7 Error analysis / constraints

AN1812

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7

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

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 will also

Then eq(5) becomes,

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

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

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’ will be 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 will be significant so the capacitor will be charged very fast. This will affect 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 basically do the averaging. So the larger the value of the capacitor, the longer