PIC17C7XX Datasheet, PDF (181/328 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM Microcontrollers with 10-bit A/D

PIC17C7XX
16.1 A/D Acquisition Requirements
For the A/D converter 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 Figure 16-5. 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),
Figure 16-5. The maximum recommended imped-
ance for analog sources is 10 kΩ. As the impedance
is decreased, the acquisition time may be decreased.
After the analog input channel is selected (changed)
this acquisition must be done before the conversion
can be started.
To calculate the minimum acquisition time,
Equation 16-1 may be used. This equation assumes
that 1/2 LSb error is used (1024 steps for the A/D). The
1/2 LSb error is the maximum error allowed for the A/D
to meet its specified resolution.
Example 16-1 shows the calculation of the minimum
required acquisition time TACQ.
This calculation is based on the following application
system assumptions.
CHOLD
=
Rs
=
Conversion Error ≤
VDD
=
graph in Figure 16-5)
Temperature
=
VHOLD
=
120 pF
10 kΩ
1/2 LSb
5V → Rss = 7 kΩ (see
50°C (system max.)
0V @ time = 0
EQUATION 16-1: ACQUISITION TIME
TACQ = Amplifier Settling Time +
Holding Capacitor Charging Time +
Temperature Coefficient
= TAMP + TC + TCOFF
EQUATION 16-2: A/D MINIMUM CHARGING TIME
VHOLD
or
Tc
= (VREF - (VREF/2048)) • (1 - e^{(-Tc/CHOLD(RIC + RSS + RS))})
= -(120 pF)(1 kΩ + RSS + RS) ln(1/2047)
EXAMPLE 16-1: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
TACQ = TAMP + TC + TCOFF
Temperature coefficient is only required for temperatures > 25°C.
TACQ = 2 µs + Tc + [(Temp - 25°C)(0.05 µs/°C)]
TC =
-CHOLD (RIC + RSS + RS) ln(1/2047)
-120 pF (1 kΩ + 7 kΩ + 10 kΩ) ln(0.0004885)
-120 pF (18 kΩ) ln(0.0004885)
-2.16 µs (-7.6241)
16.47 µs
TACQ =
2 µs + 16.47 µs + [(50°C - 25°C)(0.05 µs/°C)]
18.447 µs + 1.25 µs
19.72 µs
Note 1: The reference voltage (VREF) has no effect on the equation, since it cancels itself out.
Note 2: The charge holding capacitor (CHOLD) is not discharged after each conversion.
Note 3: The maximum recommended impedance for analog sources is 10 kΩ. This is required to meet the pin leak-
age specification.
Note 4: After a conversion has completed, a 2.0TAD delay must complete before acquisition can begin again. Dur-
ing this time the holding capacitor is not connected to the selected A/D input channel.
© 1998 Microchip Technology Inc.
DS30289A-page 181