AN1812 datasheet(5/34 Pages) STMICROELECTRONICS

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AN1812

2 Theory of operation

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Theory of operation

Vin is the Input voltage. The voltages across resistor R are the reference voltage V1 and the

input voltage Vin. Due to the properties of the op-amp, V1 is output on the inverting pin of the

op-amp. Therefore, for a given input voltage, the current flowing through resistor R will be

constant. Let this current be I.

Current I charges the capacitor C, and output starts increasing in the positive direction for the

input Vin <= V1 (for input Vin > V1 it will charge in the opposite direction).

The output is captured at two instants using the two output comparators at voltage references

V2 and V3. The time corresponding to voltage levels V2 and V3 are T2 and T3 respectively. The

final reading of time Tm is taken as the difference of T3 and T2.

The input voltage is calculated from this difference through the formulae given in the circuit

analysis.

This technique can only be used where the input voltage varies slowly, otherwise the charging

of the capacitor will be non-linear.

2.1

Advantage of using two comparators

The purpose of using the second comparator (comp2) can be understood from the diagram

below (Figure 2), which shows the relationship between the op-amp output (Amp in Figure 1)

and the time for a given input value.

Figure 2.

Relationship between Vout and time for a given input

The time is measured as the difference of the two timer readings (T3 -T2) for the same slope.

So factors like the residual voltage of the capacitor ( Vc(0+)) and any other constant errors (like

the effect of output offset voltage) on the output side of the op-amp are subtracted. So its

performance is better than a single-slope converter.

Vout

Time (t)

Point of

uncertainty

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