For the ADC to meet its specified accuracy, the
charge holding capacitor (C_{HOLD}) must be allowed
to fully charge to the input channel voltage level. The analog input model is shown in
Figure 2. The source
impedance (R_{S}) and the internal sampling switch
(R_{SS}) impedance directly affect the time
required to charge the capacitor C_{HOLD}. The
sampling switch (R_{SS}) impedance varies over the
device voltage (V_{DD}). The maximum recommended
impedance for analog sources is 10 kΩ. As the source impedance is decreased, the
acquisition time may be decreased. After the analog input channel is selected (or
changed), an ADC acquisition time must be completed before the conversion can be
started. To calculate the minimum acquisition time, Figure 1 may be used. This
equation assumes an error of 1/2 LSb. The 1/2 LSb error is the maximum error allowed for
the ADC to meet its specified resolution.

Figure 1. Acquisition
Time Example

Assumptions: Temperature = 50°C; External impedance = 10 kΩ; V_{DD} = 5.0V

T_{ACQ} = Amplifier Settling
Time + Hold Capacitor Charging Time + Temperature Coefficient

The value for T_{C} can be
approximated with the following equations:

_{CHOLD} charged to within ½ LSb

_{CHOLD} charge response to V_{APPLIED}

**Note:** Where n = ADC resolution in bits

Solving for T_{C}:

Therefore:

Important:

- The reference
voltage (V
_{REF}) has no effect on the equation, since it cancels itself out - The charge
holding capacitor (C
_{HOLD}) is not discharged after each conversion - The maximum recommended impedance for analog sources is 10 kΩ. This is required to meet the pin leakage specification.

Figure 2. Analog
Input Model

Figure 3. ADC
Transfer Function