absolute value of Vref is mod (Vref) < V1 so the input voltage range depends on

the value of V1.

1/Tm

Positive

input

Negative

input

-Vref

Total input range (+Vref to -Vref)

Vin

+Vref

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