Intel 8XC196MD, 8XC196MH, 8XC196MC manual Minimizing the Effect of High Input Source Resistance

ANALOG-TO-DIGITAL (A/D) CONVERTER

Typically, the (RF / AV + 1) term is the major contributor to the total resistance and the factor that determines the minimum sample time specified in the datasheet.

12.6.1.1Minimizing the Effect of High Input Source Resistance

Under some conditions, the input source resistance (RSOURCE) can be great enough to affect the measurement. You can minimize this effect by increasing the sample time or by connecting an

external capacitor (CEXT) from the input pin to ANGND. The external signal will charge CEXT to the source voltage level. When the channel is sampled, CEXT acts as a low-impedance source to charge the sample capacitor (CS). A small portion of the charge in CEXT is transferred to CS, re- sulting in a drop of the sampled voltage. The voltage drop is calculated using the following for- mula.

CS	× 100%
Sampled Voltage Drop, % = C---------------------------EXT+ CS

If CEXT is 0.005 µF or greater, the error will be less t han –0.4 LSB in 10-bit conversion mode. The use of CEXT in conjunction with RSOURCE forms a low-pass filter that reduces noise input to the A/D converter.

High RSOURCE resistance can also cause errors due to the input leakage (ILI1). ILI1 is typically much lower than its specified maximum (consult the datasheet for specifications). The combined effect

of ILI1 leakage and high RSOURCE resistance is calculated using the following formula.

	RSOURCE × ILI1 × 1024
error (LSBs) = -----------------------------------------------------------
	VREF
where:
RSOURCE	is the input source resistance, in ohms
ILI1	is the input leakage, in amperes
VREF	is the reference voltage, in volts

External circuits with RSOURCE resistance of 1 kilo-ohm or lower and VREF equal to 5.0 volts will have a resultant error due to source impedance of 0.6 LSB or less.

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