PIC16C71X
full scale error is that full scale does not take offset error
into account. Gain error can be calibrated out in soft-
ware.
7.5
A/D Operation During Sleep
The A/D module can operate during SLEEP mode.This
requires that the A/D clock source be set to RC
(ADCS1:ADCS0 = 11). When the RC clock source is
selected, the A/D module waits one instruction cycle
before starting the conversion. This allows the SLEEP
instruction to be executed, which eliminates all digital
switching noise from the conversion. When the conver-
sion is completed the GO/DONE bit will be cleared, and
the result loaded into the ADRES register. If the A/D
interrupt is enabled, the device will wake-up from
SLEEP. If the A/D interrupt is not enabled, the A/D mod-
ule will then be turned off, although the ADON bit will
remain set.
Linearity error refers to the uniformity of the code
changes. Linearity errors cannot be calibrated out of
the system. Integral non-linearity error measures the
actual code transition versus the ideal code transition
adjusted by the gain error for each code.
Differential non-linearity measures the maximum
actual code width versus the ideal code width. This
measure is unadjusted.
In systems where the device frequency is low, use of
the A/D RC clock is preferred. At moderate to high fre-
quencies, TAD should be derived from the device oscil-
lator. TAD must not violate the minimum and should be
≤ 8 µs for preferred operation. This is because TAD,
when derived from TOSC, is kept away from on-chip
phase clock transitions.This reduces, to a large extent,
the effects of digital switching noise.This is not possible
with the RC derived clock. The loss of accuracy due to
digital switching noise can be significant if many I/O
pins are active.
When the A/D clock source is another clock option (not
RC), a SLEEPinstruction will cause the present conver-
sion to be aborted and the A/D module to be turned off,
though the ADON bit will remain set.
Turning off the A/D places the A/D module in its lowest
current consumption state.
Note: For the A/D module to operate in SLEEP,
the A/D clock source must be set to RC
(ADCS1:ADCS0 = 11). To perform an A/D
conversion in SLEEP, ensure the SLEEP
instruction immediately follows the instruc-
tion that sets the GO/DONE bit.
In systems where the device will enter SLEEP mode
after the start of the A/D conversion, the RC clock
source selection is required. In this mode, the digital
noise from the modules in SLEEP are stopped. This
method gives high accuracy.
7.6
A/D Accuracy/Error
7.7
Effects of a RESET
The absolute accuracy specified for the A/D converter
includes the sum of all contributions for quantization
error, integral error, differential error, full scale error, off-
set error, and monotonicity. It is defined as the maxi-
mum deviation from an actual transition versus an ideal
transition for any code. The absolute error of the A/D
converter is specified at < ±1 LSb for VDD = VREF (over
the device’s specified operating range). However, the
accuracy of the A/D converter will degrade as VDD
diverges from VREF.
A device reset forces all registers to their reset state.
This forces the A/D module to be turned off, and any
conversion is aborted.
The value that is in the ADRES register is not modified
for a Power-on Reset. The ADRES register will contain
unknown data after a Power-on Reset.
7.8
Connection Considerations
If the input voltage exceeds the rail values (VSS or VDD)
by greater than 0.2V, then the accuracy of the conver-
sion is out of specification.
For a given range of analog inputs, the output digital
code will be the same.This is due to the quantization of
the analog input to a digital code. Quantization error is
typically ± 1/2 LSb and is inherent in the analog to dig-
ital conversion process.The only way to reduce quanti-
zation error is to increase the resolution of the A/D
converter.
Note: Care must be taken when using the RA0
pin in A/D conversions due to its proximity
to the OSC1 pin.
An external RC filter is sometimes added for anti-alias-
ing of the input signal. The R component should be
selected to ensure that the total source impedance is
kept under the 10 kΩ recommended specification. Any
external components connected (via hi-impedance) to
an analog input pin (capacitor, zener diode, etc.) should
have very little leakage current at the pin.
Offset error measures the first actual transition of a
code versus the first ideal transition of a code. Offset
error shifts the entire transfer function. Offset error can
be calibrated out of a system or introduced into a sys-
tem through the interaction of the total leakage current
and source impedance at the analog input.
Gain error measures the maximum deviation of the last
actual transition and the last ideal transition adjusted
for offset error.This error appears as a change in slope
of the transfer function. The difference in gain error to
DS30272A-page 44
1997 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
