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}), refer to Figure 2. 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 must be completed before the
conversion can be started. To calculate the minimum acquisition time, Figure 1 may be used. This equation assumes that 1/2 LSb error is used (1024
steps for the ADC). The 1/2 LSb error is the maximum error allowed for the ADC to meet its
specified resolution.

Figure 1. Acquisition Time Example
T A C Q = 2 μ s + T C + [ ( T e m p e r a t u r e − 25 ° C ) ( 0.05 μ s / ° C ) ]

Assumptions: Temperature = 50°C; External impedance = 10kΩ; 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:

Note:

- 1.The reference voltage (V
_{REF}) has no effect on the equation, since it cancels itself out. - 2.The charge holding capacitor (C
_{HOLD}) is not discharged after each conversion. - 3.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