32.3 ADC Acquisition Requirements

For the ADC to meet its specified accuracy, the charge holding capacitor (CHOLD) must be allowed to fully charge to the input channel voltage level. The Analog Input model is shown in the following figure. The source impedance (RS) and the internal sampling switch (RSS) impedance directly affect the time required to charge the capacitor CHOLD. The sampling switch (RSS) impedance varies over the device voltage (VDD). Refer to the following figure.

Figure 32-4. Analog Input Model
Note:
  1. Refer to the “I/O Ports” section in the “Electrical Specification” chapter.
Important: The maximum recommended impedance for analog sources is 10 kΩ.
If 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, the following equation may be used. This equation assumes that 1/2 LSb error is used (4,096 steps for the ADC). The 1/2 LSb error is the maximum error allowed for the ADC to meet its specified resolution.
Equation 32-1. Acquisition Time Example

Assumptions: Temperature = 50°C and external impedance pf 10 kΩ, 5.0V VDD

TACQ = Amplifier Settling Time + Hold Capacitor Charging Time + Temperature Coefficient

= TAMP + TC + TCOFF

= 2 μs + TC + [(Temperature - 25°C)(0.05 μs/°C)]

The value for TC can be approximated with the following equations:

V A P P L I E D ( 1 − 1 ( 2 n + 1 ) − 1 ) = V C H O L D ⁢ ⁢                                     ;[1] V C H O L D charged to within 1/2 lsb
V A P P L I E D ( 1 − e − T C R C ) = V C H O L D ⁢ ⁢                                     ;[2] V C H O L D charge response to V A P P L I E D
V A P P L I E D ( 1 − e − T C R C ) = V A P P L I E D ⁢ ( 1 − 1 ( 2 n + 1 ) − 1 ) ⁢                                     ;combining [1] and [2]
Note: Where n = number of bits of the ADC.

Solving for TC:

T C = − C H O L D ( R I C + R S S + R S )     ln ⁡ ( 1 / 8191 )
T C = − 28 ⁢   p F ( 1 ⁢   k Ω + 7 ⁢   k Ω + 10 ⁢   k Ω )     ln ⁡ ( 0.0001221 )
T C = 4.54 ⁢   u s

Therefore:

T A C Q = 2 ⁢   u s + 4.54 ⁢   u s + [ ( 50 ° C − 25 ° C ) ( 0.05 ⁢   u s / ° C ) ]
T A C Q = 7.79 ⁢   u s
Note:
  1. The reference voltage (VREF) has no effect on the equation, since it cancels itself out.
  2. The charge holding capacitor (CHOLD) 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 32-5. ADC Transfer Function