AN2615 datasheet(14/37 Pages) STMICROELECTRONICS

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Error analysis/constraints

AN2615

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Error analysis/constraints

This ADC can be used for measuring any slowly varying input (voltage/current), for example

battery monitoring, and for measuring positive and negative input voltages. But, besides the

need for accurate power supply and voltage references, the following factors also affect the

accuracy of the conversion.

7.1

Input offset voltage

As mentioned previously, the output offset voltage is subtracted from the input, but the input

offset voltage of the op-amp (Amp) still remains present and is directly added to V1. For

measurement purposes, let us refer to the input offset voltage of the op-amp as Koffset.

7.2

Correction factor for the product of R*C

As the value of the R and C changes with time and temperature, the factor R * C also

changes. Let the correction factor be Kgain.

Then eq(5) becomes,

Vin = V1 + Koffset – Kgain* (R * C) *(V3 -V2)/Tm

------------------ (6)

The coefficients Koffset and Kgain can be calculated by measuring Tm for two known input

values. These factors can also be compensated by software calibration techniques (like

using look-up tables or storing some known values). In the present example the first method

is used to calculate these coefficients.

7.3

Value of charging resistance R

If the charging resistance ‘R’ is too high then the current ‘I’ is comparable to the input bias

current of the op-amp, which can affect the output. Also if it is too low then the current

flowing through it is significant so the capacitor is charged very fast. This affects the

measurement accuracy of the ADC.

7.4

Charging capacitor C

Up to this point we have assumed that capacitor C discharges completely from the previous

conversion. However, this is not so in actual practice and a few millivolts worth of charge

(which adds to the offset voltage), may remain on the capacitor. This effect is called

capacitor dielectric absorption and varies depending on the capacitor's dielectric material

voltage to which it was charged during the last charge cycle and the amount of time the

capacitor has had to discharge. Also due to this effect, the output of the capacitor may not

be linear over the whole conversion range. So it is very important to choose the right

capacitor for your requirements. While Teflon capacitors exhibit the lowest dielectric

absorption, polystyrene and polyethylene are also excellent. Ceramic, glass and mica are

fair, while tantalum and electrolytic types are poor choices for A/D applications.

Also, as integrating ADC’s are dependent on the integration of the current flowing through

capacitor C, they do the averaging. So, the larger the value of the capacitor, the longer the

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